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JOURNAL OF GEOLOGY

JULY-AUGUST, 7593.

THE BASIC MASSIVE ROCKS OF THE LAKE SUPERIOR REGION.

Introduction. — Before the application of the microscope as a geological instrument the classification of rocks was dependent largely upon their apparent similarities and dissimilarities as noted by the unaided eye. When the use of this instrument became almost universal it was found that many rock types similar macroscopically were very different from each other in microscopic appearance, and very dissimilar genetically, while many of the apparently dissimilar types were discovered to owe their differences in appearance simply to the ordinary processes of weathering, which masked their original essential character- istics with the products of mineral alteration.

The rocks now known as gabbro are quite well characterized by peculiarities that are strikingly uniform in their essential features, though formerly the term was made to cover a large ~ number of closely related but quite different rock types. Their history affords a good illustration of the manner in which rock classification developed from its early independent form to its present highly differentiated but well defined one.

In the case of the gabbros, as well as in the case of other rock groups, there were at first included under one name all rocks whose superficial features were similar to those of the type originally described. Later, more discriminating study separated this group into a large number of subordinate groups, based on

Vol. I. No. 5 433

434 THE JOURNAL OF GEOLOGY.

slight differences noted in the characteristics of their components. The number of such groups became larger and larger until eventually there were almost as many sub-groups recognized as there were students who had investigated them. Thus the classification grew complicated, because the criteria upon which it was based were mainly unessential, though prominent, pecu- liarities in the components comprising the classified bodies. The next step, following the use of the microscope in rock investi- gation, consisted in the consolidation of several sub-groups into one larger group—a result due directly to the comparative ease with which the microscope enables the student to distinguish between the primary and secondary—the essential and unessen- tial—properties of rocks. After careful work of this kind had finally established the various varieties on the basis of mineral- ogical composition, attention was directed to the manner in which the rock components are associated —to the rock structure —and an explanation of variation in structure was sought in the environment of rock masses. The study ot the gabbros thus became a geological study rather than a mineralogical one.

The brief historical sketch of the classification of the granular basic rocks, with special reference to the differentiation of the gabbros from the remainder of the group, will thus serve to illustrate the successive steps with which rock classification in general has progressed. But the sketch is not offered here solely as an illustration of the development of rock classifica- tion. It was originally written with a view of emphasizing the distinctive differences between the gabbros and the coarse dia- bases. In the Lake Superior region there exist many coarse basic rocks that have been called indiscriminately ‘ gabbros.” Some of these possess the features of true gabbros, as defined by a study of the history of this group of rocks, and others the peculiarities of diabases. Until the distinction between these two types is clearly recognized, it will be impossible to discuss the causes of their differences. It is hoped that the present contribution will serve partly to clear the ground for a careful study of the coarse basic eruptive rocks of the Lake Superior

DDB A SHOCMNEAS SIVLE TO CLES, 1 1G. 435

province, which the writer desires to make as opportunities and time permit. The present plan proposes a series of papers ap- pearing in this Journal at irregular intervals. The first follows this introduction. The second will embrace a sketch of previous work on the basic rocks of the region, and the succeeding ones will treat of the gabbros and coarse diabases in the Huronian and Keweenawan areas on both sides of the lake.

I. BRIEF HISTORY OF THE CLASSIFICATION OF THE GABBROS AND NEARLY RELATED ROCKS.

At about the same time the names Euphotide and Gabbro were applied, respectively by Hatiy* in France and von Buch? in Germany, to rocks composed essentially of a foliated augite and a “compact feldspar.”” Haity describes the Euphotides as con- sisting of a compact feldspar and diallage, for which combina- tion he constructed the name from the two Greek words éy (blessed) and das (light), in allusion to the green and white mottling in the hand-specimens from many localities. Von Buch’s name, gabbro, was adopted from the Florentines? to cover a group of rocks that had been described at various times under a great number of different names, of which perhaps jade was the most common. Although gabbro was used by the Italians to designate what is now known as a diallagic serpen- tine, it has been accepted by nearly all geologists outside of France as the name to be applied to the group of rocks which von Buch so clearly and definitely separated from other allied rocks, and defined as made up of jade, feldspar and smaragdite.

Between the time of the appearance of von Buch’s paper and the publication of the first microscopic description of gabbros by Rose in 1867,4 many descriptions of these rocks appeared in

tTraité de Mineralogie, 2d Ed., IV., p. 535.

* Ueber den Gabbro, mit einigen Bemerkungen iiber den Begriff einer Gebirgsart. Geol. natur. f. Freund. zu Berlin, Mag. etc., 1810, IV., p. 128; 1816, VIL., p. 234.

3Cf, T. S. Hunr: Contributions to the History of Euphotide and Saussurite. Am. Jour. Sci., 2d Series, Vol. XXVII., 1859, p. 336.

4G. RosE.—Ueber die Gabbroformation von Neurode in Schlesien. Erster Theil. Zeits. d. deuts. Geol. Ges. XIX., 1867, p. 270.

436 THE JOURNAL OF GEOLOGY.

the various geological journals. In 1835 Gustav Rose* separated the rocks composed of labradorite and hypersthene, with acces- sory olivine, mica, apatite and ilmenite, from the gabbros, and ‘mat, thessame time suggesting that the term gabbro be confined to rocks con-~ taining labradorite and diallage. Many rocks were described as

hypersthenites or hypersthene rocks, because of the supposition

included them under the name “ Hypersthenfels,

that the highly foliated augite in them really belonged to this — variety of pyroxene. Delesse? and others showed that the com- pact feldspar of Haitiy, and the jade mentioned by von Buch as an essential constituent of gabbros (afterwards called saussurite by de Saussure Jr., and by Beudant) is in some cases a true plagioclase; and Hunt? showed that in other cases it consists of zoisite, of white garnet mixed with serpentine, or of meionite, and that the rocks containing these substances usually also con- tain hornblende, with the characteristics of Rose’s uralite. Hunt, further, declines to regard the rocks containing a triclinic feldspar and pyroxene (either augite, hypersthene or diallage) as true gabbros. He places them among the dolerites, and declares that the true euphotides described by Hatiy and de Saussure are mixtures of smaragdite and saussurite; a declara- tion that Cocchi+ made for the Tuscan rocks a few years later. Rocks composed essentially of diallage and saussurite Cocchi called granitones. Whatever may be the virtue of the objec- tions raised to the use of the name gabbro for plagioclase- diallage rocks, it still continued’ to be apphed to rocks thought to be of this composition, just as hypersthenfels or hypersthenite

™Ueber die Gebirgsarten, welche mit den Namen Griinstein und. Griinsteinporphyr bezeichnet werden. Poggendorf’s Annalen, XX XIV., 1835, p. 16.

? Recherches sur |’ Euphotide. Bull. Soc. Géol. d. France, VI., 1848-49, p. 547.

3T. S. Hunr.—On Euphotide and Saussurite. Am. Jour. Sci., 2d Series, Vol. XXV., 1858, p. 437; and Contributions to the History of Euphotide and Saussurite. Ibid., XX VIL. 1859, p. 326.

41. Coccut1.—Description des roches ignées et sedimentaires de la Toscane dans leur succession géologique. Bull. Soc. Géol. d. France (2) XIII., 1856, p. 267.

5Cf. P. KerrBEL.—Analysen einiger Griinsteiner des Harzgebirges. Zeits. d. deutsch. geol. Ges. IX., 1857, p. 569.

THE BASIC MASSIVE ROCKS, ETC. 437

was used to designate those in which hypersthene was supposed to occur.*

When Naumann? wrote the chapter on rocks for the second edition of his ‘Lehrbuch der Geognosie”’ he defined the gabbros _as characterized by the possession of labradorite or saussurite and platy augite, and divided them into two varieties—the gabbros, consisting of labradorite or saussurite, diallage and smaragdite, and the hypersthenites, containing hypersthene as the pyroxenic constituent, and sometimes a little secondary hornblende. Naumann recognized the difficulty of distinguish- ing between the gabbros and the diabases, even at this early day, before it was known that augite could have imposed upon it a parting as the result of pressure, for he says ‘‘ Diese Familie wurde sich vielleicht mit der nachstfolgenden des Diabases vereinigen lassen” (p. 573); and again, in a foot-note to diabase “Wenn der feldspathige Bestandtheil der Gesteine dieser Familie wirklich in allen Fallen Labrador ware, so wiirde es zweckmassig sein, die Familie des Gabbro mit ihr zu vereinigen” (p. 578). The norites described by Scheerer3 and Esmark, were thought probably to belong with the gabbros, but their true relations to the group were not known.

A few years later Kjerulf+ discussed the results reached by ‘himself and other Norwegian geologists, and ended by dividing the Norwegian rocks of the gabbro type into gabbros and norites, the former consisting of labradorite, augite, hornblende, and the latter of labradorite and diallage.

*VON RATH: Geognostische Bemerkungen iiber das Berninagebirge (?) in Grau- biindten. Ib. IX., 1857, p. 246.

RAMMELSBERG: Bemerkungen iiber den Gabbro von der Baste (Radauthal im Elarz) lbs 1850: p. Tol.

VoN RICHTHOFEN: Geognostische Bechreibung von Siid-Tyrol. 1860, p. 146. ? Lehrbuch der Geognosie. B. I. 1860, p. 573-577.

3 Geognostisch - Mineralogische Skizzen, gesammelt auf eines Reise an der Siid- kiiste Norwegens. Neues Jahrb. f. Min., etc., 1862, p. 668.

4Zusammenstellung der bisherigen Ergebnisse der geologischen Untersuchung Norwegens. Neues Jahrb. f. Min., etc., 1862, p. 144.

438 THE JOUKRNAE (OF (GEOLOGY,

The macroscopic examination of the rocks of this type con- tinued to give rise to many different methods of classifying them, but the general tendency after this time seems to have been toward the union of the gabbros and the hypersthenites into one group. Von Cotta,’ for instance, embraces the gabbros hypersthenites and norites under the single head ‘“‘gabbro,”’? and then divides this group into five sub- groups— gabbros cane tone of Cocchi and other Italians), with labradorite or saussurite and diallage, or saussurite and smaragdite (gabbro of Cocchi, Hunt and others); euphotides, equivalent to the saussuritized gabbros of later authors ; norites of Scheerer, which are regarded as gabbros containing a soda-orthoclase and some quartz; hypersthenites, consisting of plagioclase and hypersthene, and finally, Monzoni-hypersthenites, afterwards discovered by de Lapparent3 to belong to an entirely elhigteat group since they contain no hypersthene.

In the same year in which von Cotta’s classification appeared Aug. Streng+ began the task of reducing the number of varie- ties that had been separated as distinct sub - groups of the gene- ral group gabbro. In his article on the gabbros and associated rocks in the Harz he describes the former as made up of labradorite, diallage, hypersthene, augite, hornblende, brown mica, and ilmenite. Of the hornblendic constituent he says, it is ‘“‘ Kein selbstandiger Gementheil des Gabbro, und es werden daher durch ihre Anwesenheit keine besonderen Abanderungen erzielt.” It is fibrous and is intergrown with the augite and dial- lage. The labradorite is saussuritized (p. 935) and the saus- surite is therefore regarded as an _ unessential component. The hornblende- gabbros and the saussurite gabbros of the Harz

™Die Gesteinslehre. 2, Aufl. Freiberg, 1862.

?Cf. also Rocks Classified and Described. A Treatise on Lithology. By Bernhard von Cotta. An English edition by P. H. Lawrence, London, 1866.

3DE LAPPARENT: Sur la constitution géologiqué du Tyrol meridional. Annales des Mines. (6) VI., 1864, p. 259.

4AuG. STRENG: Ueber Gabbro und den sogenannten Schillerfels der Harzes. Neues Jahrb. F. Min., etc. 1862, p. 932.

DEAE BASIC MASSIVE, ROCIES, EG. 439

are nothing but altered forms of the fresh gabbro. It is rather surprising to one accustomed to the use of the microscope as a means of studying rocks to learn that such correct conclusions as to the inner constitution of rock masses could be reached without the aid of this instrument as were reached by Streng in his study of these rocks.* A few years later the same geologist examined the gabbros and serpentines of Neurode in Silesia and discovered that all of the so-called hypersthene of these rocks is probably diallage, and that the serpentine rock, which from very early times had been known under the name of forellen- stein, is really an altered gabbro, containing but a small amount of pyroxene. While Streng was examining the rocks of Silesia and deciding that the so-called hypersthenite is a true gabbro, Des Cloizeaux,? was investigating the hypersthenites and gabbros of France, with a view to their better classification. Des Cloizeaux declared as the result of his investigations that dial- lage, which is only a lamellar augite, and saussurite form eupho- tides and gabbros, and that many rocks that had been called hypersthenites or hyperites contain no hypersthene, but that the supposed hypersthene is diallage. He further proposes that dis- tinctions between gabbros and hypersthenites be made more clear by the use of the name diallagite for labradorite and diallage rocks, and hyperite for those composed of labrodorite and hypersthene or bronzite. Although the use of Des Cloizeaux’s name diallagite was not accepted by petrographers, all workers acknowledged the correctness of the statement that very many of the hypersthenites described from various localities are noth- ing more than gabbro in which the cleavage of the diallage is well marked. :

Thus far the study of the gabbros and related rocks had pro- ceeded without the aid to be obtained from the microscope. Many rocks had been described as belonging to the gabbro- type,

1A, STRENG: Ueber den Serpentinfels und Gabbro von Neurode in Schlesien. Neues Jahrb. f. Min., etc., 1864, p. 257.

2 ALF. Des CLOIZEAX: Sur les Classifications des roches dites hyperites et eupho- tides. Bull. Soc. Geol. d. Fr. XXI, 1864, p. 105.

440 THE JOURNAL OF GEOLOGY.

as defined by von Buch, and these had been given distinct names in accordance with the usual custom of distinguishing between the different varieties of a rock containing different character- istic mineralogical components. The years between r860 and 1862, perhaps, marked the height of the wave of differentiation. After this time the classification of the numerous varieties took the direction along which it was to be carried farther by micro- scopical methods. Some of the hornblende gabbros, the forel- lenstein, many of the hypersthenites and some of the norites had been shown to be altered or fresh forms of true gabbros. The characteristics of the components of the two groups of the gab- bros and the hypersthenites had been fairly well determined, and the similarity between many of the gabbros and the diabases had been pointed out.

The best résumé of the state of knowledge at this time con- cerning the rocks under discussion is to be found in Zirkel’s* “Lehrbuch,” published a year before the microscope was brought into use for the purpose of studying these rocks. Zirkel collected the observations of the different workers and incor- porated them along with his own in sucha way as to give an excellent impression of the value of macroscopic rock determi- nations, when undertaken by competent observers and aided by chemical analyses. He distinguishes as gabbros those rocks con- taining labradorite and diallage, at the same time agreeing with Bischof? in the view that the latter mineral is merely a variety of augite. Saussurite he regards as sufficiently characteristic of some gabbros to warrant their separation from others. He like- wise looked upon smaragdite, which was thought to be an inter- growth of augite and green hornblende, as an essential constitu- ent of some gabbros, and these he separated from the diallage gabbros under the name of smaragdite gabbros. The hypers- thenites are described at some length, with the appended state- ment that many hypersthenites are probably gabbros. The

{ *F. ZIRKEL: Lehrbuch der Petrographie. Bonn, 1866, p. 112. * BiscHoF: Lehrbuch der chemischen und physkalischen Geologie. Bonn, 1864. 2 Aufl. II, p. 654.

THE BASIC MASSIVE ROCKS, ETC. 441

norites of Scheerer are classed among the gabbros and the hypers- thenites, and those of Esmark are said to belong partly with these and partly with the diorites.

In the year succeeding the appearance of ZGirkel’s book, as has been stated, Rose* made the first microscopical examina- tion of gabbros that has been recorded. He found among the Silesian gabbros two varieties, one of which is black and con- tains olivine, and the other green and free from olivine. Tscher- mak? followed Rose with a description of some Austrian gab- bros, and an announcement that many serpentines are altered gabbros, and that Streng’s forellenstein is only an olivine gabbro. He concluded, further, that augite and diallage differ only in physical properties, and therefore that gabbro ‘“‘ist eine Abtheil- ung des Diabas”’ (p. 168).

In the few years succeding Tschermak’s paper several con- tributions of great importance were added to the literature of the gabbros. Zirkel3 recognized olivine varieties of these rocks among the Tertiary formations on the islands off the west coast of Scotland, and succeeded in showing that the hypersthenites described by Macculloch from the island of Skye contain no hyphersthene. He further pointed out as important the fact that the plagioclase associated with diallage is rich in inclusions, while that associated with ordinary augite is free from them. In the same year Hagge‘ continued the work that had been so ably begun by DesCloizeaux in 1864. He made a careful micro- scopic examination of all the important gabbro and hyphersthe- nite occurrences recorded, and reached a result very similar to that of Des Cloizeaux. He found that very many of the rocks

™G, RosE: Ueber die Gabbroformation von Neurode in Schlesien, Erster, Theil. Zeits. d. deutsch. geol. Gessel. XIX, 1867, p. 270.

2Die Porphyrgesteine Oesterreichs aus der mittleren geologischen Epoche. Wien, 1869.

3F, ZIRKEL: Geologische Skizzen von den Westkiiste Schottland. Zeits. d. deutsch. geol. Gessell. XXIII, 1871, pp. 58 and 92.

4R. Hacce: Mikroskopische Untersuchungen uber Gabbro und verwandte Gesteine. Kiel, 1871.

442 THE JOURNAL OF GEOLOGY.

heretofore described as containing hypersthene, have none of this mineral in their composition. He divided the gabbros into those containing olivine and those without this constituent, and from the latter separated a group which he called saussurite gabbros, recognizing at the same time, however, that saussurite is an alteration product of labradorite. He described it as con- sisting ‘‘of small. crystal needles, prisms and grains, which are colorless or light-green, and are scattered irregularly in a ground mass with the appearance of a colorless glass, which often forms clear patches in the saussurite’’ (p. 52).

Six years after Rose’s description of the Neurode gabbro, and seven years after the appearance of Zirkel’s masterly class- ification of rocks based almost entirely upon their macroscopic properties, the latter geologist was enabled to issue a second volume containing a classification of rocks based on the micro- scopical characters. In this volume‘ he defines the gabbros as granitic in structure, and consisting principally of plagioclase and diallage, usually with the addition of olivine. The plagio- clase is usually labradorite. It usually contains fluid inclusions and numerous little dark needles and prisms arranged in a defi- nite order. The diallage is filled with small brown plates and the olivine is characterized by thousands of fantastically shaped hair- like bodies. The structure of genuine gabbros is described as coarsely or finely granular. They contain no porphyritic crys- tals and no unindividualized ground mass.

The group of hyphersthenites had by this time become almost depleted of its members. Most of the hyphersthenites had been found to be diallagites, in the sense of Des Cloizeaux, so that but four undoubted occurrences of this rock were left to be included by Zirkel in the group. On the other hand, the number of ‘‘forellensteins”’ had increased to such a degree that a group was formed of the same classificatory value as that of the hyper- sthenite group. These rocks were described as having the structure of gabbros, while at the same time they contain but

‘FR, ZIRKEL: Mikroscopische Beschaffenheit der Mineralien und Gesteine. Leipzig, 1873.

THES BASIC MASSIVE ROCKS, ETC. 443

little diallage. Their separation from the gabbros and the hyp- ersthenites seems to be upon mineralogical grounds solely ; since emphasis is laid upon the fact that their feldspar is apparently anorthite. Of such great importance was the-mineral constitu- tion of rocks regarded at this time, that we find no statement made with respect to the similarity between many diabases and many gabbros. The facts pointed out by earlier investigators to the effect that augite and diallage are but slightly different varieties of the same mineral, had been overlooked, or ,had, at any rate, been regarded as of little importance, since these expressions of opinion had for the most part not been founded on the study of thin sections. The microscope was used principally for the determination of the nature of the constituents of rocks, and had therefore emphasized their mineralogical composition out of due proportion to its importance.

The influence of Zirkel’s book upon geologists in all parts of Europe was soon felt in the increased number of purely pet- rographical papers published in the journals; and this increased interest soon manifested itself in studies that included more than a mere description of rock sections. Vogelsang* had, years before, shown that there were great possibilities in the new science of petrography, but in the flush of excitement over the discovery of an easy and exact method of rock analysis, these possibilities were left unexplored until geologists became quite well acquainted with the essential components of the most im- portant rock types.

Soon after the composition of the important rock types became fixed, attention was turned more particularly to their structure. Professor Judd? examined the gabbros in the denuded cores of Tertiary volcanoes in Scotland, and found that while diallage is the prominent pyroxene of the lower portions of the

™H. VOGELSANG: Philosophie der Geologie und Mikroskopische Gesteins- studien. Bonn. 1867.

2J. W. Jupp: The Secondary Rocks of Scotland. Second Paper. On the Ancient Volcanoes of the Highlands and the Relations of their Products to the Mes- ozoic Strata. Quart. Jour. Geol. Soc., XXX. 1874, p. 220.

444 THE JOURNAL OF GEOLOGY.

masses, in their upper portions the diallage is replaced in large part by augite. Many other papers of importance were publish- ed, and in most of these the structure of the rocks described was more or less briefly alluded to. Wiik* announced the fact that many of the Finnish rocks classed by Zirkel among the hypersthenites are olivine-diabases and olivine gabbros, while Stelzner? filled the gap thus produced in this group by the dis- covery of a bronzite gabbro from the Monte Rosa district in the north of Italy. Vallee- Poussin and Renard? made a thorough examination of the plutonic rocks of Belgium and the eastern part of France, and discussed the composition and structure of some gabbros.

The result of these and other workers were collected and edited by Rosenbusch* in his well-known book on the micro- scopical characters of massive rocks, in which the fixing of rock types which had been begun by Zirkel was carried out ina scheme which was not improved upon until the same author published the second edition of his treatise ten years laters. In the scheme proposed in 1877, the gabbros were placed among the pre- Tertiary massive granular rocks. The group was made to include all pre- Tertiary rocks consisting essentially of diallage and plagioclase in their unaltered state, either with or without olivine. Saussurite was recognized as a secondary product pro- duced by the alteration of plagioclase, and green hornblende (actinolite and smaragdite) as the result of an alteration of dial- lage. The saussurite and the hornblende gabbros were no longer

‘F, J. Wik: Mineralogiska och petaografiska meddelanden. Ref. Neues Jahrb. f. Min., etc., 1876, p. 206. 2A. STELZNER: Briefliche Mittheilung. Zeitz. d. d. geo. Gessell., XXVIII. 1876, p. 623. 3 Ch. de la VALLEE-PoussIN, et A. RENARD: Memoire sur les caracteres miner- alogiques et stratigraphiques des roches dites plutoniennes de la Belgique et de I’ Ardenne francaise. Bruxelles, 1876, pp. 62-76 and 125-128.

