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Chapter XXI — THE PENNSYLVANIAN (UPPER CARBONIFEROUS) PERIOD | Index | Chapter XXIII — THE TRIASSIC PERIOD |
[p. 660]
At the close of the Pennsylvanian period much of the central and eastern parts of the United States became dry land, and the sea-covered area in the west was greatly restricted. The area of land was perhaps as large as at any time since the beginning of the Paleozoic. The waters which still lay upon the continent were partly in the form of lakes and inland seas, and partly connected with the open ocean, but the areas which the sea overspread at the beginning of the period were largely abandoned before its close. These changes in geography reflected themselves both in the distribution of the Permian formations and in their character.
East of the Mississippi. Fresh-water sedimentation continued much as before during the earlier part of the period in some parts of the east (parts of Pennsylvania, West Virginia, Maryland. and Ohio), and with the other formations there is some coal. The Appalachian belt farther south seems not to have been the Elite of deposition. In Nova Scotia, New Brunswick, and Prince Edward Island, non-marine Permian strata rest on Carboniferous beds in such a way as to show that sedimentation was not seriously interrupted, and the two systems are separated on the basis of fossils, as in the eastern part of the United States.
West of the Mississippi. The system is better developed west of the Mississippi than east of it. It is best known in Texas, Kansas, and Nebraska, where it is partly marine and partly non-marine.
In Kansas and Nebraska the lower part of the Permian is marine, and though the connection has not been traced, the Permian of these states is probably continued northwestward to Wyoming and South Dakota, for marine Permian beds in the Laramie Mountains and the Black Hills have fossils very similar to those of Kansas.[1] [p. 661] The marine Permian of Kansas is overlain by beds containing gypsum and salt, and possessing other features which show that the open sea of the region was succeeded by dissevered remnants, or by salt lakes whose supply of fresh water was exceeded by evaporation. With the saline and gypsiferous deposits and above them are the “Red Beds” formerly referred to the Trias; but most of them are now classed as Permian, as are most (though not all) of the Red Beds east of the mountains. Some of the Red Beds in western Texas, New Mexico, and elsewhere are perhaps later than Permian, and some in Oklahoma, Kansas, Colorado,[2] and perhaps elsewhere, are probably older.
In the Staked Plains of Texas the system has its greatest development. The oldest part (Wichita formation) indicates that the critical attitude which characterized the surface farther east during the Pennsylvanian period, now affected Texas, for the beds are partly of marine and partly of fresh-water origin. These beds are succeeded by a formation of limestone (the Clear Fork) of marine origin, which overlaps the Lower Permian. The Upper Permian (Double Mountain formation) which follows, indicates a reversal of relations, for much of Texas was again cut off from the ocean, and converted into an inland sea, or into inland seas, in which the phases of deposition common to such bodies of water took place. Occasional beds of limestone with marine fossils point to occasional incursions of the sea, while deposits of salt and gypsum point with equal clearness to its absence, or to restricted connections, and to aridity of climate.[3]
Throughout much of the area west of the Rocky Mountains, the Permian has not generally been differentiated. There is often conformity between the Carboniferous below and the beds classed as Trias above, suggesting the presence of unseparated Permian between. In northern Arizona, however, and in southwestern [p. 662] Colorado and perhaps at other points, there is an unconformity at the top of the Permian.[4] The presence of Permian in New Mexico,[5] northern Arizona[6] and the Wasatch Mountains, suggests that the Permian sea perhaps extended west from Texas as far as the Great Basin for a part of the period; but if so, the continuity of the beds has since been interrupted by erosion. A very considerable thickness of marine Permian (3,800 feet) is reported from Utah.[7] The Permian deposits of the far west, as well as some of those in the longitude of Texas and Kansas are often red. This color so often characterizes formations known to have been made in inclosed basins, that the connection can hardly be fortuitous.
Thickness. In the Appalachian region, the Lower Permian beds, sandstone and shale with thin seams of coal, have a thickness of about 1,000 feet. The Upper Permian is wanting. In Kansas the thickness is twice as great, while in Texas it reaches 7,000 feet.