4H. RosenpuscH: Mikroskopische Beschaffenheit der Massigen Gesteine. Stuttgart, 1877.

5H. RosENBUSCH: Mikroskopische Beschaffenheit der Massigen Gesteine. 2te Aufl. Stuttgart, 1887.

THE BASIC MASSIVE ROCKS, ETC. As

regarded as sub-groups of the gabbro family, but were looked upon merely as altered gabbros. Magnetite and titanic iron oxide as well as apatite were mentioned as accessory in all mem- bers of the group, and hornblende, rhombic pyroxene, brown mica and quartz were spoken of as occurring in many (p. 459). The difficulty of distinguishing between a gabbro and a diabase was Clearly appreciated. -The distinction between diallage and augite, upon which is based the mineralogical distinction between gabbro and diabase, is acknowledged to be of doubtful value for this purpose, since some rocks with the other properties of gab- bros have an augite devoid of the diallagic parting, while others with many of the properties of diabase possess an augitic con- stituent with the parting highly developed. ‘ Héchstens diirfen sie (the gabbros) als ein unterabtheilung der Diabase, welche sich durch eine eigenthiimliche Structur und Theil- barkeit ihres Pyroxens charakterisirem.’’ The structure of the gabbros was said to vary within narrow limits. They are always coarse-grained rocks whose different structures depend princi- pally upon the different amounts of their constituents. Since they are so well characterized by the monotony of their texture, and since no gradations* between them and porphyritic or glassy forms were known, while on the other hand the structure of the diabases varies so widely between holocrystalline and glassy, the former were regarded as a distinct rock type. Rosenbusch, however, declined to regard the gabbros as dependent for their individuality upon the mere possession of an augite with pinacoi- dal parting, but was inclined to look upon them as rocks occu- pying a position in the scheme of classification intermediate between that of the diabases and that of the norites, the latter

*MR. T. T. Groom has recently described a gabbro glass associated with gabbro at Carrock Hill in the Lake District, England, under the name carrockite. Since this glass occurs only as a narrow selvage where the gabbro has cooled rapidly in contact with preéxisting rocks, it cannot be considered as contradicting the above general statement. The structure is not one connected genetically with the rock itself, but is a local phenomenon dependent upon extraneous circumstances. See T. T. Groom: On the Occurrence of a new form of Tachylyte in Association with the Gabbro of Carrock Fell, in the Lake District. Geol. Magazine. Jan. 1859, p. 43.

446 THE JOURNAL OF GEOLOGY,

consisting of plagioclase and an orthorhombic pyroxene, and therefore corresponding in part to Zirkel’s hyphersthenites. The - principal difference between the gabbros and diabase was, then, one of structure, while subordinate to this was a difference in mineralogical composition. In his sentences closing the dis- cussion of the gabbros Rosenbusch writes: ‘‘ Man miusste aber alsdann das Hauptgewicht fur die Absonderung der Gabbros nicht auf den eigenthimlich struirten Diallag legen, sondern darauf, dass sie einen pinakoidal spaltbaren klinorhombischen Pyroxen als wesentlichen und daneben einen rhombischen Pyrox- en als accessorischen Gemengtheil enthielten.” The distinction here made is evidently a strained one, for quite a number of gabbros were known in which the structure is the typical gabbro structure, while at the same time they are entirely free from rhombic pyroxenes. The new group name “ Norites”’ is borrowed from Esmark and Scheerer, although the rocks described by these geologists are by no means typical of the group. The advantage of the name over ‘‘hyphersthenite”’ is readily appre- ciated when it is remembered that the rhombic pyroxene of these rocks is not always hyphersthene.

The publication of Rosenbusch’s classification of the massive rocks fixed the characteristics of the various types with some degree of scientific accuracy. There was, however, much to be learned concerning the less well known types, and much more to be discovered concerning the relations of the various types to each other.

The work of Judd, referred to above, was the beginning of a severe attack on the wavering line of geologists who still clung to the belief that mineralogical differences alone should deter- mine the class to which a rock should be referred. It would be unprofitable in the present place to mention all of the important articles treating of gabbros and their varieties. It will be suf- ficient for our purposes to refer briefly only to those papers in which new types of gabbro are described and a little more fully to those which treat of the classification of these rocks.

The existence of true hyphersthenites (norites), of gabbros,

THE BASIC MASSIVE ROCKS, ETC. 447

and of types intermediate between these, was established at the time that Rosenbusch’s book appeared. In this year (1877) Toérnebohm* suggested that the name hyperite be used for the latter classs, composed essentially of plagioclase, diallage and an orthorhombic pyroxene, that the term gabbro should be used to designate plutonic rocks in which the pyroxene is diallage, and that hyphersthenite (or norite) should be restricted to those containing a rhombic pyroxene as their principal augitic constit- uent. This suggestion has not met with a very wide acceptance because the gradation between the three types is very gradual, and in all cases the geological relations of the types are the same. It is convenient, however, asa descriptive name for those gabbros containing two pyroxenes.

In the same year Streng’ investigated the crystalline rocks of Minnesota and described a gabbro from near Duluth, in that State, to which he gave the name hornblende- gabbro, because of the supposition that the brown hornblende it contains is primary. Irving,3 however, has shown that much of the brown hornblende in the rocks of the Lake Superior region is secondary. He thought that nearly all, if not all, of the hornblende of the hornblende gabbros is of this nature. Williams‘ has also shown that compact brown horn- blende is often a secondary product of the alteration of augite; and Wadsworth’ holds to the view that this is the character of all the hornblende in the Lake Superior gabbros.

™A. E. TORNEBOHM: Ueber die wichtigsten Diabas und Gabbrogesteine Schweden. Neues Jahrb. f. Min., etc., 1877, p. 387.

2A. STRENG and J. H. Kioos: Ueber die krystallinischen Gesteine von Minne- sota in Nord Amerika. Neues Jahrb. f. Min., etc., 1877.

3R. D. Irvine: On the Paramorphic Origin of the Hornblende of the Crystal- line Rocks of the Northwestern States. Am. Jour. Sci., Vol. XXVI, 1883, p. 27; Ib. XXVII, 1884, p. 130.

4G. H. WILLIAMS: On the Paramorphosis of pyroxene to hornblende in Rocks. Am. Jour. Sci., XXVIII, 1884, p. 259.

5M. E. WapDsworTH: Preliminary Description of the Peridotytes, Gabbros, Diabases and Andesytes of Minnesota. Bull. No. 2. Geol. and Nat. Hist. Surv. of Minn., St. Paul, 1887, p. 66.

448 THE JOURNAL OF GEOLOGY.

If Irving, Williams, and Wadsworth are correct in their opin- ion, the hornblende-gabbro of Streng is merely an altered form of gabbro, and therefore it does not deserve a distinctive name (except for the mere purpose of description), any more than do the saussurite-gabbros.

Another type of gabbro to which a distinctive name has been given is also found in the region surrounding Lake Superior. This is an orthoclase-gabbro which has been carefully described by Professor Irving. An unstriated feldspar taken to be ortho- clase had been discovered in gabbros from European localities by various petrographers, but it was usually present in such small quantity that but little importance was attached to it. In this country Pumpelly’ and Julien? identified orthoclase in cer- tain gabbros from Wisconsin, and Irving? discovered it in simi- lar rocks from both Wisconsin and Minnesota. The latter author describes the orthoclase as often reddened and charged with secondary quartz. He mentions in detail the characteristics of the rocks containing it, and regards the differences noted between these and the non-orthoclastic gabbros as of sufficient importance to warrant their separation from the latter under the variety name orthoclase-gabbro.

Within the past few months still an additional gabbro variety has been brought into prominence by Adams‘ and by Lawson$ working in different portions of North America. This consists essentially of plagioclase with gabbro characteristics, with which is associated only now and then a grain of pyroxene or magnetite. in containing no olivine, and from

It differs from ‘‘forellenstein”’ ™R. PUMPELLY: Geology of Wisconsin, III, 1880, pp. 38, 40, 41.

7A. A. JULIEN: Microscopical Examination of eleven rocksfrom Ashland County, Wisconsin. Geol. of Wisconsin, III, 1880, p. 233.

3R. D. Invinc: The Copper-Bearing Rocks of Lake Superior. U. 5S. Geol. Survey, Monograph V, pp. 50-56.

4F. D. Apams: Ueber das Norian oder ober-Laurentian von Canada. Neues.

Jahrb. f. Min., etc. B.B. VIII, p. 419.

5A. C. LAwson: The Anorthosytes of the Minnesota Coast of Lake Superior. Geol. and Nat. Hist. Surv. of Minn. Bull. No. 8, p. 1.

THE BASIC MASSIVE ROCKS, 1G. 449

gabbro proper in the absence of diallage and orthorhombic pyroxenes. To this variety belong the norite’ of New York State, the labradorite rock of Labrador, and the ‘anorthite rock”’ of Irving? from the north shore of Lake Superior.

But if we are to regard the anorthosites as gabbros in which pyroxene and olivine are wanting, we must pass to the other end of the series and include in the gabbro group those rocks in which plagioclase is wanting, and in which the sole essential components are pyroxene and olivine, or the pyroxenes alone — the peridotites of most authors and the pyroxenites of Williams.3 Judd# has shown conclusively that the peridotites of Scotland are but phases of the gabbro with which they are associated, con- sequently they may with good reason be included within the gabbro group. But other peridotites and many of the pyrox- enites must be regarded as distinct rocks. They are the products of the cooling of magmas of an essentially different composition from that of the gabbros, hence their consideration may well be excluded from this history.

The varieties of gabbro that depend upon mineralogical com position, so far as known, have been carefully described and named by their investigators —the names referring for the most part to the nature of their iron-bearing constituents. These are gabbro and olivine-gabbro, hyperite, norite, peridotite and pyrox- enite, together with the alteration products of the first named, viz.: hornblende, saussurite, orthoclase, and perhaps quartz-gabbro,° the latter of which is more properly a quartz norite, since it con- tains no diallage. The varieties whose names have reference to

rCf, F.D. Apams: 1. c., p. 475 and 483.

2R. D. Invinc: Copper-Bearing Rocks of Lake Superior. Mon. V. U.S. Geol. Survey, p. 438.

3G. H. WitiiaAms: The non-Feldspathic Intrusive Rocks of Maryland and the course of their Alteration. Amer. Geologist, VI, 1890, p. 95. Not the pyroxenites of the French authors, which are mainly augite gneisses or schistose gabbros.

4J. W. Jupp: On the Tertiary and older Peridotites of Scotland. Quar. Jour. Geol. Soc., XLI, 1885, p. 357.

5Cf. U.S.GranT: Note on the Quartz-Bearing Gabbroin Maryland. Johns Hop- kins Univ. Circ. No. 103.

450 THE JOURNAL OF GEOLOGY.

the feldspathic component are the orthoclase-gabbro of Irving and the eukrites’ of the older authors. The latter name was proposed to designate rocks whose feldspar is anorthite. It never received a very wide application owing partly to the diffi- culty of distinguishing positively anorthite from the other plagio- clases. Since the discovery by Tschermak that the plagioclases form a series of isomorphous compounds, the value of the dis- tinction recognized by the name has disappeared and the name itself has fallen into disuse.

In addition to these there are two other varieties that seem to be sufficiently well characterized to deserve special names. One of these, the anorthosite, consists exclusively of gabbroitic plagioclase and the other ‘“forellenstein” contains olivine and plagioclase.

During the past few years nearly all the work on the gabbros has tended toward the separation of these rocks from the dia- bases by sharper lines than those based merely on mineralog- ical distinctions. All those who had attempted to separate the two groups by the methods in use had failed, and some had thought it well to include the two in one group. The views of the earlier petrographers on this subject have been referred to. Later petrographers have accorded with these in their recog- nition of the fact that the value of the pinacoidal parting of diallage is not of great importance for the purpose of rock classi- fication. The discovery of Judd, referred to above, produced a marked effect on the work of those who followed him in the same field.

In 1883 J. Roth? declared that the position of the gabbros with respect to the diabases depends upon the significa- tion given to diallage. If we regard it as an altered augite with a pinacoidal parting produced by twinning it is found, as Rosenbusch has already stated, that the parting may occur in the pyroxene of some rocks without the presence of

‘For a discussion of the eukrites see J. RorH: Allgemeine und Chemische Geol- ogy, II, 1883, p. 200.

2 Allegemeine und Chemische Geologie, II, p. 185.

WUE AHA SHOVES SUMED TAO GIES WS aC. ASI

twinning lamellae. On the other hand, the pinacoidal parting is entirely absent in cases where twinning lamellae are present. Consequently not much dependence can be placed upon this constituent as a means of distinguishing between gabbros and diabases. The former rocks are evidently related to the latter, whose typically granular, holocrystalline forms they are. Irving,’ in his work on the geology of the Keweenawan series in Michi- gan, Wisconsin, and Minnesota, was compelled to make use of coarseness of grain as a means of distinguishing between diabases and gabbros, both of which were thought by him to occur as flows. ‘It is evident,’ he writes, ‘“‘that my observations on these north Wisconsin gabbros bear out the conclusions reached by certain European lithologists, as to the subordinate import- ance of the foliated condition of augite, by which gabbro is ordinarily separated from diabase, of which it would seem to be merely a phase. Nevertheless, the name is here retained, not only because most of our rock is very close to the typical Euro- pean gabbros, but more especially because it is so sharply con- trasted with the typical Keweenawan diabase that a separate name seems necessary.” And again, when speaking of the dia- bases, he says,? “Although grading through coarser kinds into the coarse olivine-gabbros, the fine-grained rocks here considered deserve a place by themselves. The gradation into the coarser kinds has never been observed in any one bed, and they are very strongly marked by their external characteristics, both in the fresh and altered states.”

The prime distinction between the two classes of rocks is, then, one based upon structure and not upon the difference between the augitic and diallagic nature of its pyroxenic con- stituent. The structure of the most typical gabbros was recog- nized by most geologists to be granitic and that of the diabases as ophitic. Professor Judd3 proposed to restrict the name

* Geology of Wisconsin, III, 1880, p. 171.

2 Copper-Bearing Rocks of Lake Superior, p. 69.

3J. W. Jupp: On the Tertiary and older Peridotites of Scotland. Quart. Jour. Geol. Soc., Vol. XLI, 1885, p. 354; and On the Gabbros, Dolerites and Basalts of Tertiary age in Scotland and Ireland. Ib. XLII, 1886, p. 49. ;

452 LHE JOOCKRNALE OR GEOLOGY,

gabbro to granitic forms of plagioclase pyroxene rocks, and to designate as diabases the ophitic, porphyritic and glassy forms. He agrees with Zirkel* and Lasaulx? in regarding the Hebridean rocks as Tertiary in age, and at the same time as corresponding in all their characteristic features with older augite-plagioclase rocks of granitic structure. These rocks possess not only the structure of the most typical gabbros, but their various constitu- ents are marked by the same microstructure. The plagioclase, olivine, and augite contain the numerous inclusions that were so early recognized as characteristic of these minerals in gabbro, and the latter mineral, the augite, is marked by the diallagic parting, which is the result of the action of a secondary process upon ordinary augite. The process, called by Professor Judd$ schillerization, is moreover shown to be a function of the depth at which the original rock magma cooled, and the granitic struct- ure of the rock mass is demonstrated to be likewise due to the . fact that the rock possessing this structure crystallized at some depth below the earth’s surface.

The work of Professor Judd established two great facts, viz.: first, that the age of a rock cannot serve as a basis for rock classification, since it has but little to do with the development of a characteristic structure; and, second, that the geological position of a rock mass is the condition determining not only its structure, but also the peculiar features possessed by its constit- uents. The rocks which it is proposed to call gabbros are marked by both of the characteristics of deep-seated rocks, while the diabases possess neither of them. The differences between the two groups of rocks, as expressed by their structures, ~ are probably differences that are dependent upon the geological conditions under which they solidified.

Zeits. d. deutsch. Geol. Gesell. XXIII, 1871, pp. 58 and 93. ? Min. u. Petrog. Mitth. I, 1878, p. 426.

3Cf. also J. W. Jupp: On the Relations between the Solution-planes of Crys- tals and those of Secondary Twinning; and on the Mode of Development of Negative Crystals along the former. A Contribution to the Theory of Schillerization. Min- eralog. Magazine, VII, p. 81.

IED IASC: MHAS| SINE, IROCIGS, 12 ING. 453

Professor Rosenbusch* clearly appreciated the value of the work on the basic rocks of the Hebrides, for, in the second edi- tion of his Mikroskopische Physiographie, he defines the gabbros as hypidiomorphically granular plutonic rocks,-consisting of a basic plagioclase, diallage, or a pyroxene resembling diallage, rhombic pyroxenes and often olivine. The important feature in this definition is the characterization of the gabbros as plutonic rocks. The diallage no longer defines the gabbro. The condi- tions which determined the characteristic structure of the rock at the same time produced the diallagic structure in its pyroxenic constituent. The structure of the typical gabbros, as defined by Rosenbusch, is granular, with the components all equidimen- sional. Notwithstanding the fact that some plutonic rocks of this class seem to lack the granitic structure, it remains true that the typical gabbro is well described by this definition.

When, however, we seek to separate the gabbros from the diabases we are met at the outset with the same difficulties that have always stood in the way of an exact separation of these two rocks. Rosenbusch? describes the diabases as possessing some of the features of plutonic rocks, while at the same time they possess other features that are eminently characteristic of rocks that have flowed out upon the surface of the earth. He never- theless includes them with the plutonic rocks, stating, however, at the same time that they occur principally as dykes and interbedded flows ; are more frequently interstratified with schists than are any other plutonic rocks; and that their predominant structure is the ophitic. That there is a fundamental difference between the two rocks is shown by the fact that the typicial gabbro can not be traced into porphyritic or hyprocrystalline varieties, nor is it ever accompanied by tufas. Whereas the diabases are often porphyritic, and are not infrequently associated with diabasic tufas. A consideration of these phenomena, together with the great differences in the structures of the typical gabbros and diabases, have led Loewinson-Lessing to regard the gabbros as

1 Mikroskopische Physiographie, der Massigen Gesteine, 2, Auf. 1887, p. 132. 2 Mikroskopische Physiographie, 2 Auf. II, pp. 174 and 195.

454 THE JOURNAL OF GEOLOGY.

the intrusive? equivalents of the diabases, which he thinks were effusive under water, with the augite porphyrites as their equi- valent terrestrial effusives. The conclusions of Loewinson-Les- sing are not at all startling in their originality, for the wide separation in origin of the two groups of rocks here discussed has been suspected by petrographers ever since the classification of rock-types based on age, mineralogical composition and struc- ture, gave way to the classification founded on geological rela- tionships. The placing of the diabases with the effusive rocks will probably be looked upon with favor by all petrographers, especially since Professor Rosenbusch? has treated of them as members of this group in his Heidelberg Lectures, and Brauns3 has shown that a typical-lava flow of a suitable composition may have the diabasic structure developed in it but a few feet below its upper surface.

Lawson,‘ on the other hand, has shown conclusively that the coarse grained, ophitic diabases, jnterbedded with the Huronian slates and quartzites on the north shore of Lake Superior, are not effusive, but are intrusive, and that their intrusion between the fragmentals with which they are associated, must have occurred at a time when these were deeply buried under a great thickness of overlying rocks. Consequently these coarse, holocrystalline diabases must be regarded as intermediate in their geological relationships, as they are in their structural features between the hypidiomorphic, holocrystalline, plutonic gabbros, and the typi- cally ophitic, hypocrystalline effusive diabases.

But if the hypocrystalline diabases are classed with the effu- sives, their position with respect to the melaphyres and _ basalts

*F. LOEWINSON-LESSING: Quelques considerations genetiques sur les diabases, les gabbros et les diorites. Bull. d. 1. Soc. Belge. de Geol. etc., II, 1888, p. 82. * Cf, Zeits. d. deutsch. geol. Ges. XLI, 1890, p. 533.

3R. BRAUNS: Mineralien und Gesteine unf dem hessischem Hinterland II, 3, Diabas mit geflossener Oberflache (Strick oder Gekroselave) von Quotshausen. Zeits. d. deutsch. geol. Ges. XLI, 1890, p. 4o1.

4A. C. Lawson: The Laccolitic sills of the northwest coast of Lake Superior, Geol]. and Nat. Hist. Surv. of Minn. Bull. No. 8, 1893, p. 24.

THE BASIC MASSIVE ROCKS, ETC. 455

must be defined. Brauns,* in the article referred to in the last footnote, has attempted this correlation. He finds, after review- ing the opinions of various writers on the subject, that ‘It is not possible to distinguish between diabase and melaphyre on purely petrographical grounds, whether olivine is considered as an essential component of melaphyres, as Rosenbusch holds, or whether it is regarded as unessential in these rocks.” In order to construct an exact definition for these three types of rock Brauns is compelled to fall back upon distinctions of age, although Rosenbusch? in his last article, in which he refers to this subject, declares it as his opinion that ‘it requires no great foresight to prophesy that in the not very distant future, this separation [ of the effusive rocks into an older and a younger series| will be proven untenable.” In spite of the almost certainty that Braun’s classification will meet with but little favorable acceptance, it is given here in order to complete the sketch of the history of gab- bros and the related rocks. According to Brauns, the basalts are made to include rocks of this class from recent time to the beginning of the Tertiary age. The limit of separation between the melaphyres and the diabases passes through the productive ' coal measures; rocks older than this are regarded as diabases, while the melaphyres extend from the Carboniferous to the Ter- tiary. Each group is divided into varieties, according to struct- ure, and into sub-varieties according to mineralogical composi- tion. A tabular grouping of the principal divisions of the effusive rocks of the composition of diabase follows:

HEEL UO. “Oa ee Mesozoic to Tertiary to ductive Coal Terti R Measure ce ertiary. ecent. Granular - - - Diabase. Melaphyre. Basalt. Porphyritic - - Diabase-porphyrite. |Melaphyré-porphyrite.| Basalt-porphyrite. Glassy - - - - Diabase-glass. Melaphyre-glass. Basalt-glass.

It is very evident that the introduction of the diabases among

™R. Brauns: Ib. 5. Systematik der Diabas, Melaphyr und Basaltgesteine. Ib. Pp. 532.

2H. ROSENBUSCH: Ueber die chemische Beziehungen der Eruptivgesteine. Min. u. Petrog. Mitth. XI, 1890, p. 146.

456 THE JOURNAL OF GEOLOGY.

the effusive rocks has created a disturbance in the melaphyre- basalt group that can only be quieted by the ejection of one of the members of the group, probably the melaphyres, from the position it now occupies. When this is done it is probable that the diabases will take the position thus left vacant, and the plagioclase-augite rocks will be found to occupy these places with respect to each other: the gabbros, the position of a deep seated rock, the diabases that of the corresponding holocrystalline effusive, and the basalt that of the hypocrystalline equivalent. WWE Ss IBAIGLIB,

WATERVILLE, ME., June 1, 1893.

NOMES OWN Wels, SIVAIMS Islets JUN Welle, MMUNBES AND MINING BUILDING *AT.THE WORLD'S COMUINNBIVAIN IS 2XIPOSIMMIOIN,, » ClsUUCevGrO),

THE Mines and Mining Building at the World’s Columbian Exposition contains exhibits of the different mining industries of the various states of the United States and of foreign coun- tries, exhibits of many of the manufactured products derived from these industries, exhibits of various kinds of mining and engineering machinery, and many private mineralogical and pet- rographical collections of great value and interest. To describe the whole would require a volume, and it is the intention of the present paper to discuss only some of the more important features of the state exhibits, with occasional references to the foreign exhibits.

A mining exhibit should seek to show the actual resources of the region it represents, whether these resources be developed or undeveloped, and to give the different products prominence according to their present or prospective importance to the region. The products of present importance should be exhib- ited as showing what the region actually produces; the pro- ducts of prospective importance should be exhibited as show- ing what the region contains in bountiful quantities, but what is not yet utilized, either from lack of knowledge on the part of the public concerning it, from temporary inaccessibility, or from some other cause. By this means many valuable mate- rials, which have not yet been developed, are brought to the attention of the general public, and often to that of specialists on such subjects, and in this way receive quicker development than if they had not been exhibited. It is often difficult to give the proper relative importance in an exhibit to products actually being mined and those which have not yet been devel-

457

458 THE JOURNAL OF GEOLOGY.

oped, but an effort can be made in this direction, and it is always possible to state that a given material is not being mined.

A properly arranged mining exhibit affords advantage in two directions. In the first place, it benefits the exhibitor in calling attention to his products, and in the second place it is of great educational benefit to the general public as showing what different regions produce. The best interests of the exhibitor are served by a true exhibition of his products; while the educa- tional value of an exhibit depends almost entirely on the exact- ness with which the exhibit reproduces the actual state of affairs, for if the exhibit is exaggerated in one direction or neglected in another it leaves with the uninitiated a false idea of the resources of the region.

Most of the state exhibits have been collected and arranged by commissioners appointed by the state, and are supposed to fully represent the resources of the state. Many of the foreign exhibits, however, are made up of the individual exhibits of different mining companies, and often show only a certain class of the products of a given region. They are, therefore, not claimed to always represent the whole of the mining industries of a region’ and cannot be criticised for not doing so. The state exhibits, however, should fairly and honestly represent the mining industry within their borders, giving undue prominence to no one product, and neglecting nothing that should be repre- sented. In this feature some of the states have been highly successful, while others have done worse than make a failure, for they have misled those who are not sufficiently acquainted with the resources of the country to know that the exhibit is not char- acteristic. Some of the states have exhibited and made very prominent great amounts of materials which they do not possess in paying quantities; other states have actually exhibited mate- rials which they do not possess at all, and which have been obtained from other states, a proceeding which is very mislead- ing to the general public.

«A notable exception to this is the New South Wales exhibit, which is one of the best in the building.

EXHIBITS. IN MINES AND MINING BUILDING. 459

lit-bas becnmthe object of Mir oh gia Vi .Skith Chret or’ the: - Mines and Mining Building, to make the mining exhibit truly characteristic of the states and foreign countries represented, and thus to give it the greatest possible value to the general public and to the individual exhibitors. His supervision has been wise and systematic, and it is to him that a large part of the success of the mining exhibit is due. Where failures have been made they have been the fault, not of the Chief of the Mines and Mining Building, but of the commissioners under whose charge the exhibits were prepared, or else of the govern- ment of the state or country which they represent. Very often the commissioners have been so hampered by the fancies of the mine owners or others in their districts that, though entirely capable of doing so, they have been unable to make a creditable exhibit of the regions they represent. Many of the state exhib- its contain a large amount of good and characteristic material which is often rendered useless and often ridiculous by bad and ignorant arrangement; while many otherwise good and charac- teristic exhibits are rendered very unattractive by the slovenly way in which they are exhibited and the untidiness of the cases and specimens. Of course the last mentioned defects are minor ones, especially to those interested in the subject; but at the same time the neatness of presentation has a great influence on the attractiveness, and hence on the benefits, of an exhibit to the general public. An exhibit which has no natural beauty may be made very attractive by neat and systematic arrangement, while on the other hand, an exhibit of beautiful things may be made actually repulsive by a slovenly and dirty mode of presentation.

The different state exhibits have been collected and dis- played by means of the appropriations made by the various state legislatures for such work. As the amount and conditions of the appropriation varied very much in different states, the size and costliness of the exhibits vary accordingly, and often give a very great advantage to the state with the larger appropriation. In criticising an exhibit, therefore, these circumstances must be borne in mind.

460 THE JOURNAL OF GEOLOGY.

Among the best American exhibits are, beginning with the Eastern states, those of Massachusetts, New York, Pennsylvania, North Carolina, Michigan, Minnesota, Missouri, Colorado, Mon- tana, Arizona, Idaho and California; and in Canada those of the Provinces of Quebec and Ontario. Among the other foreign exhibits that of New South Wales is preéminent in the quality, nature and mode of arrangement of the exhibit. The exhibits of Great Britain, Germany, Norway, Sweden, Denmark, Spain, Greece, Austria, Switzerland, Belgium, Italy, South Africa, Cey- lon, Japan, and other foreign countries, are good as far as they go.

Many of the mining exhibits of both states and foreign coun- tries are divided, and put partly in the Mines and Mining Build- ing and partly in the individual buildings of the states or coun- tries in question. . Such a course is a great mistake, as it renders the exhibit in both buildings imperfect, and those who see the exhibit in one building without knowing that it is supplemented in another, receive an incomplete, and therefore an erroneous, idea of the products of the country represented. Each mining exhibit should be kept together, whether it be in the Mines and Mining Building or in another building.

The exhibits of the New England states are naturally repre- sentative of less economic value than those of some of the other states, because, with the exception of building and ornamental stones, most of their mining products are of subordinate import- ance ; but at the same time they display what they have ina sys- tematic and consistent manner. The Massachusetts exhibit is thoroughly characteristic and well arranged, showing not only the economic products, but also many rocks and minerals of purely scientific interest. The Maine exhibit is also character- istic of the state, while the New Hampshire and Vermont exhib- its are small but appropriate, consisting largely of building stones, with mica and other minerals from New Hampshire. The granite of New Hampshire and the granite and white marble of Vermont are displayed on a small but sufficient scale.

Coming westward, the New York exhibit is the first one we

EXHIBITS IN MINES AND MINING BUILDING. 401

find which is representative of great economic importance. It displays its clays, sands, iron ores, building stones, petroleum, salt, etc., in a thoroughly systematic and creditable manner, and gives a very good idea of the relative importance of the differ- ent products.

The Pennsylvania exhibit is somewhat more elaborate than that of New York, as it should be, on account of the greater value of its products. Its immense coal and oil resources, together with its iron, clays, glass-making materials, slates, build- ing stones, etc., are very well displayed. A model showing the method of coal mining and relief maps of the anthracite basins and of the whole state add to thé attractions of the exhibit. A large series of samples of crude and refined petroleum are an appropriate and interesting feature of the exhibit. A large col- umn of anthracite in a conspicuous position in the centre of the building, and apart from the rest of the Pennsylvania exhibit, represents a vertical section of the ‘‘mammoth seam” on the property of the Lehigh Valley Coal Company. A second column, near the main Pennsylvania exhibit, is composed of blocks of the different products of that state, varying in size according to their importance, the smaller blocks being’ placed successively higher in the column.

New Jersey makes a much less elaborate exhibit than either New York or Pennsylvania, though it is neatly arranged and in some respects it is good. The magnetic iron ores of the north- ern part of the state, and the clays, marls, and other products are well exhibited. The zinc deposits of Sussex county, how- ever, are only poorly represented, and in this respect the exhibit might have been improved. A _ glass-plate model of the zinc mines at Mine Hill, Franklin Furnace, Sussex county, is an attractive feature.

Virginia makes a very characteristic and well arranged exhibit, though the fact that the materials exhibited are not in cases detracts from their neatness. A large display of coal and coke, so rapidly becoming the most important products of the state, is made; while the characteristic brown hematite (limon-

462 THE JOURNAL. OF GEOLOGY.

ite), the manganese ores, zinc ores, clays, fire-brick, and slate of that state are represented. The Bertha Zinc and Mineral Com- pany displays the zinc ores of the southwestern part of the state and the spelter made from them, as well as statues, wire, etc., made from the spelter.

West Virginia makes a fine display of coal and coke, at pres- ent two of its most important industries. Such an exhibit is very appropriate when we consider that forty-eight out of the fifty- two counties of the state are said to contain more or less coal. The salt, mineral waters, crude and refined oils, iron ores, and building stones are also displayed.

North Carolina makes a véry neat and characteristic exhibit of iron ores, auriferous quartz, mica, kaolin, asbestos, building stones, gems, etc. The gems include diamond, sapphire, topaz, ruby, beryl, garnet, rutile, chalcedony, etc. A number of inter- esting models of gold nuggets are also displayed. A number of photographs of different districts form a part of the exhibit, which is neatly and systematically arranged.

South Carolina makes a good exhibit of its great phosphate industry, displaying the crude phosphate and also the manufac- tured superphosphate. The phosphate industry far eclipses in importance all other mining industries in that state, and the others, such as gold, iron, and manganese mining, in the western part of the state, are of very little and very unstable importance, and are not represented.

Florida, long unknown to the mining industry, has suddenly become of great importance on account of the recent discovery of its phosphate deposits. A small exhibit of these phosphates is made in the Mines and Mining Building, but it is not suffi- ciently extensive to do credit to a young and rapidly growing industry.

Louisiana makes a very appropriate exhibit of its mining products, among which are lignite, oils, salt, sulphur, marls, clays, chalk, building stones, grindstones, mineral waters, and other minor materials.

The Tennessee exhibit consists mostly of a ‘‘ Mineral Exhibit

EXHIBITS IN MINES AND MINING BUILDING. 463

of Harriman, Tenn.,” and shows the coal, coke, fossil and mag- _netic iron ores and brown sandstone produced in that district, together with the pig iron manufactured. The exhibit is creditable to Harriman, but it is a pity that the state of Tennessee in general did not make a full display of its coal, iron, marble, and many other mining resources. The Cleveland Fire Brick Company of Cleveland, Tenn., makes an exhibit of its clays and bricks.

Kentucky makes a good and extensive exhibit of coal and coke, with smaller collections of iron ores, building stones, clays, Diteks,e ete. oy felich imap) otythe state is alsoman jattractive - feature of the exhibit. The exhibit contains a large amount of good material, but it might be displayed to better advantage.

Ohio makes a good exhibit of coal, its most important min- ing industry, and also displays on a smaller scale its crude and refined oil, its salt, clays, iron ores, whetstones, etc. It presents a good model of a salt refining works, and makes a very attrac- tive display of the bricks, tiles, etc., made from its clays.

Indiana makes a good business-like exhibit of just what it has and no more, including a display of coal, clays, building stones, oil, mineral waters, and tiles, and glass manufactured from native products. The exhibit is well arranged and shows all that is necessary.

Illinois makes an extensive display of its clays and the various manufactured articles made from them. A much more exten- sive mining, mineralogical, and geological exhibit of the state is made in the large state building elsewhere on the World's Fair grounds. This exhibit is well arranged, and truly indicative of the products of the state.

Michigan makes one of the most elaborate exhibits of all the states. The three great mining products of this state are iron, copper, and salt. The first two are excellently repre- sented ; the last is much neglected. The different kinds of iron ore are illustrated with numerous specimens ; anda large colored cross-section of the Cleveland Cliffs Iron Company’s mine is given. A wooden model of the No. 4 Ore Dock at Marquette,

™;

404 THE JOURNAL OF GEOLOGY.

on the Duluth, South Shore and Atlantic Railroad, gives a good idea of the method employed in handling large quantities of ore. The different modes of occurrence of the copper of Michigan are shown by a number of well selected specimens ; ‘while the copper in ingots, sheets and wire is well displayed. . Interesting wooden models are given of the shaft house and mills of the Calumet and Hecla mine, and of the rock and shaft house of the Tamarack mine. Other interesting features of the exhibit are a number of pre-historic copper implements from Michigan, and © arches and columns of brown sandstone produced in the state.

The Wisconsin exhibit contains some good material, but it seems to be arranged more to give prominence to fine specimens than to show systematically the products of the state. The lead and zinc industries of the southwestern part of the state are well represented, but the great iron interests of the northern part of the state are neglected, one pile of ore indefinitely marked ‘iron ore” and a few other odd specimens being all that are displayed. Some good specimens of granite and columns of red sandstone are also exhibited. - In addition, various mineral specimens are displayed, some of which have come from other localities than Wisconsin, and are therefore misleading to the uninitiated.

Minnesota confines its exhibit almost entirely to its greatest mining industry, z. e., the iron of the northern part of the state, and in this department the exhibit is very good. Some building stones and a few mineral specimens are also displayed. A wooden model of the Chandler mine, and a number of maps showing the mines and the geology of the state also form a part of the exhibit.

Iowa makes a small but fairly characteristic exhibit, consist- ing mostly of coal, building stones, etc. A feature of the exhibit is an artificial “drift” in a coal mine, showing the mode of working and transporting coal on underground tramways. A model of a coal shaft and breaker is also given.

The Missouri exhibit is excellently arranged, and is thoroughly indicative of the resources of the state. The lead, zinc and iron industries are well represented, and pig lead and

EXHIBITS IN MINES AND MINING BUILDING. 405

zine spelter are displayed with the ores from which they are derived. A model of the dressing works of the Saint Joseph Lead Company and a relief map of Iron Mountain, colored to show its geology, are interesting features of the_exhibit. The coal industry of the state is also represented, together with a number of building stones, ochres, etc., and a fine collection of calcite and other mineral specimens from the lead and zinc mines.

The South Dakota exhibit consists largely of tin ore, aurif- erous quartz, mica and some argentiferous galena, and is essentially a Black Hills exhibit. ‘ Lode” tin ore and stream tin, as well as pig tin manufactured from the ores, are exhibited in large quantities. A large column of tin ore, from the property of the Harney Peak Consolidated Tin Company, contains a placard stating that the capital invested is $3,500,000, a fact it is difficult to understand they should wish to make so promi- nent in view of the unproductive history of their operations. The auriferous quartz is a good exhibit and characteristic of the quartz deposits of the Black Hills. Some beautiful pieces of Arizona silicified wood, which were polished in Dakota, are exhibited, but in lack of the proper explanation as to their source, they are misleading, as they suggest Dakota as the region from which they were derived.

Kansas makes a very good exhibit of lead and zinc ores with the pig lead and zinc spelter derived from them. The exhibit also includes a display of rock salt, gypsum, building stones and other minor products. The exhibit is small, but it is character- istic of the state and is well arranged.

Montana makes a good exhibit as far as it goes, but many localities and many important deposits are not represented. The best exhibits are from the great mining camp of the state, 7. ¢., Butte City. The great copper and silver interests of this district —especially the former—are well presented, and large quan- tities of sulphide copper ores, and the metallic copper made from them, are displayed. A quantity of gold quartz, and an inter-_ esting collection of gold nuggets are also a part of this exhibit.

400 THE JOURNAL OF GEOLOGY.

The most prominent feature of the exhibit, however, is a solid silver, life-size statue of the celebrated actress, Ada Rehan, stand- ing on a globe which in turn rests on a base of solid gold. The whole work represents several hundred thousand dollars worth of precious metals, all the products of Montana mines.

Wyoming makes a neat and effective exhibit. It consists largely of coal in columns and blocks, jars of petroleum, blocks of sulphate of soda and sulphate of magnesia, “lode” tin ore and stream tin ore from the northeastern part of the state, adjoining the Dakota tin region, iron ore, copper ore, auriferous quartz, lead carbonate, asbestos, agates, clay, sulphur, building Stones CLG:

Colorado makes a fairly good display of its silver-lead ores, copper ores, gold ores, coal and manufactured lead and copper. Some of the building stones and iron ores of the state are shown, but these materials are not fully represented. An instructive feature of the exhibit is a series of cases of gold nuggets, dust gold and sheet gold from Breckenridge, Colorado. Many of the important mining camps in the state are represented, especially Aspen, Leadville, Creede, Cripple Creek, etc. The exhibit is fairly good, but a state of such immense mining wealth as Colo- rado could have made a much better one.

The Utah exhibit contains a large amount of valuable material, but it is too much crowded and badly arranged. The desire for a display of brilliantly contrasted colors has in some cases entirely upset the systematic arrangement of the exhibit, and has given part of it the appearance of the toy boxes with pieces of minerals glued on the outside that are sold to confid- ing tourists in our western states as works of art and value. The exhibit represents the varied mining industries of the terri- tory, among the most important substances being coal, gilsonite, albertite, elaterite, asphalt, oil shales, sulphur, salt, iron ores, copper ores, silver and gold ores, building stones, etc. The exhibit of articles “‘japanned” by the gilsonite varnish are of interest. Some large specimens of silver-lead ores and ores containing chloride of silver are characteristic of the mines producing them.

EXHIBITS IN MINES AND MINING BUILDING. 467

The New Mexico exhibit contains some good material, but is not very well exhibited. The silver ores, one of the most important products of the territory, are well represented, being grouped according to the localities from which they came. A small cabin in the centre of the exhibit is composed of silver, lead and gold ores from different localities. A stuffed burro carrying a prospector’s camping outfit is a somewhat sensational feature of the exhibit. A column of coal from Blossburg and Los Cerillos represents the growing coal industry of the terri- tory.

The Arizona exhibit is very good and well arranged. It is truly indicative of the products of the territory. The most im- portant features of it are the copper ores, the silicified wood and the gold ores. The copper ores especially are well represented, and a beautiful column of green and blue carbonates of copper from Bisbee forms the most prominent feature of the exhibit. While in the Michigan exhibit we see only native copper, in the Montana exhibit only sulphides of copper, here in the Arizona exhibit we see mostly carbonates of copper with some silicate and oxide of copper. Thus in these three copper districts we have representatives of three great classes of copper ores. An interesting feature of the Arizona copper exhibit is a series of models showing the underground workings of the Copper Queen Consolidated Mining Company at Bisbee. The Old Dominion Copper Company whose mines are at Globe, Arizona, makes a very excellent exhibit of its ores and its copper ingots in a cabinet alongside the main Arizona exhibit. The gold ores of Arizona are well represented, and some of the silver ores are also shown, while the beautiful polished sections of the celebrated silicified wood of Arizona form an attractive and interesting feature of the exhibit. Some of the so-called “onyx” in polished slabs.

Nevada makes a fairly good exhibit of its mining products, mostly the silver ores abundant in this region, and the accom- panying minerals. A ‘special exhibit”’ from Eureka, Nevada, contains a number of interesting specimens.

is also exhibited

408 THE JOURNAL OF GEOLOGY.

The Idaho exhibit is fairly good, but not thoroughly charac- teristic of the state. The most prominent features are the silver- lead ores from the northern part of the state, green copper car- bonates, and a mineral water known as ‘‘Idanha”’ from Soda Springs. A number of photographs of different mining dis- tricts are of interest.

Washington makes a fairly good, but poorly arranged, ex- hibit of gold ores, silver ores and silver-lead ores, and a few other products. The coal resources of the state are entirely neglected, though they are well represented in the Washington state building. This separation of the mining products of a region, and their distribution partly in one building partly in another, is a great mistake, as it gives a person who sees only one of the exhibits an incomplete and therefore an erroneous idea of the resources of the state. The exhibit should be all in one or the other building.

Oregon makes a large exhibit of auriferous quartz and shows a very good working model of hydraulic mining. Some build- ing stones are also represented. The exhibit is very good so far as it goes, but it does not do justice to the state, as many of its developed and undeveloped resources such as iron, coal, etc., are not represented.

California makes a good exhibit, and one characteristic of the resources of the state. It is very appropriately composed largely of gold ores and a display of the methods of gold min- ing. The auriferous quartz of the celebrated Grass Valley and other localities is well represented. An interesting feature is a wooden model by A. C. Hamilton showing a system of mine timbering. Stibnite from San Benito county and the metallic antimony derived from it are also represented. Among the other prominent features of the exhibit are iron ores, asphalt, oils, slate and a beautiful display of ornamental and_ building stones. The so-called “onyx” from San Luis Obispo county, and the col- ored marbles from Inyo County are exceedingly beautiful. The exhibit is entered through arches built of the various ornamental stones of the state, while blocks of rock containing the beauti-

EXHIBITS IN MINES AND MINING BUILDING. - 469

ful rubidolite, or pink tourmaline, are displayed at the entrance. Elsewhere in the building is a fine and beautiful exhibition of the so-called “onyx” from New Pedrara, in Southern California.

Besides the California exhibit in the Mines and Mining Building, an interesting collection of the mining products of the state, especially gold ores and native gold, are contained in the California state building. Somewhat similar specimens, how- ever, are in the Mines and Mining Building, so that the division of the collection in this case is not especially injurious.

Among the foreign collections, that of New South Wales stands preéminent. The great mining wealth of this province is exhibited in a very systematic and thorough manner, and an excellent idea is given of the resources of the region. There is no attempt at a display of a sensational character as is seen in some of the exhibits, but everything is shown in a plain business way, in large quantities and in properly selected samples. Among the most prominent features of the exhibit are its tin, gold, silver, lead, antimony, copper, iron, manganese, and chromium ores, its coal, graphite, building stones, etc. The ores exhibited are average samples such as are sold in the market, and there- fore give a true idea of the deposits represented. In many cases, as in antimony, tin, etc., the metals are exhibited in blocks or pigs, with the ore from which they are derived. The ores of the great Broken Hill silver mine and the statistics of its pro- duction are of interest to those acquainted with this famous mine. The exhibit of the tin industry is of great interest as rep- resenting the development of this comparatively new tin region, which has only been much developed since 1872; while the coal exhibit shows not only the bituminous coal of the region, but also the kerosene shales, etc.