Correlation. In the region east of the Mississippi, the Permian is so closely associated with the Coal Measures that the two were formerly classed together. Were this region only considered, this classification would appear to be satisfactory. In the western part of the continent, on the other hand, the separation of the Permian from the Carboniferous will probably prove to be more distinct, when details have been worked out, and its relation with the Trias close. The Permian period is best looked upon as a transition period from the Carboniferous to the Trias, and so from the Paleozoic to the Mesozoic. Its close relationship to the underlying system in some places, and to the overlying system in others, is therefore to be expected.
Europe
In Europe, as in America, the Carboniferous period was brought to a close by very considerable changes, for much of the area which [p. 663] had been receiving deposits during that period was exposed to erosion at its close. Subsequently, through further changes, much of the same surface was again brought into a position for renewed deposition, partly from fresh and partly from salt waters. The Permian system is on the whole much more distinct from the Carboniferous, than the Permian of the eastern part of North America is from the Pennsylvanian.
The Permian of Europe has two somewhat different phases known as the Dyas and the Permian respectively. The former name had its origin in the twofold division of the system, characteristic of western Europe, and the latter came from a province of Russia where the formation is well developed. Except in Russia, its extent at the surface is not great.
The Dyas Phase
Lower Permian. Where the Dyas phase of the formation is developed, as generally in western and central Europe, the Lower Permian (Rothliegende) consists of a series of fragmental beds made up of shale, sandstone, conglomerate and breccia, and a large amount of igneous rock, in the form of lava-sheets, dikes, and pyroclastic material.
The character of the formations and of their fossils is such as to show that much of the sediment was accumulated in inland seas, and in salt and fresh lakes. Gypsum, salt, and a meager fauna of dwarfed and stunted species, some of which are marine, are among the distinctive marks of the series. But the sea sometimes had access to the inland areas of sedimentation, as some of the fossils show. The shallow-water or subaerial origin of much of the Permian is shown by the sun-cracks, rain-pittings, ripple-marks, tracks of terrestrial and amphibious animals, etc. In keeping with the conditions of its origin the Rothliegende in various parts of Europe contains coal.
Especial interest attaches to the conglomerates and breccia of the system because of their likeness to glacial drift.[8] This likeness [p. 664] is found not only in the presence of large bowlders, but in their character and in the matrix in which they are set. Furthermore, the stones have now and then been observed (Midlands and west England) to carry marks which have been thought to be glacial striae. This origin of the marks, however, has been called into question. The conglomerate is wide-spread, and in some < contains bowlders which have been transported considerable distances.
The Upper Permian. The Upper Permian of western an< central Europe (the Zechstein of Germany) is unlike the Lower ii several important respects. It contains much more limestone am dolomite, but neither coal, igneous rock, nor, except at its very base, [p. 665] conglomerate. From the stunted aspect of the fossils, and from the association of the dolomite with gypsum, salt, etc., it has been thought that the limestone and dolomite may be largely chemical preciptates. Some parts of the Permian are, however, of marine origin.
The Upper Permian of central and western Europe contains the thickest salt-beds known in any part of the earth. Near Berlin, one of these salt-beds has been penetrated about 4,000 feet, and its bottom has not been reached. In addition to common salt, salts of potash and magnesium ware locally (Strassfurt) deposited in such quantity as to be commercially valuable. The world’s supply of potassium salts, with the exception of saltpetre, comes from these beds. Like rock salt, these salts probably represent precipitates from waters of enclosed basins under special conditions.
The Permian Phase
The typical Permian phase of the system underlies the larger part of Russia (in Europe), and appears at the surface over a large area in the southeastern part of that country. It is generally conformable on the underlying Carboniferous, and is partly marine and partly non-marine. It contains salt, gypsum, etc., and also, in some horizons, marine fossils.
In southern Europe the Permian is of aarine origin, for the most part, and is generally conformable on the Carboniferous.
Summary. Great areas in both Europe and North America seem to have maintained a halting attitude near the critical level during much of the period, while in both continents there were considerable areas of dry land. In both continents there are beds which accumulated in fresh water, in salt lakes or inland seas, and on the floor of the epicontinental seas. But the differences between the continents are not less instructive than the likenesses. In Europe the Permian period was distinguished by great igneous activity, while in America activity of this sort is unknown. The Permian of Europe seems to be more closely allied, stratigraphically, with the Trias than with the Carboniferous, and while the same is perhaps true of the western part of North America, the opposite is true for the eastern part.