Among some of the other foreign exhibits those of the prov- inces of Ontario and Quebec are very good, showing as they do the various products of those provinces in a thorough and syste- matic order. The other provinces of Canada do not make such good exhibits. A collection of the rocks of Canada by the Geo- logical Survey is of great interest. Mexico exhibits a great

470 LLL] OORAN ALES OTHE GTR OTE OG NG

amount of material, but it is so arranged that it loses much of the benefit that it might afford to the exhibitors and to the public. Brazil makes a fairly good exhibit, while Chile, Ecuador and other South American countries are also represented. The South Africa diamond exhibit is very interesting as showing the mode of occurrence, methods of mining and washing, and cutting the diamonds. The exhibits of Great Britain, Germany, Japan and other foreign countries are also of interest. La Societé ‘Le Nickel” of France makes a very interesting exhibit of its nickel ore in New Caledonia, the nickel derived from it, pictures of the mine and various other interesting features of the industry. Many others of the numerous exhibits of American and foreign products in the Mines and Mining Building might be mentioned, but lack of space forbids further elaboration. The same cause makes it necessary to discuss in another article the extensive and excellent exhibition of the United States Survey

in the Government Building. R, AvP: PENROSE, JR

=

Waa; ILA) SUNIOWUANS). (GuEVAC CIDE IR

One of the largest of the extinct glaciers of the Rocky Mountains was that which occupied the valley of the Las Ani- mas river. This stream originates in the San Juan mountains in southwestern Colorado, and flows nearly south to its junction with the San Juan river in New Mexico. The San Juan moun- tains, with their outlying spur, the La Platas, are the first high mountains encountered by the moist winds from the direction of the Gulf of California on their way northeastward; and although so far south, this region has perhaps the heaviest snow fall in Colorado, as Fremont found to his cost. His expedition up the Rio Grande attempted to penetrate the snowiest part of the mountains.

Silverton -is situated about fifteen miles from the head of the valley, and Durango about sixty. About one mile north of Du- rango, near Animas City, two well defined morainal ridges extend across the valley of the Las Animas, and from thence a plain or series of terraces of water-washed morainal matter extends for several miles down the river. I have not explored far below Durango, and do not know the extreme limit of the ice. At Durango the ice rose to about the same height as the mesa lying east of the city, on which is the reservoir of the water- works, 300 or more feet above the valley terrace. This is proved by the fact that a thin sheet of morainal matter covers the slopes of the bluff and extends back for a short distance on top of the mesa (up to 100 feet); whereas, beyond that the top of the mesa is a base level of erosion in the sedimentary rock, with none of the far- traveled bowlders that abound in the moraine stuff. The glaciated bowlders are largely composed of rocks found only near the head of the valley, such as volcanic rocks, Archean schists and granites, Paleozoic quartzites, etc. Most of these must have traveled thirty to sixty miles.

471

472 THE JOURNAL OF GEOLOGY.

About a mile above Durango, at the most distinct of the ter- minal moraines thus far noted, the valley widens to about one mile, and continues pretty broad for twelve miles or more north- ward. The valley is here covered with rather fine sediment. It is marked on Hayden’s maps as alluvium, but the glacial char- acter of the terraces near Durango is not recognized, though deposits substantially the same, situated a few miles northwest of Durango in the La Plata valley, are markedly morainal. :

The post-glacial history of the valley was as follows. The terminal moraines near Durango formed a dam that held ina lake. This lake was partially filled with sediments, and at the same time the river was cutting down through the morainal bar- rier. The outlet is now so low as to drain the lake, except there are some low, marshy flats where the water stands only a short distance below the surface of the ground.

I have visited many of the tributary valleys of this river above Silverton. Every cirque had its glacier that flowed down into the larger valleys. The volcanic rocks of that region weather readily, so that one seldom finds glacial scratches except at recent excavations for roads and mines. It has there- fore been a matter of considerable difficulty to determine the depth of the glacier of the main valley. By degrees the esti- mated depth increased until a few months ago, when I found scratches well preserved on quartzite at a height estimated at 1,500 feet above the Las Animas river. This was near the Mabel mine, about four miles southeast from Silverton, and not more than 500 to 800 feet below the top of the ridge which here borders the valley on the east. The glaciated rock is situated on a long gentle westward slope, while the scratches have a north and south direction. Local glaciers would have flowed westward. These scratches are therefore parallel with the movement in the main Las Animas valley, under conditions where no local glacier could have produced them.

It thus appears that near Silverton (elevation of valley about gooo feet) the Las Animas glacier was 1,500 or more feet deep, while at Durango (elevation about 6000 feet) it had a thickness

LHE LAS ANIMAS GLACIER. 473

of about 350 feet and a breadth of one-fourth mile or more. Its extreme length was more than sixty miles, probably about seventy miles. The average slope of the upper surface was eighty-three feet or more per mile. For fifteen miles its breadth was one or more miles.

From the terminal moraines near Durango, the valley of the Las Animas is for several miles southward covered by a plain of water-washed material, from coarse gravel up to bowlders three to five feet in diameter. Some of these have glacial scratches, though most have been so much rolled and polished as to pre- serve no distinct scratches. The lower terraces at Durango are of this character. They are typical of the overwash gravels found in many of the Rocky mountain valleys. The subglacial streams poured out their load of sediments in the valley in front of the ice, where they were mixed with some material dropped directly from the ice, and hence not rolled far enough to obliterate the glacial scratches. More or less of this glacial gravel is found in * all the wider parts of this valley and its tributaries above Silverton until we reach within five or ten miles from the heads of the val- leys. During the retreat of the tributary glaciers they poured out much less glacial gravel after they came to be ten miles or less in length, and what there was is usually but little water-worn.

Since the above was written further exploration reveals the fact that a large glacier originated on the eastern slopes of the La Plata mountains, and flowed southeastward down the valley of Junction creek and joined the Animas glacier in the northern part of Durango. Five hundred or more feet above the creek it left a lateral moraine on the top of the narrow ridge which borders the valley on the south. The moraine consists chiefly of the eruptives and metamorphosed sediments found in the La Platas, and but little of the local rocks.

The drift terraces near Durango are found at different levels. The lowest terrace is that above described, and consists of glacial gravel mixed with matter that has been but little rolled. The higher terraces have the appearance of ordinary valley terraces as seen from the river, but in some cases do not extend

A74 THE JOURNAL OF GEOLOGY.

back to the sides of the valley. The largest of these lies on the east side of the Animas river, between Animas City and Durango. It is more than a mile in length, and the outer or distal side ends in a bluff twenty to forty feet high.» At its north and south ends this curved terrace approaches near to the mesa bordering the valley, thus enclosing a depression several hundred yards wide that is occupied by a small lake in time of violent rains. A basin of this kind could not have been hollowed out by the river, and, besides, the terminal moraines of Animas City extend across the north end of the basin. It is evident that this terrace was formed laterally to the glacier in substantially its present form. It contains great numbers of boulders up to fifteen feet in diameter, but a large portion of it has been very much water- rolled. The most probable interpretation is that these higher terraces began to be deposited at the outer edge as a lateral moraine. Then as the ice gradually receded morainal matter and glacial gravel were simultaneously deposited in the space between the moraine and the retreating ice. This hypothesis well accounts for the fact that morainal and water-rounded matter are so intimately mixed in the terrace, also that the overwash did not spread laterally back to the margin of the valley. We thus have the terraces ending distally in the steep slope characteristic of the moraine rather than the more gentle slope of the overwash apron. Most of these higher terraces end proximally (next the river) in rather steep slopes or bluffs rising twenty to seventy-five feet above the lower terraces. No city of Colorado has so much of glacial interest within its limits as Durango, unless it be Leadville.

It is an interesting fact that the cols of the mountain ridges of this region are glaciated almest or quite to their tops. Thus at Stoney Pass, the first pass north of Cunningham Pass, I saw well- glaciated rocks within 200 feet (horizontally) from the top of the pass. From the top of this pass the mountain slopes steeply northwestward toward the Las Animas valley, and in the opposite direction down the Rio Grande valley. The rocks at the summit were weathered, and it was not evident whether

THE ELAS ANIMAS GEA CTER. 475

the top of the ridge had been glaciated, but it is certain the ice or snow flowed in opposite directions from the col. On each side of the pass, peaks of the Continental Divide rise above the col to a height of 1000 to 2000 feet. It is evident that the snow from these peaks would flow or slide from each side down into the pass, and maintain a supply of névé or ice right on top of the ridge in the pass. The pass is about 11,800 feet high. It thus appears that the snow fields reached nearly to the tops of the mountains, say about 12,000 feet in the cirques and passes, while above this the discharge was probably in large part by avalanches.

Durango city is situated in about N. Lat. 37° 16’, a few miles north of the end of this glacier. It is to be carefully noted, in the study of the climates of the glacial epoch, that a glacier nearly seventy miles long reached so far south. Apparently the most snowy part of Colorado now was also the most snowy then.

During the retreat of this glacier it left numerous small retreatal moraines, both in the main valley and in the tributary valleys above Silverton. One of the most accessible is near the junction of the two branches of Mineral creek, about three miles northwest from Silverton.

It is noticeable that the proportion of moraine stuff left by this glacier is small as compared to the glacial sediments. No- where have I yet found very noticeable ridge or terrace lateral moraines. This is in part due to the steepness of the hills that border the sides of the Animas valley. There is usually a scattering of glaciated matter on these hill slopes, and where they are less steep, or in lee of ridges projecting out into the valley, local morainal sheets are sometimes found that have a depth of twenty feet or more. Small terrace-like lateral moraines extend for a mile or two north of the terminal moraines of Animas City near Durango. Probably the snow avalanches and flowing névé carried down débris and incor- porated it with the glacier proper, so that there were no large surface lateral moraines as in some of the valleys of the Alps, or in the Arkansas and some other valleys of Colorado. In other words, the débris of this glacier was largely englacial and basal. GEORGE H. STONE.

STUDIES BOR SUD ENES

CONDITIONS OF SEDIMENTARY DEPOSITION.

EROSION.

Erosion consists of fragmental reduction and abrasion of rock masses, chemical disintegration of rocks and transportation. The three sub-processes may be called rock-breaking, rock-decay and transportation. They are conditioned by declivity, lithologic character and climate.

ROCK-BREAKING.

Favorable conditions:

(ay Steep slopes. (0) Bare rocks. (c) Cleaved and jointed rocks. (dz) Alternation of hard and soft beds. (e) Rapid changes of temperature. (f) Aridity and high winds. g) Abundant rainfall, in the absence of vegetation. (2) Sea cliffs. Products: Shingle, gravel and sand of mixed mineralogical composition. ROCK-DECAY. Favorable conditions : (z) Gentle slopes. (6) Porous soil. (c) Soluble rock constituents. (@) Carbonic acid and other acids of organic decay. (e) Abundant rainfall in the presence of vegetation. (f) Prolonged transportation of gravel and sand. 476

CONDITIONS OF SEDIMENTARY DEPOSITION. 477

Products: Rock cores of disintegrated masses, sand, (chiefly quartz-sand), residual clays, and lime, magnesia, iron, etc., in solution.

TRANSPORTATION. =

Favorable conditions:

(az) Steep slopes.

(6) Abundant rainfall.

(c) Absence of vegetation. (d) Floods

(@) Fine detritus.

By comparison of the statements of favorable conditions for rock-breaking, rock-decay and transportation it becomes apparent that breaking and decay are favored by opposite conditions in nearly all respects, while breaking and transportation are most efficient under like conditions. But breaking promotes decay, and decay aids transportation, by reducing the size of the parti- cles to be decomposed and carried, and the maximum effect of erosion is probably attained when rock-breaking is active among greater elevations, and rock-decay and transportation are both proceeding energetically on lower slopes. *

The amount of material furnished by erosion is an important consideration in reference to the rate of accumulation of sedi- ments over a given area, and is a condition not to be overlooked in comparing thicknesses of deposits with the lapse of geological ages.

SEQUENCE OF SEDIMENTS.

Shingle, gravel, sand, clay and silt are products of ero- sion of rock masses. They are produced either by mechanical breaking or by chemical disintegration. These two sub- processes of the general process of erosion are favored by unlike conditions. Those conditions which render breaking most efficient are unfavorable to immediate disintegration ; and those conditions which promote disintegration limit breaking. Breaking, the reduction of a rock mass to small pieces, is usually

tGilbert, Henry Mts. p. 105.

478 THE JOURNAL OF GEOLOGY.

the antecedent of disintegration, of decay, but the two are not most efficiently active at the same time. Now their products differ. Rock breaking yields shingle, gravel, coarse sand of mixed mineralogical composition, and no chemical ‘solutions. Rock-decay yields directly no shingle or gravel, but produces sand, chiefly quartz-sand, clay, silt and chemical solutions. Hence, if the products of rock-breaking are deposited unchanged in the sea, there will result one class of sediments from which we may infer corresponding conditions of erosion of the parent land ; and if the products of rock-decay are deposited we must infer other conditions of erosion.

Declivity is the chief factor which determines either rock- breaking or rock decay. Rock breaking occurs on steep slopes, that is, among hills or mountains ; rock-decay takes place chiefly on gentle slopes, that is, in valleys or on plains. Hence the sediments may indicate the topographic phase of the parent-land.

They may indicate topographic phase, not permanent topo- graphic character, for relief of the land surface is transient. The steeps of mountains become the slopes of hills, the hill slopes sink to plains and plains to base-level; and erosion pauses till renewed by uplift. So the conditions of rock-breaking pass into those of rock-decay, and the product of the two processes may appear in sediments, the older gravel and sand beneath the younger sandy clay and clay.

The possible sequence of unlike sediments does not stop with the finer mechanical products of disintegration; chemical solu- tions may be related to chemical or organic deposits, and these have their place among strata. The amount of lime and mag- nesia carried annually from a given land area is directly related to the efficiency of rock-decay, and so among other factors to slope. Rock-decay is limited on the one hand by declivities, which promote the rapid running off of rainfall, and on the other hand by the accumulation of a deep covering of soil, which pre- vents percolation. Other things being equal, it is probably most efficient during the period corresponding with the life of low hills and sloping plains. If at any time chemical solutions from

CONDITIONS OF SEDIMENTARY DEPOSITION S. - 479

the land determine the deposition of calcareous formations they will do so most efficiently during this topographic phase, and in the absence of mechanical sediments the corresponding de- posits will be limestones or dolomites. As the topographic phase passes to its close and the sloping plains sink to base-level, the power of streams to transport mechanical sediment fails, and rivers finally carry only silt in lessening proportion; hence the upper portions of a great limestone deposit may be less clayey than the lower. Furthermore, the mantle of residual clays, accumulating upon the extended base-level, will check solution, and thus, in so far as the deposition of limestone is influenced by contributions from the land, will limit the growth of the for- mation; and with the cessation of both mechanical and chemical supply, terrigenous deposits will cease to form beneath the sea. Then, while these conditions endure geologic ages may pass with- out record in sediments unless there is a marine source of supply.

Thus far this statement has tacitly assumed a constant relation of elevation between coast and ocean. Assume that the long quiet, which has been necessary for the reduction of a mountain range to base-level and the deposition of the corresponding sedi- ments, is interrupted by sinking or heaving of the landarea. The surface is low, flat and covered by a mantle of residual sand and clay intimately mingled. Moderate subsidence must lead to ex- tensive transgression and the invading sea, margined by tide flats, will spread arenaceous, clayey deposits, bearing the marks of shallow water formations and resting unconformably upon the aneient rocks. If the residual soil be red, the sediments will be of similar color, since the process of deposition on tide flats does not involve much attrition and the ferruginous coating of the grains will remain.* The base of the deposit may be a zone of transition, composed of cores of undecomposed rocks, imbedded in more or less re-arranged products of partial decomposition.’

‘Bull. U. S.G.S. No. 52. I.C. Russell, Subaérial Decay of Rocks and Origin of Red Color of Certain Formations.

2R.Pumpelly. The Relation of Secular Rock Disintegration to Certain Transitional Schists. Bull. Geol. Soc. of America. Vol. IL., p. 209.

480 2 HE, JOORINALG OFA GTSOL OGM

Or, on the other hand, moderate uplift of the base-leveled conti- nent, must cause the revived streams rapidly to sweep into the sea the mass of insoluble clay and sand which formed the resi- dual mantle. Thus the limestone deposits will be succeeded by a thickness of shales of a more or less arenaceous or clayey Characters

From these considerations it follows that a complete topographic cycle may be related to a sedimentary sequence composed of a sandy base, a limestone middle and a shale top. Newberry first noted the frequent recurrence of this sequence, and sought an explanation in conditions related simply to the sea; its advance, presence and retreat. When he made his gen- eralization the base-level had not been recognized as a result of continued erosion, nor had Gilbert analyzed the process of erosion; and Davis had not described a topographic cycle. These contributions to the science have widened the field of in- ference, and the topographic phase of the land can no longer be disregarded in the discussion of the deposits of the sea.

But it should not be forgotten that the inference from sedi- ments should be confined to the topographic phase of a belt of land extending back from the shore to a moderate distance only. The products of rock-breaking disintegrate during prolonged transportation and mountains remote from the coast are not in- dicated in deltas of great rivers. A student of the deposits of the Mississippi would not infer the height of the Rocky moun- tains, but the sands of the Klamath river bear witness to the near- » ness of the coast range.

The analysis and discussion of conditions which govern the character of the material contributed from land to sea might be extended in detail, and illustrated by descriptions of sediments in existing rivers, but the subject is worthy of independent treat- ment.

SEDIMENTATION.

Sedimentation consists of three sub-processes, sorting, distribution and deposition. These are effected by waves and undertow, tides, winds and oceanic currents and are modified

B

CONDITIONS OF SEDIMENTARY DEPOSITIONS. 48%

by the relation of volume of sediment to the force of waves or currents. If the analysis be based on the sub-processes and conditions which favor them, it may be stated and discussed as follows:

=

SORTING.

The conditions under which sorting is more or less efficiently carried on are three in number.

Favorable conditions:

(z) Vigorous wave action accompanied by strong undertow.

(6) Prolonged transportation in consequence of deep water and continuous currents.

(c) Moderate volume of sediments.

The conditions under which sorting is not accomplished are the reverse of these, namely:

Unfavorable conditions :

(z) Feeble or diffused wave action. (6) Concentrated deposition. (c) Excessive volume of sediments.

It will be profitable briefly to discuss these positive and neg- ative conditions.

(a) Vigorous wave-action.—The force of waves is determined by their fetch and the strength of winds. In the study of mod- ern beaches the latter is important, since it controls the form and the greatest storm’ fixes the maximum size of detritus moved; but in considering fossil beaches as strata we deal with sands which have been so rearranged during submergence that the beach form is lost. However the former condition, the fetch of the waves is more constant, and the force of the waves determined by it may be inferred from the nature of the beach deposits.

The efficiency of waves of a given force is determined by the concentration of their blows, and this is conditioned by the slope against which they break. If relatively deep water prevails

For full discussion of wave erosion and deposition, see Lake Bonneville, by G. K. Gilbert. Monograph 1, U.S.C. 5.

482 LE JOUTANALE OL NGHOLO. GAVE

to the shore, whatever force the waves may have is expended at the water’s edge. On a bold coast they carve sea-cliffs and grind shingle with sand. Such are the coasts of New England, Oregon, California, and of all the Pacific side of South America. The resulting sediments are com- posed of worn but fresh rock fragments and thus bear witness to rapid mechanical erosion, like the products of rock breaking on steep declivities. Ona shore of incoherent materials waves stir, wash and separate fine and coarse, light and heavy particles. Under favorable conditions of depth of water and long fetch, waves thus sort a heterogeneous mass of gravel or of residual sand and clay more efficiently than any other agent, and leave clean cross-stratified beach sand and gravel with boulders, while the finer materials are swept away. The southeastern shore of Long Island presents a conspicuous example of this, and the westward drift of the beach-sands is illustrated in the fact that shingle beaches prevail toward the eastern end of Montauk point, and the sands there washed from the bluffs of glacial gravel form long barriers along the coast to the westward.

If, on the other hand, waves break in shallow waters at a dis- tance from shore they there build a barrier, and the height to which they build it above high tide is the measure of their max- imum power during great storms. Within the barrier then ex- tends a lagoon. The whole Atlantic coast from Long Island to Florida is thus fringed by the features of prevalent wave action, due to the great fetch from off the ocean and the gradual slope of the continental platform.

(6) Prolonged transportation—Sorting is also accomplished to some extent, though less perfectly, by deep water and con- tinuous currents. Sediments settle unequally according to size and specific gravity of particles; therefore the largest and heav- iest reach bottom first, the finer and lighter later, and the finest and lightest last. If the conditions of supply or current be in- termittent over any area then each incident of deposition will be marked by a layer composed of coarsest grains below and finest grains on top. This is the nature of deposition in tidal estua-

CONDITIONS OF SEDIMENTARY DEPOSITION. 483

ries. If,on the other hand, currents be continuous and constant, the zones of sand, clay and silt deposits will occur each beyond the former. But this is a question of distribution as well as of sorting of sediments. ae

(c) Moderate volumes of sediments.—Sediments are also more or less completely sorted by waves or currents according to the relation between the volume of sediment and the force of the sorters. When waves breaking upona coast have only the product of wave erosion to handle they sort most completely; the material is washed again and again until no trace of clay remains mingled with the sand grains ; and the under-tow, bur- dened only with the clay washed out by the waves and the fine products of abrasion, ¢arries them all away. But where a river pours out a large volume of sediment, and waves or currents are consequently overloaded, both sorting and transportation fail to a greater or less degree. Deposition takes place too rapidly for the separation of fine from coarse and the deposit is of mixed character. _The effect of waves is then seen in ripple-marked and ill-assorted beds of tide flats.

DISTRIBUTION.

The conditions under which sediments are more or less widely distributed, depend upon movement of the waters and the nature of the sediment; those favorable to distribution are:

Favorable conditions :

(a) Efficient wave action prevailing from one direction oblique to the shore.

(6) Continuous currents.

(c) Uniform or gradually increasing depths of water.

(2) Fine or light sediment.

The reverse of these conditions favor deposition, and will be discussed in that connection.

(a) Efficient, oblique wave action.— Distribution of shore drift is fully discussed by Gilbert, and has already been referred to in stating the effect of sorting by waves of the Atlantic on

484 LPHE, JOOKINALE OF NGE OLOGY.

the south shore of Long Island, and the formation of barriers of wave-washed sand.

(2) Continuous currents.—Distribution by continuous cur- rents is the condition usually assumed as having controlled the arrangement of sediments in seas of past geologic periods. In consequence of the sorting which results from different rates of settling clay is carried beyond sand, and silt is distributed more widely than clay. The prevailing current, which thus distrib- utes, is under-tow more or less checked and assisted by tides. If the submarine slope descends from the shore steeply into oceanic depths, the force of undertow must rapidly be dissi- pated, but pebbles and sand move easily down the steep incline, and form a sequence of continually smaller particles, which is usually not very extended. This is the case on themenwestern coast Oks SOuUrhna Americas If on) thesrother hand, the seaward slope is very gentle, undertow loses force more gradually and fine sands may occur to great dis- tances from the shore, with clay and silt deposited beyond them. This is the case off the Atlantic coast of the United States where tides probably form a powerful alternating influ- ence; there the continental plateau is covered with sand to its outer rim, as is shown by soundings by the Coast Survey. But the force of undertow is determined in the first place by the force of waves, and it can be effective in distributing only where waves are powerful. It fails in limited seas except in a very narrow zone along shore.

Ocean currents also distribute sediments very widely. The terrigenous deposits of the Bay of Bengal and Arabian sea, mapped by Murray," covering 1,600,000 square miles, owe their wide spread distribution apparently to the ocean currents which circulate east and west alternately with the changes of season in these great bays.

(c) Uniform depths —Changes in depth of water affect the velocity of a current and thus modify its power to distribute sed- iment. Narrowing channel or shallowing water may cause a

*Scottish Geogr. Mag., Vol. V. No. 8, Aug. 1889.

CONDITIONS OF SEDIMENTARY DEPOSITION. 485

current to scour and take on more load ;. but broadening channel or deepening water tends to cause it to deposit. The Gulf stream scours the straits of Florida and the Blake plateau, but deposits a silt bank on the lee side of the latter.?~ Only in the broad expanse of deep water does it widely distribute sediment.

_(d)_ Size of particles —Fine or light sediment is most widely distributed. The ‘blue muds” which form the terrigenous de- posits beyond the littoral zone consist of particles of an average diameter of .05 mm.

Deposition occurs whenever a body of water becomes over- loaded with substances in suspension or in solution. According to the condition which determines the result the deposits may be classified as mechanical, chemical and organic.

MECHANICAL DEPOSITION.

Favorable Conditions :

(a) Arrest and retreat of waves; beaches and sand deposits from undertow.

(0) Current entering still water and slowing; lake-deposits.

(c) Alternating currents in fresh and salt water; estuarine deposits.

(d) Rise of salt water surface at a river’s mouth in conse- quence of winds, long continued from one direction ; delta of the Mississippi.

(e) Flotation of fresh water on salt; bars of the Mississippi.

(f) Floculation of sediments in salt water.

(¢) Expansion and diffusion of a current in rapidly deepen- ing water; silt deposits on the edge of continental plateaus.

(Z) Final subsidence from oceanic circulation.

Arrest of Waves.—(a) Beaches are formed where waves break. The rotary oscillation which constitutes waves in deep water becomes a motion of translation when the water shallows sufficiently and the mass of the broken wave, rushing forward,

t Agassiz. Three Cruises of the Blake. Bull. Mus. of Comp. Zodlogy. Har- vard College. Vol. XIV.

486 THE JOURNAL OF "GEOLOGY,

carries up material stirred from the bottom. The finer particles are swept back by the undertow, the coarse are placed by the greater waves beyond the reach of the lesser. Thus waves, con- stantly in advancing, take material from the lower part of the slope to carry it up, and in retreating sweep back more or less of their load with them. If the slope be gentle they thus take from the lower to add to the upper part, and therefore they increase the declivity until the seaward profile becomes so steep that the load carried in retreat balances that advanced. This is the profile of equilibrium, which waves perpendicular to the trend of the beach do not change, unless they are of unusual force. Waves oblique to the beach-slope, scour, transport and deposit the same sands repeatedly, and if the oblique advance be prevailingly from one direction the effect is to move the beach longitudinally. _Then the beach, in any one section, continues, while the supply of sand is continuous; but when the supply ceases the beach is gradually moved onward in the direction of the prevailing wave action, and the material beneath the beach sands is exposed to Wave erosion.

A beach itself is but a narrow zone; it cannot constitute a wide-spread formation any more than a line can constitute a plane. But if a line be moved in one direction parallel to itself it will develop the plane, and in the same manner if a beach ad- vances landward it may spread a formation. This advance may be a result of wave erosion, which carving a sea cliff on a bold shore planes a surface of marine denudation. The beach deposit is then a basal conglomerate. Or, the land reduced to a low sur- face by subaérial erosion may subside slightly in reference to sea level,and the sea, transgressing, will rearrange the superficial for- mations. If the waves have power to handle the material the sea is margined by beach sands. If they cannot efficiently sort it the land will merge in tide-flats with the water.

A beach is not only narrow, it is also shallow; waves build on the surface over which they break, and the height to which they may build does not exceed a few feet. Therefore, beach deposits cannot form thick strata.

CONDITIONS OF SEDIMENTARY DEPOSITION. 48 7

The undertow rolls coarser sand and. pebbles down the slope of the bottom, and carries out in suspension silt and clay with more or less fine sand. The rolling of coarser sands is promoted by a steep slope. The transportation of finer sands depends on the endurance of the undertow of a given initial strength; and this endurance will be the greater the more gradual the seaward slope and the stronger the tides. The amount of sand thus de- posited is limited only by the supply, and sandy strata may, therefore, attain great thickness and have great extent seaward from a fixed beach line. If the coast be continually maintained by uplift or renewed by volcanic flows the work of the waves may be of like duration and the record will be correspondingly voluminous. Professor Chamberlin mentions the great conglom- erates of Lake Superior in this connection.

Beach deposits, strictly speaking, are usually of quite coarse sand, clean and characterized by marked and irregular cross- stratification. Sand deposits from undertow graduate from clean to muddy sands, becoming ever finer seaward, and are horizon- tally bedded or massive.

Therefore the interpretation which may be put on strata, de- posited by the arrest and retreat of waves, are:

(1) A basal conglomerate is significant of an horizon of wave erosion, due to transgression of the sea and probable sub- sidence of the land. If the basal contact be clean and sharp the waves probably carved a shore cliff in hard rocks. If, between the parent rock and the later sedimentary formation, there pera zone of transition composed of boulders, sand and clay of mixed mineral composition, the waves probably rearranged the cores and finer products of a surface of partial subaérial rock decay. A basal conglomerate of any variety is a definite proof of an un- conformity by erosion; it is often the only fact by which such an unconformity can be distinguished from an overthrust fault.

(2) A deposit of clean sands is proof of the former existence, somewhere, of a beach on which they were washed; but the place of deposit may have been remote from the line of the beach. Coarseness of grain suggests proximity of land and vice

488 THE JOURNAL OF GEOLOGY.

versa, but such suggestions need to be qualified by considering the probable fetch of the waves, the corresponding initial strength of the undertow and the declivity of the seaward slope.

A thin stratum of coarse cross stratified sands may represent a transgression by a beach-building sea over a subsiding land. A thicker stratum may have been formed by deposits from under- tow behind a stationary or advancing beach line, and if such a deposit shows cross-stratification throughout, it was washed by conflicting currents, probably tidal, during its accumulation. ;

The deposition of beach-washed sands is consistent with con- stant or subsiding level of the land in relation to the sea. It does not appear that it is likely to occur during uplift from the sea except in the comparatively rare case of the rapid ele- vation of a bold coast range with preponderance of rock-break- ing over rock-decay.

The occurrence of a stratum of sandstone is not evidence that during its formation the land furnished no other detritus. If the sands be of mixed mineralogical composition, bold declivities on land and prevalence of rock-breaking are indicated; but if the sands be chiefly quartzose it is more probable that the waves have sorted the waste of a residual mantle. :

Quiet Water—(6) When a current enters a body of quiet fresh water, unvexed by tides or winds, as a stream enters a lake, the inertia of the greater mass and the diffusion of the stream in the greater volume checks the current, and it drops whatever sediment it may have carried. The laws of this simple case can be formulated mathematically, and Babbage has calcu- lated the distance to which sediments of an assumed character

would be transported by a river current of assumed velocity entering a salt-water body, whose bottom has an assumed slope ; he neglects the difference of density between fresh and salt water, and assumes an off-shore current equal to that of the river at its mouth. The conclusion is determined in advance, and cannot be applied to the interpretation even of lake sediments, since the assumed conditions of sediment and current are hypo-

‘Hand Book of Physical Geology, 1884. A.J. Jukes-Browne, p. 185.

CONDITIONS OF SEDIMENTARY DEPOSITION. 489

thetical. An existing case, which approaches the conditions assumed by Babbage, is that of the Rio Uruguay, which is described by Revy.* .

“The little town of Higueritas, also called Nueva Palmira, is situated in latitude 33°52’S., long. 58°23’ W.,in the Banda Ori- ental, at the junction of the Uruguay with various branches of the Parana, all of which discharge jointly their volume into the La Plata. Three miles below Higueritas, at Punta Gorda, the La Plata proper commences; three miles above Higueritas the Uruguay opens into a lake from 4 to 6 miles wide and about 56 miles long. There are no islands on this lake, although, with the exception of a deep channel half a mile wide of steep sides and submerged, the lake is shallow ; it may be called the estuary of the Uruguay. A little above Fray Bentos, 58 miles from Hig- ueritas, the first islands appear within the lake; and, their num- ber soon increasing, we enter the delta of the Uruguay, which for 25 miles more retains the width of the lower lake, breaking, however, up into a great number of large and small islands, until, a little below Paysandu, the river proper commences within a con- fined channel. At Paysandu, a commercial town of importance, 125 miles from Higueritas, the delta of the Uruguay commences. At Fray Bentos the visible delta terminates ; and from the latter place to the La Plata the future delta of the Uruguay is now in COURSE OloOnmMations @ a...

. During the survey of the Uruguay there was a period- ical rise of the river, viz., on February 3, 1871, and a sample of -water was taken on that day at the Salto section, about 200 miles above Higueritas. The water was turbid, of deep brown color ; and the analysis of the sample showed that it contained one part by weight of solid matter in suspension in 9524 parts of water. There was no perceptible change in the color of the water or in its analysis, until we reached Fray Bentos [142 miles below Salto | on the 5th February, 1871, and here it contained 1 part solid matter in 11,200 of water by weight in suspension. At Higueritas, on the same day, the waters of the Uruguay ap-

t Hydraulics of Great Rivers. J.J. Revy, pp. 134-135.

490 THE JOURNAL OF GEOLOGY.

peared clear, and we could only trace one part of solid matter held in suspension by 25,925 of water. Nothing could more forcibly illustrate the formation of deltas. The river retains matter held in suspension by its water within its ordinary chan- nel as long as its velocity is maintained ; as soon as itenters a lake or an estuary checking regular currents, the matter held in sus- pension is dropped.”

That is to say, in flowing 142 miles in its navigable channel and through its delta the river dropped about 15. per cent. of the load which it bore at Salto; and beyond the delta in still water it dropped 48 per cent. more; leaving it but 37 per cent. of the original load to be carried past Higueritas to the estuary of the La Plata. Or stating the proportions in terms of the sediment brought through the delta to the head of the lake, 57 per cent. was deposited and 43 per cent.escaped. It would be desirable to determine in what ratio the deposit is made in the upper and lower reaches of the lake, but Revy gives no data between Fray Bentos and Higueritas. He states however that the lake is without islands, although it is shallow with the exception of a deep channel half a mile wide ; but just above Fray Bentos islands indicate the present front of the delta. The occurrence of these advance elements of the delta only in a limited distance indicates that the bulk of deposition is on the delta’s front, and that the sediment which passes beyond is that which the slower current of the lake can hold in suspension.

The deposits of the extinct lakes Bonneville and Lahontan have been fully described by Gilbert and Russell, but the lake beds of the west still present rich fields for study of deposition under simple conditions in fresh and salt water.

(c) Alterations of Current—Whema land-locked water body is open to the ocean it is subject to influx and reflux of tides, but the rivers pouring into it may possess volume sufficient to exclude salt water; it is then a fresh-water estuary, which receives the sediments as well as the waters of its tributaries. The currents in such an estuary are periodic, changing with the flood and ebb, and the conditions of deposition vary accordingly. The

CONDITIONS-OF SEDIMENTARY DEPOSITION. 491

Atlantic coast is fringed with estuaries which are carefully mapped by the Coast Survey, but variations of deposit with changes of current have apparently not been described. Writing of the La Plata, an estuary 125 miles long, where the tide from the Atlantic contends with the current of the rivers Paraia and Uruguay, Revy says: * |

‘At this point, where the power of the tidal wave balances that of the rivers, there will be no current; the level of the estuary will rise slowly like that of a lake receiving supply from all round its border. It is here—where the rivers and the tidal wave contend for supremacy, each trying to establish its own current, and where for hours the power of either of them trembles in the balance without any sensible movement in any direction—that deposit copiously takes place ; matter, held in suspension by the rivers as long as their currents are maintained agitating their water, is dropped as soon as they cometo rest. It is here, within about 10 or 20 miles of the river's mouth that banks are most rapidly growing and islands are forming, and the ultimate result of these daily contests is invariably in favor of the rivers which slowly but steadily encroach on the estuary and ultimately annex its whole territory. The progress of the tidal wave is, however, never checked an instant, the rivers only check the currents orig- inating with the wave. . . . . A tidal wave is never visible to the eye, and can only beconceived from observation, by a suc- cessive measurement of its dimensions, which are very large. We may, from an elevated position, see 10 or 15 miles, but a tidal wave onthe La Plata is about 258 miles long... . .

“.. . . During the second half of the tidal wave, viz., from flood to ebb when the surface of the La Plata is falling, there is much more uniformity in the directions of the currents, which for a time will be the same for the whole estuary, all tending to the Atlantic. The wave will again proceed faster in the deeper than in the shallower portions of the estuary, and will accord- ingly make the level fall a little faster in the deeper channels, and

‘Op. cit. pp. 29-30.

492 THE JOURNAL OF GEOLOGY.

the current will now set from shore into the estuary; the reverse of what happened with the rise of the La Plata.

‘By degrees the level of the estuary will again adjust itself to mean sea-level. All the water which the tidal wave brought from the sea will now have to be returned, and in addition the whole volume which the great rivers have discharged into the estuary ; and the currents will not only be stronger, but they will also last longer, of which circumstance the outline of the tidal wave bears evidence, the duration of the rise of the La Plata being about six hours, its fall continuing for about seven hours.”’

Revy further calls attention (page 23), to the fact that the current with a given fall of the river is swifter in deeper, slower in shallower water therefore deposit during flood-tide is more copious over shallows, and is there less liable to scouring during the ebb. It follows that the shallows become tide-flats, tide-flats are raised to rush-grown islands, and the islands unite to extend the river’s banks. Thus the Parana has filled two-thirds of the La Plata, which was 325 miles long, and the river will ultimately replace the estuary, so that the future delta will be built into the Atlantic, as that of the Mississippi extends into the Gulf.

If the sediment thus deposited consists of mingled sand and clay it will be sorted to some extent by the alternate checking and starting of currents. As with rising tide the current slows, sand will first be dropped; during the period of quiet water both sand and clay will sink together, though at unequal rates; and when the ebb restores the outward current, the surface of the latest deposit may be scoured, removing clay and leaving sand. Furthermore the swifter currents of the channel may carry clay, ; even though dropping sand, while the slower currents of the shallows drop both. Hence there must be a tendency toward alternation of more sandy layers with more clayey ones, and of horizontal passage of sands into clays.

Where rivers enter bays of such depth or expanse that the fresh water does not displace the salt water, other conditions than those governing estuarine deposition prevail. It is there

CONDITIONS OF SEDIMENTARY DEPOSITION. 493

probable that the influence of tides is often subordinate to that of winds, of the difference of density between fresh and salt water, of mechanical and chemical reactions of salt water on sediments, and of currents prevailing along shore.-

The influence of tides upon undertow, tending alternately to retard and accelerate the seaward current, may be important and may lead to alternate episodes of deposition and scouring as it does in estuaries; this is probably the case on all submerged continental platforms, and particularly where tides sweep in from a great expanse of ocean, as on the Atlantic coast of the United States. The effect, where conditions favor it, would be more regular than among the shoals and channels of an advancing delta, and the alternation of strata would be more distinct and even; it is possible that thinly interbedded strata of unlike character may be thus interpreted.

The well recognized characteristics of tidal formations are the evidences of shallow water, ripple marks, sun cracks, organic trails, etc., peculiar to sections of the shore where sediment is abundant. The strata are shales, and shaley sandstones irregularly bedded and often red. Such deposits are direct evi- dence that:

(1) The land from which they came presented gentle slopes and was mantled in residual formations to a distance from the sea.

(2) Since the zone of tide-flats along any shore is limited in width, if the distribution of such strata be wide, either great rivers gradually filled a shallow basin, as the Mississippi, the Amazon and Parafia have done, or the sea transgressed upon a low-level land. In the former case the land was built outward by volumes of muddy fresh water, and the deposits would be of fresh or brackish water types. In the latter case the sea pre- vailed and the deposits would be of marine character.

(3) Since the level of tidal deposits is near the surface of the water, and they are therefore limited in thickness, if a con- siderable thickness shows the characteristic marks throughout, the area of deposition subsided at a rate approximating to that of accumulation.

494 THE JOOKINATIZOFNGEOLOGM,

(4) Since tidal deposits are imperfectly sorted, they form under shelter from waves or in the presence of waves of force insufficient to handle the volume of sediment. The shelter may be a point of land before a bay or a barrier of beach sand before a lagoon; in either case clean sands and mud deposits may be contemporaneous. Or the feeble waves may be unequal to the task of sorting, because of short fetch in a nar- row sea.

(2) Long continued or powerful winds.—The fall of a river determines its current, other things being constant, and therefore its transporting power. The fall near the mouth is lessened in any given stream if the level of discharge is raised, and vice versa, and the influence of tides in this respect has just been discussed. Winds may exercise a no less important influence. Revy (p. 27) describes an instance in which the effect upon the tides of a storm approaching from the east, combined with its subsequent direct effect in heaping up waters, was to raise the level of the La Plata fifty inches at ebb tide, and to reverse the current of the Parafia fora hundred miles. An extraordinary result like this is probably balanced in its effect upon deposition by the scouring which takes place when the wind changes direc- tion, or calms, and the mass of water returns to its normal level. But the influence of long continued winds blowing periodically during certain seasons of the year must be effective in causing deposition from silt-laden rivers. Humphreys and Abbott briefly discuss the nature of winds affecting the level of the gulf at the mouth of the Mississippi, and assign an important share of the results from deposition to the influence of the southeast winds.*

(¢) Flotation of fresh water on salt-—Fresh water is lighter than salt water, hence a river discharging into the ocean rises and spreads over the surface. The volume of the river, advanc- ing, holds back the salt water, and the fresh water flows up an incline which is the surface of contact between the media of unlike densities. This checks the river’s current and forms a

‘Physics and Hydraulics of the Mississippi. Page 450.

CONDITIONS OF SEDIMENTARY DEPOSITION. 495

vertical eddy or ‘‘ dead angle,” in which material rolled on the river’s bottom is left and some sediment is dropped. Thus bars are formed in advance of deltas.* With rising tide or on shore winds the elevation of the salt water surface will-increase this effect and force the zone of maximum deposition shoreward, while the reflux with the ebb or change of wind will lower the incline and assist wider distribution of sediment. Hence there is most rapid accumulation in the comparatively narrow strip of deposition during rising tide.

Flocculation in salt water.— Acids and salts in solution cause fine particles of sediment to draw together in flocculent form and therefore to settle more rapidly than they would in fresh water. W. H. Brewer states that clay which has been in sus- pension thirty months in fresh water had not settled out as clearly as the same clay from a solution of common salt in less than thirty minutes,” and he describes a number of experiments tending to show that ‘‘when a muddy river enters salt water chemical laws interfere with the purely mechanical ones. Then the rate of deposition is affected by the salt more than by the current, and velocities which would be more than sufficient to carry the finer suspended matter indefinitely, if the water were fresh, entirely fail where the water is brackish or salt. Practi- cally it is the degree of saltiness which controls deposition.”’

Brewer applies this principle to a discussion of the formation of the bars of the Mississippi and concludes that the zone of maximum deposition retreats and advances as the greater or less volume of the river changes the position of the opposing salt water. It is obvious that this condition would be combined with that of the ‘dead angle” produced by the rise of the fresh water on salt.

The phenomena of flocculation have been attributed by Hilgard, Brewer and Barus to chemical reactions, but Milton Whitney finds a readier explanation in the forces of attraction or

*Humphreys and Abbott; op. cit. p. 445.

* Memoirs of the National Academy of Sciences, Vol. II, 1883, p. 168.

496 THE, JOCKNALE VOR (GHOLOGN.

tension existing among the fine particles of a solid in suspension, which are modified by the presence of salts.* But whatever the conclusion may be as to the nature of the controlling law, the influence of salt water in this respect is an important cause of deposition of clays at the mouths of rivers.

(g) Inequalities of depths; lee banks. —When any volume of flowing water expands, it loses velocity and, if muddy, deposits sediment. This well recognized condition of river deposition has been considered in reference to a river entering a lake; it is equally true of an ocean current or of undertow, where the for- mer passes from a narrow strait to the broader sea, or where either one flows from shallow into rapidly deepening water. The condition needs no explanation—it requires only illustration.