[p. 666]
Other Continents[9]
In other parts of the world the Permian is widely developed. In countries about the Indian Ocean, including South Asia, Australia, and South Africa, there is usually a less distinct break between the Carboniferous and Triassic systems than in Europe, and locally at least, the Permian seems to bridge the interval.
Permian glacial formations. The most remarkable fact about the Permian system outside of North America and Europe is that it includes formations of glacial origin, and that these occur down to and even slightly within the tropics. Such formations are found in Australia, Asia, Africa, and South America, all the continents which have large areas in low latitudes.
In Australia the Permian formations of glacial drift (locally nine or ten of them) are interbedded with marine formations, the aggregate thickness of the whole being not less than 2,000 feet. Not less than 30 or 40 beds of coal are included in the system. The recurrence of the bowlder beds points to the repeated recurrence of glacial conditions, and the great thickness both of clastic beds and of the included coal point to the great duration of the period through which the several glacial epochs were distributed.
Counting Tasmania, where glacial deposits are also found, the glaciation of Australia had a known range of nearly 22° in latitude, and about 35° in longitude, though it is perhaps not probable that all the area within these limits was glaciated. The glacial phenomena are known chiefly at low levels, descending in some places nearly to the sea. Not only is the altitude of the region low now, but it was probably low during glaciation, as shown by the relation of the glacial deposits to the marine beds. Whatever the difficult iefl in the way of its explanation, the fact of a long period during which glacial conditions recurred many times, must be accepted.
The marine beds associated with the glacial deposits seem to match approximately those of the Carboniferous period elsewhere, but the plants of the associated coal have the general facies of the Triassic flora. Permian fish remains are found ahove all the bowlder [p. 667] beds, suggesting that the glacial conditions were over before the end of the Permian. The testimony of the plant fossils is therefore that the period of glaciation was late Permian or early Triassic; that of the marine fossils that it was late Carboniferous or early Permian.
In India, too, there are glacial formations (Talchir conglomerate) of about the same age, with fossil plants like those of Australia, in associated beds. The bed on which the glacial formations rest is in some places striated and roche-moutonneed, as beneath modern glacial deposits. These formations are in some respects even more remarkable than those of Australia, for they reach below latitude 18°, and are, therefore, several degrees within the Tropic of Cancer; not only this, but they occur at low levels, descending in places nearly to the level of the sea. Similar formations, believed to be of the same age, appear in the Salt Range of India (Lat. 32°), in the central Himalayas, in Cashmere, and Afghanistan. In the Salt Range, a marine Permian formation overlies the glacial series.
In South Africa many of the bowlders of the glacial beds (Dwyka conglomerate) are striated, and the bed on which the glacial conglomerate rests shows indisputable marks of ice action in many places. The glacial beds are believed to have extended as far north as 26° 40’ in the Transvaal. Glacial conglomerates are also present in South America in the southern part of Brazil. The associated coal formations carry the same flora (glossopteris flora) as in the other continents.
The known Permo-Carboniferous glaciation of Australia, India, Africa, and South America, is found in two zones, the one north and the other south of the equator. In neither zone have the limits of glaciation been accurately determined; but in the former it is known to have extended from latitude 18° to about 35°, and probably still farther north, while in the latter it is known to have extended from latitude 21° to 35°. In an equatorial zone about 40° in width, glaciation has not been discovered. The glaciation can hardly be said to be limited in longitude. Glacial conditions must, therefore, have prevailed about the borders of an area many times as large as that covered by ice in the northern hemisphere during the Pleistocene glacial period.
[p. 668]
The marked likeness of the floras associated with the glacial deposits throughout these four continents is believed to be evidence that there was land connection between them at the time of the glaciation. The age of these glacial beds is not absolutely established; for the Carboniferous and Permian are not clearly differentiated in the regions where the glacial formations occur. Perhaps the best judgment that can be formed now is that the Paleozoic glaciation culminated in the early part of the Permian period.
The close of the Paleozoic era was marked by much more considerable geographic changes than the close of any period since the Proterozoic, though they may be said to have been in progress during the Permian period, rather than to have occurred at its close.