From the Atlantic the southern equatorial current sweeps past the mouth of the Amazon and Orinoco; as the Gulf stream it crosses before the Mississippi delta, and pouring out through the Straits of Floridaenters the North Atlantic. From the rivers tribu- tary to its course it receives fine sediment escaped beyond the deltas. In its passage through the Caribbean sea and the Gulf of Mexico it flows over the eastern Caribbean deep, Bartlett’s deep and Sigsbee’s deep, and where it leaves the Blake plateau north of the Bahamas it falls over the continental rim into ocean depths. Between these basins it traverses relatively shallow seas, whose bottoms are floored with modern limestone and green sand. These deeps of 2,500 to 3,000 fathoms and shoals at 100 to 500 fathoms are result of epeirogenic forces probably, but they are now floored with deposits which consist of the shells of pelagic organisms mingled with terrigenous silt, forming ‘ modified pteropod ooze.’”’? This deposition, if it has gone on long enough since the depression at the deeps, or fast enough to mask the details of deformation, possibly continued up to a recent time, determines the profiles of the slopes from shoal to abyss. In

*U. S. Dept. Agric. Weather Bull. No. 4, 1892, ‘Some physical properties of soils,” pp. 19-23. Milton Whitney. $

? Geologic and bathymetric maps of the Atlantic in ‘“ Three Cruises of the Blake,” by Alex. Agassiz, Vol. I.

CONDITIONS OF SEDIMENTARY DEPOSITION. 497

the Eastern Caribbean deep the declivities are such as would thus be determined; the northern and southern slopes between which the current flows are approximately equal and steep; the slope of the eastern side is also steep and lies at right angles to the course of the current in the position of a bank forming in the lee of a terrace, and the rise from the abyss westward in the direction of the current is relatively gradual. *

This basin is the one most advantageously situated to exhibit slopes of deposition. Bartlett's deep lies like a narrow cafion across the course of the current, and the small triangular basin immediately east of Yucatan, while it shows a steep slope north- ward in the direction of the current, presents similar declivities along its other two sides which are possibly scoured by the waters converging to pass out at the apex, the Yucatan channel. The steepest slope of the Gulf of Mexico from the 1ooth to the 2000th fathom line, is in the position of a lee-bank northwest of the Yucatan plateau, and the contours elsewhere are appar- ently modified by the scouring action of the current as it sweeps around the basin, and by terrigenous deposits from the adjacent shores and rivers. The Blake plateau, over which the Gulf stream sweeps north of the Bahamas, is clean, hard limestone, but a lee-bank of mud and ooze is forming on its short, steep slope into deep water. Agassiz says (p. 277): ‘There we pass from the comparatively coarse shore mud to finer and finer ooze, which becomes an impalpable silt in the deeper water beyond one or two thousand. fathoms, assuming at the same time a lighter color.”

Another illustration may be found in the deposits of silt which form the edge of the continental plateau off the North Atlantic coast of America. Agassiz has mapped the width of the plateau as covered with “silicious shore deposits,” and examination of some of the samples of bottom in the Coast Survey office, for which opportunity has been most courteously extended to the writer, shows that the surface of the plateau is

*See bathymetric map opp. p. 98, “Three Cruises of the Blake.”

498 THE JOURNAL OF GEOLOGY.

composed of sands which are indeed fine near the eastern edge, yet are distinctly granular and incoherent. But soundings on the steep slope beyond the 100 fathom line have brought up very fine silt from the bank of which that slope is the surface, and this silt passes at its foot into globigerina ooze. The zone of transition from clean sand to silt is as sharp as the edge of the slope and is coincident with it. It is evident that the sus- pended mud which escapes beyond the estuaries and sounds of the littoral is swept out until the undertow expands over the edge of the escarpment, and is diffused in deep water; there the silt forms a great bank 10,000 feet high, with a slope of 3 to 8 degrees, which has grown seaward during geological ages, and continues to expand as erosion continues on the land.

The structure of this deposit can only be inferred, but it is worthy of consideration. The surface of accumulation, to which bedding planes are probably parallel, is inclined at a considerable angle, and traverses the bank from top to bottom obliquely to the vertical thickness. The direction of the growth is outward, not upward. The conditions of deposition are similar to those of a delta advancing into fresh water, and the structure of the deposit is probably similar to that shown by Gilbert for a fresh - water delta. (Fig. 14, p. 68, Lake Bonneville). If the detritus was sand, instead of silt, the conditions would be identical, and the bedding which would be exposed by removal of the hori- zontal upper layers would represent an enormous thickness of strata, inclined at a dip corresponding to the slope of the bank. Russell rejects explanations of the attitude of the Newark beds so far as they are founded on sedimentation," but it seems possi- ble that they may present the structure of lee banks. It may also be probable that isoclinal structure, where repetition of strata does not occur, is evidence of this form of deposition and of the conditions essential to it.

Deposits of this character, consisting usually of clay or silt, are significant of extended rock decay on the land, of currents

‘Bull. U.S. G.S. No. 85, Correlation Papers—— The Newark System, p. 78. I. C. Russell.

CONDITIONS OF SEDIMENTARY DEPOSITION. 499

capable of distributing the sediment, and of shoals and deeps in the sea. The amount of difference in depths is not indicated, but the rapid descent from the edge of the bank to the foot is essential to diffusion of the current and the consequent deposi- tion. A lee-bank is a submarine terrace of construction. Where such a terrace extends into an abyss it argues prolonged devel- opment, and, therefore, antiquity of relation between continental platform and oceanic basin.

(h) Subsidence from oceanic circulation—The greater part of terrigenous sediment must be deposited in deltas and estuaries, on continental platforms, and in silt banks along great deeps. But a very considerable amount of fine silt brought out by rivers and undertow, quantities of volcanic dust fallen on the ocean, and the calcareous and silicious parts of pelagic organisms are taken into oceanic circulation, and find a resting- place more or less remote from their place of origin. These deposits constitute the deep- sea formations; they are not clearly recognized among the strata of past geological periods now exposed in land surfaces, and on this fact rests the principal argument for the antiquity of the con- tinents and oceans. They have been fully described by Murray,' and their mode of deposition need here be indicated only by reference to the blue muds of the Bay of Bengal and the Arabian Sea.

The blue muds are composed of minute mineral fragments derived from the disintegration of the land, of a diameter of .05 -mm., or less, which may contain calcareous remains amounting to 50 per cent. of the whole, or may be almost free from lime. The description of a typical sample, taken about 275 miles south of the mouth of the Ganges, is given by Murray? in an article which is accompanied by a map showing the distribution of dif- ferent formations. From this map we may gather that terrigen- ous deposits form a belt, 50 to 125 miles wide, along the eastern coast of Africa, the western coast of Australia, and the Malay

™Challenger Reports; Narr. of the Cruise, Vol. I, Part II.

2 Scott. Geog. Mag., Vol. V, No. 8, Aug., 1889, p. 420. John Murray on “‘ Marine Deposits in the Indian, Southern and Antarctic Oceans.”

500 THE JOURNAL OF GEOLOGY.

archipelago, but in the Arabian Sea and the Bay of Bengal they extend to distances of 800 miles from the mouth of the Indus and Ganges, and cover areas of more than 700,000 and g00,000 square miles, respectively. By reference to a map of the ocean currents it may be seen that their courses affect the distribution of these deposits. Sweeping at all seasons past the west coast of Australia and directly toward the east coast of Africa, paral- lel to which it then diverges, the principal current prevents any extended distribution of sediments in a direction normal to these coasts. But the currents of the Arabian Sea and the Bay of Ben- gal, flowing alternately east and west around these great embay- ments, past the mouths of the two great silt-bearing rivers, distribute fine material in suspension throughout the area of their circulation. CHEMICAL DEPOSITION.

Favorable conditions:

(a) Evaporation from an enclosed sea.

(6) Precipitation of lime and magnesia from ocean waters, charged by solution from the land, through evaporation, through reaction of salt water on fresh, and through varying atmospheric conditions at the surface of the sea.

(a) Evaporation of an enclosed sea—When a limited body of water, such as a lake, is subjected to a change of climate, so that evaporation exceeds precipitation of rain, the volume will shrink, outflow will cease, and the solution of salt will be concentrated. If the process is sufficiently continued the solution will become satur- ated, first for one salt, then another, and they will be deposited in the order of their insolubility. This process is important as an indication of climatic variation in the past; it has been fully described by Gilbert, Russell and Chatard for Pleistocene lakes and the chemical relations, and these studies suggest the conditions to which appeal must be made to explain the less exact facts known in ancient formations of the kind.

(0) Precipitation from brackish waters—The chemical precipi- tation of lime and magnesia from sea-water is a much mooted question. There are two lines of evidence relating to it which

CONDITIONS OF SEDIMENTARY. DEPOSITION. Sol

are apparently opposed. On the one hand, the scientists who have described material obtained by soundings on modern lime- stone deposits have recognized only organic remains. The Challenger in the open océans, remote from great rivers, the Coast Survey vessels in the Caribbean, the Gulf of Mexico and off the Atlantic coast, the Norwegian expedition in the North Atlantic and English vessels in the Indian ocean have found cal- careous oozes of various kinds and rocky limestone formations, but in every case the calcareous matter is described as composed wholly of the tests of pelagic organisms, many of them of micro- scopic size. It is known that carbonates of lime and magnesia are to a greater or less extent soluble in waters containing car- bonic acid, and that the proportion of these carbonates dissolved in ocean waters is small. According to Dittmar the salts in solu- tion in ocean waters contain 0.345 per cent of carbonate of lime and 3.600 per cent of sulphate of lime,t and the ocean is capable of dissolving all the lime poured into it by rivers.2 This view being accepted, it follows that pelagic organisms, which possess the power of secreting solid carbonate of lime from solution, alone can cause lime deposits. Chemical precipitation is, accord- ing to this view, impossible, or, if it occurs, is followed by speedy re-solution, and all limestones deposited under conditions of the existing oceans are of organic origin. On the other hand, there are many limestones, deposited at different periods of geologic time, from Algonkian to the present, including some now forming, which consist of more or less clearly crystalline calcite, devoid of organic structure. If this calcite was originally built into organic forms they have been entirely obliterated. Such limestones do indeed contain fossils which sometimes exhibit more or less crystalline texture, but the occurrence of these organic forms in the holo- crystalline matrix only raises the question: If the mass was originally all organic and has undergone secondary crystallization after lithifaction, why was the process so complete in the matrix

*Report on the Scientific Results of the Voyage of H. M.S. Challenger. ‘‘ Physics and Chemistry.” Vol. 1, p. 204.

Z@psiciteaps22ir

502 THE JOURNAL OF GEOLOGY.

and relatively so ineffective in structures whose delicate anatomy can still be traced even to microscopic details? Thin sections of limestone which show a mass of interferant crystals suggest that this was the primary structure of the rock, and organic remains appear to be foreign bodies which are accidentally of the same substance as the matrix. If this view be correct, then only the alteration of the organic carbonate is the measure of the alteration of the rock-mass. If it can be shown that limestones now forming by chemical precipitation possess a crystalline structure, which resembles that of ancient limestones, the resemblance will constitute a presumption in favor of similarity of origin for the modern and ancient formations. And the fact that limestone is now being precipitated would, if it be established, leave the geologist free to weigh the evidence in the case of any ancient limestone for and against its organic or chem- ical origin. It is not proposed here to argue that limestones are prevailingly of one origin or the other, but only to show that the assumption of organic origin for all the calcareous deposits of the stratified series is too sweeping. To this end it is desirable to consider the chemical and mechanical conditions which affect the precipitation of carbonate of lime, to estimate the solubility of the carbonate in salt water, to review the conditions under which lime is contributed to, and distributed in, the sea, and to describe several cases of modern limestone formation by precipitation.

Schloesing made a number of experiments on the solubility of carbonate of lime in carbonic acid and water ; he thus describes his method and results. 7

‘‘ Experiments :—The method adopted was to cause to pass through pure water, which was maintained at a constant temper- ature and contained an excess of carbonate of lime, a mixture of air and carbonic acid, of a composition varied at will, but con- stant, for each experiment; this mixture was constantly supplied until a perfect equilibrium was established between the substances

*Comptes Rendus, Vol. 74, 1872, pp. 1552-56, and Vol. 75, p. 70.

CONDITIONS OF SEDIMENTARY DEPOSITION. 503

entering into the reaction, then the quantities of carbonic acid and of lime were determined in the filtered solution.

“Then to run through the scale of pressures of the car- bonic, acid from the most feeble to the strongest-that could be obtained. 5

“Then to change the temperature and re-commence anew the series of experiments in order to eliminate the influence of heat.

“The experiments establish the fact that pure water in the presence of carbonate of lime, and of an atmosphere containing a determined proportion of carbonic acid, dissolves simultaneously free carbonic acid according to the law of absorption of gases, neutral carbonate according to the solubility of this salt in water free from carbonic acid, and bicarbonate of lime.”’

The relation found between the tension of the carbonic acid and the proportion of bicarbonate formed is such that: “ Equil- ibrium being established in the solution, the slightest diminution of the tension of the carbonic acidin the atmosphere determined the decomposition of a corresponding quantity of bicarbonate, with precipitation of the neutral carbonate and the emission of carbonic acid gas.”’

The veteran chemist Dumas, in an article on the normal car- bonic acid of the atmosphere, says: *

“In recent times, by a happy application of the principle of dissociation, M. Schloesing has shown that the proportion of carbonic acid contained in the air was in relation with that of bicarbonate of lime held in solution in the waters of the sea. When the amount of carbonic acid (in the air ) is diminished the bicarbonate of the lime in the sea is dissociated, the-half of its carbonic acid passes into the air, and the neutral carbonate of lime is precipitated from solution” ( ‘deposé”’ ).

Another condition which may decompose bicarbonate of lime is simple mechanical agitation of the water holding it in solution. Dittmar in examining samples of ocean water for car-

*Comptes Rendus, Vol. 94, 1882, p. 70.

504 LTTE J OULINATE OLA GIOLO GN.

bonic acid, was led to make a series of experiments on the effect of shaking with air an artificial sea-water, Containing a known amount of carbonic acid. He found that he shook out 27 per cent of the carbonic acid originally present, and this did not represent the greatest possible loss. After describing the experi- ments he says :*

eosin experiments reported inthis chapter . . . are sufficient — to prove . .. that, supposing a sea-water which contains its carbonic acid as bicarbonate, associated or not with free carbonic acid, to be exposed to the air even at ordinary temperature, such a water will soon lose not only its free but part at least of the loose carbonic acid of the bicarbonate (7. ¢., of what is present over and above that existing in the form of normal carbonates ).” Dittmar also discusses the dissociation tension of bicarbonates in sea-water and suggests that the water of the tropics constantly gives out carbonic acid to the air, and water of cooler and of arctic zones constantly absorbs it. *

Thus the chemists describe two conditions under which bicar- bonate of lime may be decomposed into neutral carbonate and carbonic acid: Ist, by diminution of the tension of the carbonic acid in the atmosphere; 2d, by agitation of the solution.

Theoretically, either one of three things may occur to the neutral carbonate of lime if it be thrown out of solution by either one of these processes, which we may admit are active on some portions of the salt water surface. The carbonate may be redis- solved, or deposited as a calcareous mud, or built into organic structures. We may discuss these alternatives in turn.

The solvent action of sea-water has been the subject of direct observation in the ocean and of experimental determination. Deep-sea shells, dredged from the bottom of the Pacific and now in the Smithsonian collection are corroded, some of them on the outside only, some of them through and through. In the former

‘Report on the Scientific Results of the Voyage of H. M.S. Challenger. ‘“ Physics and Chemistry,” Prof. Wm. Dittmar, F. R. S. Vol. I, p. 115.

ZOp a cit. pp e2t2-213%

3 For an opportunity to examine these my thanks are due to Dr. Dall, B. W.

CONDITIONS. OF SEDIMENTARY DEPOSITION. 505

case the creature still inhabited the shell and preserved the essential parts of its house; in the latter case the decomposition of the fleshy parts may have assisted the solution of the cal- careous skeletons. To this last point Murray calls attention :?

“Tt is probable, however, that carbonic acid does play an im- portant part in the solution of shells of animals sinking through the water. The organic matter of the animal on being oxidized produces carbonic acid, which, being itself liquid at all depths over 200 fathoms, will form a locally concentrated acid solution inside the shell, which it will attack with vigor.”

The shells which were corroded while still inhabited were also exposed to unusually active solvent influences since they lay upon the bottom, of which Agassiz writes :*

“The pelagic animals derive a large part of their food supply from the swarms of large and small pelagic alge covering the surface of the sea in all oceans. On dying, both surface animals and plants drop to the bottom, and still retain an amount of nutritive matter sufficient to serve as food for the carnivorous animals living on the bottom. A sort of broth, as it has been called by Carpenter, collects on the bottom of the ocean, and . probably remains serviceable for quite a period of time; the decomposition of the organic material which has found its way to the bottom takes place gradually, and its putrefaction must be very slow.” Thus these more or less corroded shells, dredged from the deep sea, bear witness to the solvent evolved ina bottom layer of decomposing organic matter.

A more direct line of evidence as to the solvent action of the sea-water itself is afforded by observations on the depths to which calcareous skeletons will sink undissolved. The pelagic pteropods and foraminifera, living at the surface, sink on dying and are slowly dissolved; if the water be too deep they never reach bottom. The limits below which they are not found are about 1500 fathoms for pteropods, thin shells exposing large

* Narrative of the Cruise of the Challenger, Vol. I, Second part, p. 981. * Three Cruises of the Blake, Vol. I, p. 313.

506 THLE, JOUKNALE OF IGEOLOGV.

surfaces to solution, and 2800 for globigerina, smaller shells, relatively more massive. Commenting on this, Dittmar says :"

‘At the greatest depths of the oceans all these calcareous shells disappear from deposits in all latitudes. The cause of this, in my opinion, is not that deep-sea water contains any abnormal proportion of loose or free carbonic acid, but the fact that even alkaline sea-water, if given sufficient time, will take up carbonate of lime in addition to what it contains.”

The solvent action indicated by the disappearance of delicate and microscopic shells, which enclose decaying organic matter, yet sink through gooo to 16,000 feet of water, is very moderate.

Dittmar says:? ‘‘Sea-water is alkaline; all the alkalinity must be owing to carbonates, and of these carbonate of lime is one.” Now the very moderate solvent power of this alkaline solution may be satisfied so far as carbonate of lime is concerned by two sources—by organic tests in suspension, and by chemical precipitate. The lime used by organisms is derived from the solution to which it is partly returned by re-solution, but an- other part is deposited, and the sea thus suffers constant loss. This loss is supplied by the land. If this terrigenous supply is less than the amount of organic deposit the sea will become less - alkaline, and will more efficiently dissolve calcareous tests until the solvent is satisfied. If the land contribution is continuously equal to the amount organically subtracted, there will be equili- brium. If the land yields more carbonate of lime than that which is being locked up in organic limestones, the alkalinity of the sea will gradually increase until there is chemical precipitation. This condition is favored by the entrance of lime-bearing fresh water into a sea free from active currents and exposed to evapora- tion which balances the inflow.

Since the amount of lime in the ocean is thus balanced be- tween that contributed by the land, and that precipitated by organic or chemical means, it is worth while to review the con-

VOpwcit-sapa22ue

2Op. cit., p. 206.

\ CONDITIONS OF SEDIMENTARY DEPOSITION. 507

ditions under which lime is carried from the land, and to consider how it is distributed in the sea. As was stated early in this paper, the amount of lime carried annually from a given land area is directly related to the efficiency of rock-decay; rock-decay is most efficient over surfaces which have suffered prolonged de- gradation, and on such surfaces the development of drainage systems has usually resulted in the growth of great rivers. Hence the lime contributed from continents to oceans is delivered chiefly at a few places, the mouths of extended systems, and there is great inequality in the distribution of these along differ- ent coasts and among different seas. Of this fact South America is the most conspicuous example, with all its great rivers pouring into the Atlantic, and not one considerable stream entering the Pacific. More limited seas, which receive vast quan- tities of solutions are the Caribbean and Gulf of Mexico, Arabian Sea, Bay of Bengal and Yellow Sea.

At the mouths of great rivers there exist several conditions which influence the solubility and distribution of lime in the adjacent seas; these are: Ist, the amount of lime in solution in the river water; 2d, chemical reactions between substances in fresh and salt water; 3d, the relative solubility of lime in fresh and salt water; 4th, the conditions of evaporation and agi- tation of the brackish water; 5th, the effects of currents.

The proportion of solids in solution ina river is dependent not only on the extent and slopes of its basin, but also on the nature of the rocks exposed, and the influence of climate on decay. Under like topographic conditions, silicious schists and a cold climate probably yield a minimum contribution; crystal- line rocks and a warm, moist climate yield more; limestone areas, though resistant in a dry climate, suffer most rapid degradation under a humid atmosphere, and the deposits of the later geo- logic periods, including as they often do quantities of soluble salts, charge the drainage most strongly. The following analyses present specific contrasts, traceable to these geologic and climatic conditions. Each analysis represents but one phase of composition, which varies in each river with high and low

508 THE JOURNAL OF GEOLOGY.

stages, and the analyses of our great rivers are incomplete, but, strange as it seems, no other analyses of their waters have been found, after diligent search.

SAMPLES.

(2) Ottawa river; sampled March 9g, 1854, before the melting of the snow, at head of St. Anne lock; water was pale amber yel- low, free from sediment and derived froma region of crystalline rocks covered with forest and marsh vegetation." ;

(6) St. Lawrence river, sampled March 30, 1854, before the melting of the snow, on the south side of the Pointe des Cas- cades, Vaudreuil; water was clear, colorless, and represents the drainage of areas of glacial drift, crystalline rocks and paleozoic sediments, clarified by passage through great lakes."

(c) Mississippi river;* sampled in the autumn of 1887 at very low water, inthe main channel above the mouth of the Missouri; water represents drainage from areas of glacial drift, crystalline rocks and paleozoic sediments, including large expanse of lime- stone and cultivated lands. |

(@) Missouri river;* sampled on the same day as the preced- ing ; water represents drainage most highly charged with the sol- uble salts of the more recent and little consolidated geologic for- mations; potash was no doubt present but was not determined.

(¢) Mississippi river;* six miles below the mouth of the Mis- souri ; sampled on the same day as the preceding in the current of Mississippi water as shown bya float dropped on taking sam- ple c; sample represents Mississippi and Missouri waters, appar- ently with excess of the latter.

(f) Mississippi river ;* twelve miles below the mouth of the Missouri, above St. Louis ; sampled on the same day as the pre- ceding inthe current indicated by the float; sample represents Mississippi and Missouri waters apparently more thoroughly mixed.

"Geology of Canada, 1843-63, Logan, pp. 565-568. *Annual Report of the Water Commissioner, St. Louis, 1888, pp. 309-310. Anal- yses by St. Louis Sampling and Testing Works, Wm. B. Potter, Manager.

CONDITIONS OF SEDIMENTARY DEPOSITION.

ANALYSES——PARTS PER I,000,000 OF WATER.

509

St. Missouri and Missouri and Constituents. Ottawa. Lawrence. | Mississippi.| Missouri. Mississippi. Mississippi. a C (oa Morale Solids: ss | awe see 253.69 | 1207.66 1058.98 787.12 Filtered sedi- DOGIME sa 69.75 167.80 20.90 638.26 622.33 389.36 Tn 8 2 ae cee 1.52* I.15* | notgiven | notgiven | not given not given UN fees reel ap fat 2.39% 5.03% B Aas 2707 g.16* 10.37% Mig @ aes 2.30*)| 12.08% 28.26 41.96 37.51 39.40 CaO se 13.88*| 44.92* 52.93 IIO.15 109.63 94.90 (Gils een) .76 2WA2 5.31 19.53 14.22 1S 3) SO peak awee 1.61 6.87 10.28 89.76 73.66 69.89 SiOpgses esse 20.60 37.00 | notgiven | notgiven| not given not given 69.75 167.80 20.90 638.26 622.33 389.36 Iron and al- umina ____ traces traces none 55-84 20.90 26.80

According to Gooch* the combination in these analyses should be calculated im the order ME), NaC) AG SO; Na, sO), Me sSOy, Cas Oj iceO nr Cac@nwNae EO. Tete: vam tints is) the order, followed in the Canadian analyses. Hence the following is the

hypothetical combination.

a b C ad e if etal Solbiglgsi 2254 |) “eesece 253.69 | 1207.66 1058.98 787.12 Filtered sedi-

TAME — 552 69.75 167.80 20.90 638.26 622.33 389.36 Klee ane 1.60 2.20 | notgiven | not given| not given not given INE Cobos os Mi een el D LPs 8.57 32.03 23.30 26.38 Kan ofece Tye22 tah |e en not given | not given} not given not given Nias SOZ 222= 1.88 12.29 | notgiven | notgiven| not given not given Me SOn S522 none none 15.41 125.90 118.49 104.83 Ca, SOnsesse none none none 9.93 none none Mg CO, ___- 6.96 25.37 19.63 none 137, 4.28 CACORE 24.80 80.83 94.56 189.35 195-79 169.47 Nas COe 2222 4.10 0.61 none none none none Fe,O3-++ ----

Als On 2 traces traces none 55-84 20.90 26.80

The chemical reactions which take place between substances dissolved in river waters and those contained in salt water are no doubt complex; but that which is most significant in relation to possible precipitation of carbonate of lime depends upon the

fact that organic matter may decompose sulphate of lime. Ac-

* Calculated from combinations given in the original publications.

t Analyses of Waters of the Yellowstone National Park, Bull. U.S. G.S., No. 47, p. 24.

510 THE JOURNAL OF GEOLOGY.

cording to Dittmar,’ the greater part of the lime in ocean water is there combined as sulphate, which in contact with organic mat- ter would be reduced to sulphide with evolution of carbonic acid; the latter would attack the sulphide with formation of carbonate of limeand sulphide of hydrogen. Thus organic matter in river waters tends to increase the proportion of carbonate of lime in the zone of brackish water. The carbonate thus formed is added to that already existing in the river water.

The solubility of carbonate of lime in fresh water and inl salt has been an object of consideration by several experiment- ers. Sterry Hunt testing artificial solutions found that 1 litre of water which contained 3 to 4 grams of sulphate of magnesia could dissolve 1.2 grams of carbonate of lime and 1 gram of carbonate of magnesia; but after standing a long time all the lime was deposited as hydrated carbonate. Thus it would appear that the presence of the sulphate assisted the solution of the carbonates.

Experiments made by Daubrée, which contradict Hunt's re- sults, led Thoulet to conduct a series to determine the question.? He took several minerals, marble, shells, coral and globigerina ooze, and subjected the comminuted samples of each separately to the action of filtered ocean water and distilled water during five weeks in each case. The solutions were shaken several times each day and the water waschanged from time to time. At the close of the experiments the samples had lost in weight and the amount taken into solution, reduced to that dissolved per cubic decimeter per day, was found to be, in grammes

In ocean In fresh

water. water. Shells, - . - - - .000039 001843 Coral, - - - - - .000201 .003014 Globigerina, - - - - .000137 .003091

Opry Cite pier 204. 2 Dittmar, op. cit., p. 209.

3 Oceanographié (Statique) par M. J. Thoulet, 1890, p. 263, and Comptes Rendus, t. CVILI, April, 1889, p. 753.

CONDITIONS OF SEDIMENTARY DEPOSITION. S511

‘“‘One sees that the solubilityin ocean water, itself very feeble, is also notably more feeble than the solubility in fresh water.”

When river water enters salt water it is. exposed in different form and under different physical conditions from those which existed in the river. As the fresh water is lighter than the salt, it floats upon it and spreads out in a sheet not unlike a fan. As compared with its depth and width in the river the layer is very shallow and widens from the mouth. Through waves and cur- rents the fresh and salt water mingle, and the expanse of brack- ish water may be of great extent. Forchhammer attributes the minimum salinity which he found for surface water from the north Atlantic, goo miles from the mouth of the St. Lawrence, to the volume of that river, and he found the ocean water fresh- ened at a similar distance from the La Plata. This thin sheet of brackish water is exposed to variations of temperature and baro- metric pressure produced by changing winds, now on, now off shore, and is in constant agitation with the rise and fall of waves. Thus the conditions which produce varying tension of carbonic acid, and which aid the escape thereof, exist at its surface, and the bicarbonate of lime in solution must be in unstable equili- brium, with constant formation of neutral carbonate and more or less constant recombination of it. If the neutral carbonate be present in sufficient quantity it will remain in suspension, undis- solved and unused by organisms, and will ultimately be deposited as calcareous ooze.

Oceanic circulation maintains an approximately uniform com- position of ocean water in all parts of the open seas, and great currents sweeping past river mouths distribute the contribution of fresh water and its solid matters, whether in solution or sus- pension. Thus the lime brought down by rivers, though meas- urable by hundreds of thousands of tons per annum, is so widely diffused in the vast volume of the ocean that it escapes recog- nition.

There are, however, several instances of modern limestone formation which, though local, illustrate the processes of chemi- cal deposition on a large scale. The descriptions of these may

512 THE JOURNAL OF GEOLOGY.

close these suggestions concerning limestone deposition by other than organic means. f

Chemically deposited limestone is forming in the southern part of Florida, probably over extensive areas. The Everglades, 4,000 to 5,000 square miles in extent, lie nearly at sea level, mar- gined by barrier reefs which confine the surface waters; in the dry season the drainage consists of numerous small streams—in the wet season the region is all submerged save the numerous muddy islands. Explorations on the western side, from Cape Sable north to Punta Rasa, were made by Mr. Joseph Wilcox, whose observations are stated by Dall as follows :*

“At the north end of Lostman’s Key (on the west coast, in about latitude 25° 30’) they entered the river of the same name and succeeded in penetrating 12 or 15 miles inland. No hard ground was seen except near-the mouth of the river, and the highest land at the latter place was not over 3 feet above high tide. Wide, shallow bays, with muddy bottom, interspersed with low, muddy mangrove islets, comprise the scenery. The boat frequently grounded, and was obliged to wait for the rise of the tide. A small fresh-water stream was finally reached, the cur- rent of which had scoured a channel 4 to 6 feet deep, with a rough, hard, rock bottom, fragments of which were broken off. It consisted of large masses of Polyzoa more or less completely changed into crystalline limestone, the cavities filled with crys- tals of calcspar. The rock is very hard and compact.”’

‘“Allen’s creek, emptying into Walaka inlet, anarm of Chuko- liska bay, was also visited. Ata point 8 or 10 miles east from the Gulf of Mexico the party were able to land on soft, wet soil, a little higher and drier than that at the head of Lostman’s river. A third of a mile eastward from the head of the creek specimens were obtained of a few rocks which project above the soil. They presented molds of recent shells with the interior filled with calc- spar, and an occasional Pecten dislocatus or Ostrea virginica, still retaining its shell structure. The cavities between the shells

* Bull. U.S. G.S. No. 84. Correlation Essays—* Neocene,” by Wm. H. Dall, pp. 99— IOI and 154.

CONDITIONS OF SEDIMENTARY DEPOSITION. 513

were filled with hard, coarsely crystalline limestone. The rock was not coquina modified, but looked more like a fossilized oyster reef. It contained no corals, and was obviously Pleistocene. The rock formed the base of small islets of drier soil amid the marsh, on which islets grew pine trees. The marsh, apart from these islets, is probably entirely submerged in the rainy season.”

In the bulletin referred to Dall speaks of the rock obtained by Willcox as being of organic or of partly organic and partly chemical origin, but at the time that manuscript was prepared the observations were less complete than now. Ina recent let- ter he says: ‘‘Mr. Willcox’s observations on the deposition of the flocculent mud from lime-bearing water were later than the original statement. The precipitated mud is more or less me- chanically mixed with masses of the corrallia of Polyzoa and bivalve shells driven in shore by the sea, but these creatures do not live in the muddy water, but in the clearer water outside.”

Through the courtesy of Dr. Dall the writer has examined specimens of this rock. It is a light cream-colored mass of crystalline calcite formed around the included fragments of shells. Under the microscope the unaltered structure of the organic fragments is strikingly different from that of the coarse holocrystalline matrix, in which it is apparent that the crystals developed in place. Were this a limestone of some past geo- logic period it would be concluded, on the evidence of the crys- talline texture of some parts of it, that it had been metamor- phosed and that the organic remains now visible had: escaped the process which altered the matrix. But the observed condi- tions of its formation preclude the hypothesis of secondary crys- tallization. Apparently, the crystalline matrix is one primary product from solution, a rock formed in contact with the bottom, the calcareous mud is another, which, being precipitated in the solution, remains an incoherent sediment.

- These results may perhaps be thus explained: The drain- age of the peninsula contains an unusually large amount of lime, in consequence of the abundant supply of carbonic acid and other products of vegetable decay in the sub-tropical climate and

514 THE JOURNAL OF GEOLOGY.

of the calcareous nature of all the rocks of Florida. In the Everglades this water is exposed in broad shallow sheets to ac- tive evaporation, agitation and variations of atmospheric temper- ature and pressure. Concentration of the solution and escape of carbonic acid, including some of that in the bicarbonate in solution, follow, and the neutral carbonate is produced in excess of the amount that can be retained in dissolved form. It is therefore precipitated in two forms—first, from the mass of the water as a flocculent mud ; second, from the lower layers of the water in contact with limestone as crystals forming an integral part of the solid rock..

The alternation of dry and wet seasons is accompanied by concentration and sluggish flow, alternating with dilution and flood currents. Therefore there are seasons of more active pre- cipitation interchanging with those of more vigorous transporta- tion and, perhaps, partial re-solution. In these latter seasons the calcareous mud is swept beyond the shallow basin where it forms, and enters as a suspended sediment into the Gulf circula- tion. What part, if any, is dissolved, what is deposited as mud in the lagoons along the coast, and what is swept into the silt banks of the Atlantic, is not known.

Conditions which produced similar results are described by Gilbert as having existed in Lake Bonneville.t Tufa was depos- ‘ited on the shores of the lake at various stages, but most abund- antly at the Provo stage, during which the water lingered longest at one level. The occurrences are thus described:

“The distribution of tufa along each shore is independent of the subjacentstenrane; “ys, a) No deposit is found in sheltered bays, and on the open coast those points least protected from the fury of the waves seem to have received the most gener- ous coating. These characters indicate, first, that the material did not have a local origin at the shore, but was derived from the normal lake water ; second, that the surf afforded a determining condition of deposition.”

"Monographs of the U.S. G.S. Vol. I, p. 167-168.

CONDITIONS OF SEDIMENTARY DEPOSITION. 515

Dittmar’s experiments in decomposing bicarbonate of lime by agitation indicate the nature of the condition afforded by the surf, and it appears that the neutral carbonate is capable of lithifying at the point of, and immediately upon, separation. Gil- bert also says that: ‘‘Calcareous matter constitutes an important part of the fine sediment of the lake bottom, and this was chiefly * and to explain the forma- tion of the coherent and incoherent deposits of the same mater-

or wholly precipitated from solution,’

ial from the same water he suggests that ‘‘separation was pro- moted by aération of the water. All precipitation being initiated at the surface during storms, coalescence at the shore may have resulted from contact at the instant of separation.”

Mr. Gilbert states (pp. 178-179), that the concentration of the waters of Lake Bonneville at the Provo stage is not definitely known. The lake had an outlet at the northern end of Cache bay, and the principal tributary, Bear river, emptied into this bay near the outlet. Cache bay was connected with the main body of the lake only by a deep but narrow strait, and it is possible that evaporation from the greater expanse of the lake exceeded the inflow of fresh water into it, while the overflow at the outlet was supplied by Bear river. In that case there would have been circulation through the strait between Cache bay and the main body, an upper current from Cache bay and an under- current from the lake. The straits were the scene of peculiarly copious deposition of tufa.

The tufa deposited in Lake Bonneville is of the variety described by Russell as ‘‘lithoid tufa,’’ ‘‘of a compact and stony structure” and he concludes that it was formed when the lake waters were moderately concentrated (pp. 210-222). A limestone of similar structure is now forming on the shores of Florida, where the waves break on the beaches under conditions quite like those which determine the growth of tufa, where the surf dashed against the shores of Lake Bonneville. This rock is deposited in irregular layers, sometimes three or four feet thick, on the quart-

* “Geological History of Lake Lahontan,” p. 190.

516 THE JOURNAL OF GEOLOGY.

zose beach sands. Like the tufa, it is independent of the material upon which it gathers, but the possibility of a local supply of lime exists in the discharge of surface waters below low tide. Under the microscope the material shows a dense, fine-grained groundmass of lime with admixture of fine clay, including grains of quartz and cavities filled with coarsely crystalline calcite.

A case, which is probably more typical of what may occur now, or may have occurred in past ages at the mouths of rivers and in shallow seas, is that of the limestone deposited beyond the delta of the Rhone: This is) referred to by Thoulet anders described by Lyell,’ who says: ‘In the museum at Montpelier is a cannon taken up from the sea near the mouth of the river, imbedded in crystalline calcareous rock. Large masses also are continually taken up of an arenaceous rock, cemented by calcare- ous matter, including multitudes of broken shells of recent species.” Lyell attributes the precipitation of lime to evapora- tion of the Rhone water, which, when it is spread upon the salt water, he compares toa lake. But this one cause is no doubt combined with the chemical and mechanical conditions which have been suggested in the preceding discussion. These con- ditions are favored at the mouth of the Rhone by the salinity of the Mediterranean and the absence of strong currents.

The examination of a few thin sections of limestone of dif- ferent ages, from Cambrian to the present, shows that they have three principal types of structure. There are those which re- semble the Everglades limestone in that they consist of more or less coarsely crystalline calcite, yet include unaltered organic remains. Of these the Trenton limestone and the marbles of corresponding age in Tennessee, which occur interstratified with unaltered calcareous shales, are the most striking examples examined. Cambrian limestones and the Knox dolomite show similar crystalline structure. The second type, the precipitated sediment which forms the muds of the Everglades and which was deposited in Lake Bonneville is represented by specimens

2@pcit.5)p. 27,0. ? Principles of Geology, Vol. I, p. 426.

CONDITIONS OF SEDIMENTARY DEPOSITION. 517

composed of exceedingly fine grained, apparently pulverulent, material; the best of these are from the Knox dolomite and the Solenhofen lithographic stone. The third variety of limestone consists of the thoroughly crystalline marbles, which contain no unaltered material, and which occur in such field relations that they are known to be completely metamorphosed. Extended study is required to determine the nature of deposition of the first and second types. They may have been organic and have suffered moderate alteration only, but there is a reasonable presumption that they did to some extent crystallize in place from sea-water, and were, to a still greater extent, precipitated from the outspread fans of fresh water, radiating from rivers’ mouths, whence they spread as fine silt over the bottom of the sea.

ORGANIC DEPOSITION.

Since deposits of this character are composed chiefly of the calcareous or silicious remains of marine organisms, their formation is conditioned primarily by the circumstances con- trolling marine life, and secondarily by the insolubility of the skeletons under circumstances of wide distribution and gradual sinking.

Favorable conditions —(a) Warm waters.

(6) Clear waters.

(c) Abundant food supply.

(@) Depths less than 1500 fathoms.

(e) Expansion and diffusion of currents in rapidly deepening

Conditions favorable to life.

water.

For a description of the oceanic deposits and of the biological conditions which promote their accumulation, the reader may be referred to the Narrative of the Challenger Expedition, Vol. I, second part, pages 915 to 926. The oozes which are character- ized by the predominance of remains of globigerina, pteropods, diatoms or radiolaria are there described, and it is shown that the nature of the deposit is determined by the conditions of tem- perature, light and motion which favor the generation of multi- tudes of the minute creatures whose living forms swarm at the

518 THE JOURNAL OF GEOLOGY.

surface of the sea, and whose remains only enter into deposits when they have escaped being used by other creatures, or being dissolved in the ocean waters.

Agassiz, writing of the physiology of deep sea life,? points out that in marine, as in terrestrial, life the primary source of food for animals is in plants. The lower types of marine life, it would seem, must derive their sustenance from the water, as land plants get theirs in part from the air, and the silica and lime thus ab- sorbed is taken directly from solution; but the creatures which live on these forms, and the carnivorous animals that feed on them, may get their lime and silica at second hand by digesting and assimilating that which the lower types take from solution. Thus the solids built from solution into organic tests may go through numberless changes before they come to rest on the bottom.

Without pursuing the discussion of biological conditions favor- able or unfavorable to deposition, and without entering upon the question of coral formations, which are rarely of prominent inter- est in stratified deposits, the writer wishes to consider only the circumstances of limestone formation from organic remains, as : ‘that from chemical precipitates has been considered.

In discussing the solubility of shells in sea-water it has been pointed out that the layer of organic matter which accumulates at the sea bottom contains a solvent formed by the evolution of carbonic acid in the process of decay. Through this layer all substances must pass before they can become part of a lithified stratum ; if they are plant tissue or flesh they will become more or less oxidized; if they are calcareous tests they will be more or less completely dissolved, and, if there be any chemically pre- cipitated lime, arriving on the sea bottom it, too, would be dis- solved in this menstruum. The earlier forms of dredge which scooped into the sea bottom, brought up a mass of ooze, formed of fine particles, burying organic forms. The later forms of dredge, arranged to skim the surface of the bottom, bring up

*Op. cit., pp. 312-313.

CONDITIONS OF SEDIMENTARY DEPOSITION. 519

shells and organisms remarkably free from mud. Now it may be conceived that the layer of mud on which the creatures live, die, and with sunken organic remains decay, grades from the fresh surface of recent accumulations downward into a much more completely decayed and dissolved mass, and that this rests upon a surface of limestone. In the upper part of this unconsolidated stratum carbonic acid may most abundantly be evolved; in its lowest part the more concentrated solution of lime may accumulate. Then it is conceivable that lithifaction by crystallization of the carbonate of lime from the more con- centrated solution is constantly proceeding on the limestone surface. If this conception be correct the formation of lime- stone by organic means involves the re-solution and crystalliza- tion of more or less of the calcite in the primary formation, and only those organic forms can remain unchanged which resist the solvent action. If they are delicate, as the trilobites’ branchia from the Trenton limestones, described by Walcott, they give evidence that they were rapidly buried and protected.

It is thought by some that limestones are evidences of organic life at whatever period of sedimentary history they were deposited, but it has here been shown that the source of all lime in the sea is the land, and that, under conditions existing in certain localities, both crystalline limestone and calcareous mud are now forming chemically. It has also been shown that lime converted into organic forms is subtracted from that which would other- wise go to saturate the sea-water. If, then, in any early age of the earth’s history, lime-using organisms were not present to subtract and deposit lime from sea-water, and if the atmospheric agencies worked then as now, the contributions from the land must have continually added to the alkalinity of the sea until chemical precipitation occurred. Such a process must have been limited to seas rather than extended to oceans, because the con- ditions of delivery of lime from the land were then, as now, localized. With the development of marine life and the increased demand for lime for organic use, and with the corresponding deposition of organic limestone, the sea-water must have become

520 THE JOURNAL OF GEOLOGY.

less alkaline and the conditions of chemical precipitation must have been still more restricted. In time it might occur that pe- lagic organisms should demand so much lime for circulation from the water to calcareous alge, to herbivorous and then to carni- vorous forms, and so back into solution, that lime could escape from solution by precipitation only under exceptional conditions. If it be true that the oceanic oozes, the muds of the Caribbean, the mud-flats of Florida, and similar calcareous deposits in differ- ent seas the world over, be wholly organic, then marine life has locked up more lime than the continents could concurrently sup- ply, and the balance is now turned against chemical precipitation. But it has not always been so. BaILEY WILLIS.

TO DIELOR TAL.