The more important geographic changes in North America which were far advanced by the close of the Paleozoic, were (1) the development of the Appalachian mountain system at the western border of Appalachia; (2) the deformation of the surface of Appalachia; (3) the development of the Ouachita Mountains; (4) the final conversion of the larger part of the area between the Great Plains and Appalachia from an area of deposition to an area of erosion; and (5) the restriction at the west of the area of sedimentation in the western interior.
Such extensive geographic changes, involving the conversion of extensive areas from sea bottom into land, must have caused profound changes in the circulation of ocean waters, in the climate of many localities, and in the distribution of terrestrial and marine life.
The life of the Permian must be interpreted in connection with the extraordinary physical conditions which formed its environment. Between them and the life there must have been reactions and adaptations of the utmost significance, if we could surely read them. At no period save our own were the phenomena so pronounced, and hence, with little doubt, so rich in possible instruction as to the adaptation of life to extreme conditions.
[p. 669]
The salient facts in connection with the physical conditions of the Permian were glaciation and aridity. In view of these facts, certain questions relative to the life arise: (1) Did it possess such powers of adaptation as to meet its extraordinary environment by adjusting itself to it? (2) Was it destroyed co-extensively with the changes in environment? (3) Did it elude adverse conditions by migrating from one area to another as the adverse conditions shifted (hypothetic ally)? (4) Did its composite experience embrace all these alternatives, and if so, what measure of each?
The impoverishment of life. In the early days of geology it was commonly held that a complete destruction of all things living on the face of the earth attended the close of the Paleozoic era, and that a re-creation followed; for in the state of knowledge of that time, no Paleozoic species was known to have lived on into the following era. Had it been known that glaciation pressed upon the borders of the tropics from either side, and that aridity prevailed over large areas elsewhere, it would have added great strength to the conviction of a universal catastrophe to life. It is now known that some species bridged the interval, and it is believed that others underwent modifications which enabled them to live. The progress of investigation is bringing more and more evidence of this kind to light, and reducing the disastrous implications of the record. Not only this, but the compensating effects of the strenuous conditions in calling into play the powers of adaptation and resistance of the organisms are coming to be recognized. Notwithstanding all this, it appears that the life of the period was greatly impoverished. A census made not many years ago gave the known animal species of the Carboniferous period as 10,000, while those of the Permian period were only 300. A census to-day would probably increase the Permian ratio, but the contrast would still be great.
I. The Plant Life
The change of the vegetation from the Carboniferous to the Permian was rather marked in America, though not, at the outset, radical. Of the 107 species of plants recorded from the lowest beds referred to the Permian in West Virginia and Pennsylvania, 22 are found in the Coal Measures below, and 28 in the Permian [p. 670] of Europe. This and other similar facts show that a rather profound change was in progress, but that it was not abrupt .
Only a small part of the total floral changes of the Permian appears in the American record, as now known; but the nature of the early change is distinctly indicated. The Lepidodendrons disappeared, the Sigillaria became rare, and the Calamites were greatly reduced in importance. The general features of the fern group remained much as in the preceding period, but most of the species and many of the genera were new. The Cordaites continued, and initial forms of ginkgos appeared, giving to the flora a Mesozoic cast.
In Europe the residual Carboniferous species declined as the period advanced, and the general aspect of the flora was that of poverty. Two new types of much interest came in and became characteristic. One of them (Walchia, Fig. 458), probably a conifer, resembled an Araucarian conifer in its foliage, though its seed organs were apparently different. The second type (Voltzia , h, Fig. 459) is a supposed forerunner of the group to which the giant sequoia and the bald cypress belong. Both these types had a pauperise aspect.
The Glossopteris flora. The most remarkable vegetal event of the period was the evolution of the Glossopteris (tongue-fern) flora in the southern hemisphere, and its migration into the northern. Many features of this flora give support to the view that it was evolved to meet the adversities of climate in and about the glaciated [p. 671] regions. Developed thus amid adverse surroundings, if not under adverse conditions, the flora not only took on a resistant aspect, in simple outlines and compact forms, but soon gave evidence of its vitality by spreading northward into east Africa and Asia, and then Europe. It reached northern Russia in the later part of the Permian period, and was there associated with forms typical of the European Permian flora. It is also found in Brazil and Argentina.[10]
[p. 672]
Its vitality is further shown in that its descendants became a dominant feature in the Mesozoic floras that followed.