In AN article on ‘‘ Englacial Drift,” in the July number of the American Geologist, my friend, Mr. Warren Upham, referring to my article in the first number of this Journal on the Englacial Drift of the Mississippi Basin, takes exception to the impression conveyed respecting his views in the matter of rising glacial cur- rents. The present writer, he says, ‘“‘ several times speaks of the opinions of writers who believe in the considerable volume of the englacial drift, as if they supposed the glacial currents to move gradually upward from the ground to the ice surface. Such a supposition, however, seems to me quite untenable. Instead, in my own writings and those of most, if not all, of these authors, the exposure of the drift on the surface of the ice- sheet near its border, whence much of it was washed away to form the eskers, kames, and valley drift, is ascribed wholly to the superficial melting of the ice sheet, which is called ablation.” I very much regret to have given expression, or to have seemed to have given expression, to the views of these writers in any other terms than they would themselves have chosen, and | cheerfully reproduce the corrective statement which Mr. Upham makes. Until my attention was called to the matter, no other interpretation of the views of these writers than that the supposed rising glacial currents moved on gradually to the surface of the ice occurred to me as possible, as no logical stopping place short of that suggested itself. I do not see any other consistent view now, but that does not affect the obligation to present accurately the views actually held. I hope these writers will credit me with attributing to them what seemed to be the most logical aspect of the hypothesis entertained by them. The sup- posed upward movement is attributed to differential motion

521

522 THE JOURNAL OF GEOLOGY.

between the successive layers of ice, as stated by Mr. Upham on_ pages 38-9 of the article referred to (quoted below). This dif- ferential motion arises from friction at the bottom and extends to the summit. It was natural, therefore, to take it for granted that the supposed rising current extended as far as its postulated cause. It was to be assumed, of course, that the current would rise less rapidly in the upper part if the difference of movement of successive ice layers were less there than below, but it would seem that the rise must be supposed to continue a some vate so long as the differential motion continued, 2z. ¢., until the sur- face was reached. The accession of snow-fall within the zone of accumulation would, to be sure, prevent erratics from reaching the new surface thus continually formed, but it would not pre- vent their reaching the surface in the zone of wastage. It is this latter zone with which our problems of deposition and many of our problems of derivation have chiefly to do. The career of some erratics is wholly confined to it. It goes without saying that ablation brings the surface down and is a factor in every ex- posure within the zone of wastage, but this does not prevent the erratics rising (by hypothesis) until they meet it. This conception of rising currents met by a plane of ablation I supposed without question to be that entertained by Mr. Upham and others. To be sure, in a strict and complete statement under this view the ex- posure of englacial erratics at the surface would be attributed to the joint result of the upward movement and the downward melt- ing, but the liberties of brief and convenient statement would permit it to be referred to in terms of either factor,and I have inter- preted the expressions of these writers on this basis. The cor- rection does not, so far as I can see, in any serious way affect the main question under discussion. If there were rising currents bearing erratics to heights of 500 or 1000 feet above the base of the ice the result in ultimate deposition would be essentially the same as if the currents rose to the surface. If the rising currents are a misinterpretation, it is immaterial whether they be sup- posed to bear erratics to varying heights up to 500 or 1000 feet, whence these erratics move forward parallel with the base of the

EDITORIALS. Rae

ice, or whether they be supposed to continue to rise (more and -more slowly) till they meet the descending plane of ablation.

If currents rise by reason of differential movements to cer- tain heights, but not beyond them, notwithstanding the extension of the differential movements all the way up to the surface, a very distinct statement of this limitation and of the dynamics invol- ved, qualitative and quantitative, would be appropriate. Perhaps such an explanation is intended in the following quotation from Mr. Upham, which I introduce to give ampler expression to his views, though I dissent from his interpretations of the crevasses of the alpine glaciers and of the esker, Bird’s Hill, as well as from his fundamental proposition.

“The conditions of the flowing ice which seem to me to have been efficacious to carry drift upward into it from tracts of plane or only moderately undulating contour, were the more rapid onflow of the ice-sheet in its upper and central parts and even in the portion near the ground but not in contact with it, than upon the bed of the ice-sheet where its movement was much retarded by friction. A very good analogy with the slowly rising cur- rents which I believe to have existed in many portions of the base of the ice-sheet is afforded by the edges of alpine glaciers, where the crevasses extending diagonally up stream into the glacier testify that the movement of its friction-hindered border is from the side of the valley into the ice mass. But the arched surface of the glacier and the great supply of its central current prevent the drift so worn off and borne away from being carried into the axial portion of the ice stream. Similarly the steady accession to the mass of the ice-sheet over any place by onflow from its thicker central part and by the accumulating snowfall forbade the drift of the upwardly moving basal current from being carried far into the ice in comparison with its total thick- ness. The evidence of the esker called Bird’s Hill, near Winni- peg, Manitoba, shows that much englacial drift had there been uplifted from a nearly level country to a height of more than 500

524 DEE VOOLINALV OF NGEOLO GN.

feet in the ice-sheet.t Probably some of the englacial drift there was as high as 1,000 feet or more in the ice, but doubtless a larger part was below than above the altitude of 500 feet; and this was on an area where the ice-sheet had attained probably a thickness of 5,000 or 6,000 feet, its lower fifth or sixth part bear- ing considerable enclosed drift. In like manner the outer por- tions of the ice-sheet, where its thickness was less, had probably at its time of culmination no englacial drift above its lower sixth or fourth or third part. Whatever boulders and other drift became incorporated in the higher portion of the zone reached by the currents flowing upward would be thence carried forward in some regions, as from the Huronian and Laurentian areas north of Lake Huron to the boulder belts in Illinois, Indiana and Ohio, de- scribed by Chamberlin? without intermixture with other englac- ial drift brought into the ice by less powerful currents on all the intervening extent, which in the case mentioned is about five hundred miles.” 3

DCs

‘Geol. and Nat. Hist. Survey of Canada, Annual Report, new series, vol. iv., for | 1888-89, pages 36-42E.

2 Boulder Belts distinguished from Boulder Trains—their Origin and Signifi- cance,’ Bulletin, G. S. A., vol. i, pp. 27-31. ‘‘ The Nature of the Englacial Drift of the Mississippi Basin,” Journal of Geology, vol. i, pp. 47-60.

3The American Geologist, vol. xii, No. 1, July, 1893, pp. 38-39.

REVIEWS.

Correlation Essays, Archean and Algonkian. Bulletin of the U. S. Geological Survey, No. 86. Pp. 549, 12 plates. By CHARLES RICHARD VAN HIsE.

In order of publication, this is the seventh of the correlation essays originally planned by the survey for the International Geological Con- gress of 1891. If the long delay in the appearance of the present essay is in any measure responsible for its excellence, no one will regret that it did not appear ontime. This is not the first piece of good work which Professor Van Hise has done; but he has done nothing which has been of greater utility to the geological world than the present vol- ume will prove to be.

In no department of geology has there been more rapid progress during the last decade than in the department in which Professor Van Hise is a specialist. In no department is it more difficult for those who are not specialists to follow current progress. But so successfully has Professor Van Hise written his essay that the reader will have little difficulty in knowing the present status of pre-Cambrian geology in America. He may know definitely what is definitely known, and he may know definitely what is not known. More than this, he may know definitely the limitations and imperfections of facts and_princi- ples which are but partially worked out, without finding himself con- fused between fact and possible fact, or between established principles and unverified hypotheses. Consciously or unconsciously, the author has given definite shape to the uncertainties and indefinitenesses of his sub- ject, and in so doing has rendered an invaluable service to students.

A mere summary of what has been done in the various areas of pre-Cambrian rocks would be valuable. But the present essay does much more. The author is personally familiar with much of the ground brought under review in the volume, and he has given, always without a suggestion of dogmatism, what every reader is glad to have, his own opinion concerning the interpretations to be placed on the phe-

525

526 THE JOURNAL OF GEOLOGY.

nomena of each of the regions with which he is familiar, together with the reasons therefor. ‘The failure to summarize and interpret the summaries of the literature reviewed has lessened the value of some of the essays of this series.

The plan of the volume is simple. It consists of, first, a digest of all the papers on the pre-Cambrian geology of North America which had appeared at the time the manuscript left the author’s hands; sec- ond, a discussion of the literature ; and, third, a discussion of the gen- eral principles involved in the study of pre- Cambrian rocks, together with a statement of the results which have already been attained in America in the application of these principles.

The digests of the literatures are grouped on a geographical basis. The digest of all publications bearing on the pre-Cambrian geology of the original Laurentian and Huronian areas constitute one chapter, and the digests of the literature of the Lake Superior region, of the great northern areaof Eastern Canada and Newfoundland, of the isolated areas in the Mississippi Valley, of the Cordilleras, and of the Eastern United States, constitute each a separate chapter. Within each area the digests are arranged chronologically. At the close of each chapter, or in some cases at the close of their subdivisions, are summaries of the results thus far attained in the respective areas. In all cases the digests appear to be as nearly absolutely impartial as it is possible for human work tobe. The total number of papers summarized is between 700 and 800. Many of them are papers of considerable length, some of them being elaborate reports. When it is remembered that these papers are not roughly abstracted, but that carefully considered digests are presented, the amount of labor involved in the preparation of the bulletin will be apparent.

It is the final chapter which, together with the maps, will attract most attention. This chapter gives a concise outline history of the development of pre-Cambrian geology in America, and a clear exposition of its present status. Professor Van Hise concludes that it may be accepted as demonstrated that in North America there is an intricate system of granites and gneisses and crystalline schists, which represent the oldest rocks of the continent, and that this system under- lies all known sedimentary rocks and their derivatives, and that if it ever contained sedimentary materials of any sort, all evidence of their existence has been obliterated.

It is to this system of rocks that the name Archean is restricted.

REVIEWS. B27,

The minerals composing these rocks, wherever found, generally agree in showing evidence of extensive dynamic changes, as do also the rela- tions of each sort of rock composing the system, to each other. So closely do the rocks of this system resemble each other in different regions, that Professor Van Hise says that a suite of specimens of Archean rocks from any one of the regions examined by him, if not labeled, “could by no possibility be asserted not to have come from any other.” The system is a unit, both in its positive and negative characters.

To the Archean system thus defined are referred the basement com- plexes of Arizona, of the Wasatch Mountains, of certain ranges of Nevada, of Southwest Montana, of Texas, of the Lake Superior region, of the Hudson Bay region, probably the basement complex of New- foundland, and much of the great area of Northern Canada, known as Laurentian. The basal complexes of the Front range, and of the Quartzite Mountains of Colorado, are referred to the Archean with less confidence. Still other areas not yet definitely classified may prove to be Archean in whole or in part.

With reference to the origin of the Archean, Professor Van Hise inclines to a modification of the theory that the system represents a part of the original crust of the earth. He believes that the Archean rocks were originally igneous, and that they may include not only such remnants of the pre-sedimentary crust as may exist, but those deeper parts of the crust which became lithified in later times, and which have reached the surface by denudations. He suggests that the banded and contorted granite-gneiss which serves as a background for the Archean may represent the rocks having such an origin, while the other parts of the system may be subsequent eruptives, assignable to no other system, and physically a part of the Archean.

The author does not overlook the fact that this suggestion con- cerning the origin of the Archean may make the system include rocks which crystallized below the outermost crust after sedimentation began, and that the date of this lithification may therefore be Algonkian, or even post-Algonkian. Their crystallization at such a date is not looked upon as sufficient reason for excluding them from the Archean group. It is manifestly impracticable to have an Algonkian system below the Archean, representing crystallization or lithification synchronous with the Algonkian sedimentation above.

This being the conception of the Archean, it is evident that strati-

528 THE JOURNAL OF GEOLOGY.

graphical methods are not applicable to it. The only division which seems applicable is a bifold one, based on lithological characters and relations, viz.: 1, the more schistose rocks, generally dark colored, and 2, the more massive rocks (granites and granite-gneisses), generally light colored. To the latter class it is proposed to restrict the name Laurentian. For the former class, the codrdinate name Mareniscan is proposed, the term being derived from the name of a township (Mare- nisco) in Michigan.

The necessity for a group between the Archean and Cambrian has come to be generally recognized during the last decade. But to all except those engaged in the study of pre-Cambrian rocks, the names which have been used to designate this group, or parts of it, have always been confusing, because of their multiplicity, their lack of defi- nition, and the lack of uniformity in their use. ‘This bulletin makes clear the nomenclature which has been adopted by the survey, and sets forth the relation of the various names which have been used to desig- nate parts of the post-Archean (as here used) and pre-Cambrian group. Whether or not those not connected with the survey agree that the nomenclature officially agreed upon is the best possible, it is to be hoped that it may be uniformly adopted in the interest of intelligibil- ity. It has the merit of simplicity and definiteness, and of avoiding disputed questions, so far as this is possible. ;

To the post-Archean pre-Cambrian group is given the name Agnotozoic, or, preferably, since its fossils are becoming known /yo- ferozoic, a term coordinate with Archean, Paleozoic, ete. Since it is impossible to divide this group into systems codrdinate with Cambrian, Silurian, etc., which can be correlated with each other throughout the various areas of Proterozoic rock, the term Algonkian is used for the present as a single system term to cover the whole Proterozoic group. In many areas the group is distinctly divisible into two or three sys- tems comparable with the Cambrian, Silurian, etc. ‘Thus in the orig- inal Huronian area there are probably two unconformable series of rocks, the lower unconformable on the Archean, and the upper uncon- formable below the Cambrian. These may be correlated with some degree of confidence with the Lower and Upper Huronian of the Lake Superior region. But here a third series, the Keweenawan, inter- venes between the Upper Huronian and the Cambrian, and is uncon- formable with both. In the Grand Cafion region again, three series are recognized. But their relation to the three series of the Lake

REVIEWS, 529

Superior region is not known. The same is true of other regions. For this reason, the various terms, Huronian, Keweenawan, Vishnu, Chuar, etc., which have been used to designate definite parts of the group, will still be retained, for in the absence of criteria for the satis- factory correlation of the subdivisions of the group in the various regions where they occur, these parts must continue to bear local names.

The group is so extensive as to be comparable in thickness to the Paleozoic, Mesozoic and Cenozoic combined, and inferentially to represent an equal lapse of time. It contains great systems, sepa- rated by great unconformities. Concerning the two unconformities in the systems in the Lake Superior region, those between the Lower and Upper Huronian and between the latter and the Keweenawan, Pro- fessor Van Hise says: ‘‘Each represents an interval of time suffi- ciently long to raise the land above the sea, to fold the rocks, to carry away thousands of feet of sediments, and to depress the land again below the sea. That is, each represents an amount of time which is perhaps as long as any of the periods of depositions themselves.” In parts of the region the Lower Huronian is known to be unconform- able on the Archean. In other parts the relations are unknown. This statement of the case gives some idea of the thickness of the group, as well as of its complexity and importance.

The delimitation of the Algonkian is theoretically easy, after the definitions of the Archean and Cambrian. It includes all pre-Cam- brian sedimentary rocks, and their igneous equivalents. Although a great unconformity generally separates the two groups, helping to render their distinction clear, it is not always easy of recognition. Locally parts of the Algonkian have undergone such profound meta- morphism at the hands of dynamic forces which affected the Archean as well, that they seem to be structurally one. In such cases it is believed that the apparent conformity is in reality apparent only, the original structural relations being obscured or even obliterated by the structures superinduced by dynamic forces on both series involved. Even where there is a common structure in rocks regarded as Archean and Algonkian, there is sometimes inherent evidence that one part of the rocks concerned is clastic, while similar evidence is wanting in the other.

Not the least instructive part of the volume is the discussion of the principles applicable to Algonkian stratigraphy. It would be useless

530 THE JOURNAL OF GEOLOGY.

to attempt to summarize this discussion, since it is as brief as is con- sistent with adequacy, in its original form. Suffice it to say that while, as applied to Paleozoic rocks, the value of lithological characters and structural relations are well understood, they have a somewhat differ- ent meaning And a greater relative value when applied to the pre- Cambrian formations. At the same time this application is more difficult.

One of the most valuable parts of the volume consists of the twelve maps, covering most of the areas where pre-Cambrian rocks are known or suspected. Nowhere else does Professor Van Hise succeed better in making the indefiniteness of our knowledge definite, than on the maps. On but two of the twelve maps does he rep;esent Archean rocks, viz., on the maps covering the original Huronian area and its sur- roundings, and on the map of the Lake Superior region. Within the United States, Archean rocks are mapped in but three states — Minne- sota, Wisconsin and Michigan. This does not mean that Archean rocks do not exist elsewhere, or that they are not known elsewhere, but that their areas elsewhere, so far as covered by the maps, have not been defined. Some of the areas which we have been accustomed to see represented as Archean on maps made before the Algonkian was differentiated, are now represented as ‘“‘unclassified pre-Cambrian.” Of this the Adirondack region may serve as an example. The maps tell us only that the rocks of this region may be Algonkian, or Archean, or both. In the text Professor Van Hise’s opinion concerning the area may be found. This is to the effect that the Algonkian is cer- tainly represented in the region, and Archean possibly, but that existing knowledge on the point is not sufficiently definite for carto- graphic representation. Other areas which have been mapped as Archean are represented simply as ‘unclassified partly or wholly crystalline rocks.” Of the areas thus represented, the whole of the crystalline schist belt of the Appalachian region may serve as an exam- ple. The author’s map does not even assert that these rocks, or any part of them, are pre-Cambrian. Here again we find the author’s opinion in the text, where it is indicated that parts of this area are pre-Cambrian, while other extensive portions may, or may not be. Such pre-Cambrian areas as are known are not defined, and therefore cannot be represented on the maps.

Algonkian rocks find definite representation in more regions than the Archean. They appear upon the maps in Arizona, New Mexico,

REVIEWS. 531

Utah, South Dakota, Minnesota, Iowa, Wisconsin, and Michigan. They are known, but their areas not defined, in various other localities.

The summaries of the several chapters, or their sections, the final chapter, and the maps, should serve as a text-book on -pre-Cambrian - geology for all advanced students in our universities. Not only will the best information available be thus put into their hands, but the whole treatment of the subject is such as give an intelligent insight into the methods of geology, and into the methods of science as well.

ROLLIN D. SALiIspuRY.

ANALYTICAL ABSTRACTS OF CURRENT LITERATURE.

SUMMARY OF CURRENT PRE-CAMBRIAN NORTH AMERICAN LITERATURE.*

Cross! describes a series of hornblendic, micaceous and chloritic schists, on the eastern side of the Arkansas river, near Salida, Col. In places these grade into massive rocks. They are cut by granitic and pegmatitic veins, as well as by dykes of porphyry. A detailed microscopical study leads to the conclusion that the rocks are a metamorphosed volcanic series. The whole constitutes a part of a single anticline. The schists are unconformably below the Silurian, and as the known Cambrian in Colorado is a thin series of quartzites and shales conformable with the Silurian, the Salida schists are considered as pre-Cambrian. The relations of the schists to the Archean complex are not exposed, but they are probably a continuation of the hornblende-schists of Marshall Pass. Greenish schists are found at Tin Cup Pass, and near the town of Tin Cup is a highly crystalline marble inter- bedded with the green schists, and fine grained gneissoid rocks, showing that metamorphosed sedimentary rocks do exist among the crystalline schists of the Sawatch Range. Taking into account all the facts it is thought that the schists and massive rocks of the Salida section probably ‘represent a great series of surface lavas, erupted in Algonkian time.

Smyth (C. H.)? describes the rocks near Gouverneur, New York, as con- sisting of gneiss, granite, limestone, and sandstone, with small amounts of associated schists. The gneiss is the oldest rock of the region, underlying the other formations. It sometimes grades into a true granite, the passage being gradual. The two are regarded as different phases of the same rock, either the granite being an unchanged remnant of a Plutonic mass from which the gneiss is derived, or the result of fusion of the gneiss. Evidence of unconformity between the beds of the limestone and the foliation of the

* Continued from page 314.

™Series of Peculiar Schists near Salida, Col., by Whitman Cross. In Proceedings of Col. Scientific Soc., Jan., 1893, pp. I-10.

2A Geological Reconnaissance in the vicinity of Gouverneur, by C. H. Smyth, Jr.

In Transactions N. Y. Academy of Sciences, vol. xii., April, 1893, pp. 97-108. 532

ANALVTICAL ABSTRACTS. 533

gneiss was found in two localities, and was indicated in several others; there is no evidence of irruptive contacts between the gneiss and limestone; the gneiss shows no evidence of sedimentary origin; therefore, the simplest hypothesis, but requiring more proof, is that the gneiss is ah eroded meta- morphosed plutonic rock, upon which the limestone was deposited. The marble is coarsely crystalline, and in age is next to the gneiss. Near the base of the limestone, and interbedded with it, are peculiar schistose rocks, which, while completely crystalline and resembling igneous rocks in composi- tion, are indicated by their field relations to be of sedimentary origin. Near Gouverneur an outcrop of limestone contains abundant fragments of black schist, scattered through the limestone in a most irregular manner, and making up, perhaps, one-third of the rock. This and other outcrops show that the schist fragments are remains of once continuous schist layers, which have been completely shattered in the course of metamorphism, since between the continuous belts of schist and the Gouverneur locality there is every possible gradation. While the schists show the effects of foldings, contor- tions, stretchings and shattering, the limestone shows no traces of it, it appearing to have been a plastic mass in which the schists moved with con- siderable freedom. The conspicuous result of metamorphism in the lime- stone is crystallization. In the limestones are also pegmatitic veins, which have been much shattered by the dynamic action, reducing them to small lumps of quartz and feldspar, scattered through the limestone. So far as observed the pegmatite yields to strain only by fracturing, not showing pre- liminary contortions, so general in the schistose layers.

In the southern part of the area examined is a granite, not grading into gneiss, and which breaks through the limestone, causing great disturbance in ‘strike and dip, enclosing masses of the rock many feet in diameter, and metamorphosing this rock to some extent. The sandstone at Gouverneur was found in direct contact with the limestone. Here it appears that the lime- stone surface has been subjected to erosion before the sandstone was deposited upon it. In confirmation of this are seen narrow irregular cracks extending several feet into the limestone, which have been filled with sandstone. The limestone was evidently completely lithified when the sandstone was deposited and sifted into it, and this implies discordance. This unconformity proves that the limestone is older than the upper Cambrian, the data being wanting for any more definite determination of its age. The metamorphism of the rocks of the limestone- bearing series occurred before upper Cambrian time, but the sandstone is metamorphosed, and this metamorphism must therefore belong to post-Potsdam time.

Comments.—The inquiry rises whether the second metamorphism spoken of, that of the sandstone, is produced merely by interstitial cementation, or is dynamic metamorphism. If the first is found to be the explanation, so far as

534 THE JOURNAL OF GEOLOGY.

the paper gives any evidence, all of the igneous activity and dynamic metamorphism are pre-Potsdam.

* Wadsworth gives a sketch of the iron, gold and copper districts of Michi- gan. The Azoic or Archean rocks are divided from the base upward into Cas- cade, Republic and Holyoke formations. These divisions are placed in order as equivalent to the fundamental complex, lower Marquette series and upper Marquette series of Van Hise. They are unconformable and represent three different geological ages. The Keweenawan is divided into two divisions, both of which are placed in the Cambrian ; the Lower Keweenawan, 25,000 ft. of interbedded conglomerates and lava flows, with some intrusives; Upper Keweenawan, 12,000 feet of sandstones and shales, not separable from the Potsdam or Eastern sandstone.

The Azoic or Archean system consists of rocks formed (1) by mechanical means, (2) by eruptive agencies, (3) by chemical action.

The Cascade, or oldest formation of sedimentary and eruptive rocks, con- sists, commencing with the oldest, of gneissoid granites or gneiss, basic eruptives and schists, jaspilites and associated iron ores, and granites, although the above arrangement may be considered no more than a hypothesis, and it is probable that the jaspilites and iron ores will be found to belong to the Republic formation. It is also probable that the Cascade formation itself will prove to be composed of two or more distinct geological formations, as shown by the fact that the chief rock of the Huron