II. The Land Animals
The Amphibians. The amphibians which reached their climax in the later portion of the Pennsylvanian period, were still abundant in the early Permian, but before the end of the period, they were overshadowed by the rise of the reptiles, which were without doubt their descendants. The Permian amphibians were much like those of the preceding period, but showed some advance in organization, and some reptilian tendencies. The amphibians of this period included the earliest known type of modern amphibians (Lysorophus) , so far as now known, peculiar to the North American Permian.
Primitive reptiles.[11] While the reptiles probably began to differentiate from amphibians earlier, the oldest certain relics of reptiles go back but little beyond the beginning of the Permian. Before the close of the period the group was large and complex. At least three distinct phyla are known to have existed. One of them (Pelycosaurus), pronouncedly reptilian in character, had branched off before the close of the Pennsylvanian period. Another (Cotylosaurus) had a singular development of dermal carapace, strongly suggestive of turtles, and unknown outside of North America. The third phylum included small, crawling reptiles, with large heads, short tails, oowerful and short limbs, whose nearest and yet rather remote relatives (pareiasaurus) are found in South Africa. The American forms were probably derived from the same stock as their African allies, but the types in the two continents had, as a result of long isolation, become somewhat distinct. The origin of the branch of the reptiles which gave rise to the mammals, was probably in Africa.
Some of the reptiles possessed peculiar interest because of the mammalian aspect of their skulls, their teeth, and some other parts of their skeletons (Fig. 460). These were especially abundant [p. 673] in South Africa (Karoo beds[12]), but they have been found also in Europe.
The rapid and diverse deployment of the early reptiles in a period of general life-impoverishment is not a little remarkable, but as the reptiles were air-breathers, the key to their rise may lie in a more oxygenated atmosphere, a point to which we shall return.
The Permian of Texas and Oklahoma affords the richest Permian vertebrate fauna now known. In contrast with the vertebrate fauna of the Pennsylvanian system, the Permian vertebrate fauna of North America is so unlike the corresponding faunas of other continents as to imply the absence of migration of land animals between North America and other continents. This isolation [p. 674] seems to have lasted from the later part of the preceding period, until well into the Triassic.
The Permian record of the arthropods and of the terrestrial mollusks is very poor, and probably represents an impoverished state of these classes, but local exceptions will doubtless yet be discovered.
III. The Fresh-water Life
The amphibians and some of the reptiles constituted, in a sense, a portion of the fresh-water as well as the land life. Besides these, fishes were abundant, locally at least On the whole they had a rather modern aspect.
There were fresh-water mollusks, some of which resembled unios. The arthropods, so far as known, show but little change from those of the preceding period.
IV. The Marine Fauna
The withdrawal of the epicontinental seas from considerable portions of the continents reduced the territory availal >le for shallowwater sea life, and of such life there was a great reduction. It is to be noted that this reduction came at a time when conditions were unfavorable for land life (p. 660). In North America the restricted marine fauna lived in tracts just previously occupied by the expanded [p. 675] seas of the Pennsylvanian period, and the Permian faunas were lineal descendants of predecessors occupying the same area. At first, nearly all the species were the same as those of the preceding period, and hence difficulty has always been experienced in drawing a dividing line between them. The known species of the Permian of the Great Plains are only about 70, and of these about one-half are pelecypods. Among the brachiopods, the product ids were the most characteristic forms. This system records the l.i-t flourishing stage of the productids, the orthids, the spirifers, and the arthyrids, types which had a long history.
[p. 676]
The increasing complexity of the sutures of the coiled cephalopods has been noted in previous chapters. In the later part of the Permian, the complexity became still greater (Fig. 463), foreshadowing the intricacy of the Mesozoic ammonites; but older types (goniatites and nautiloids) mingled with the new. Interestingly enough, there was also the ancient straight form (Orthoceras, f, Fig. 463), in the last stage of its prolonged career. The contrast (compare a,Fig. 463) between the disappearing straight type, in its depauperate form, and the robust youthful ammonites, about to become a ruling dynasty, is marked.
The retreatal tracts of the marine life. As in previous transition epochs when epicontinental waters were largely withdrawn, the marine faunas found special refuge in certain embayments or border tracts which, in connection with the coastal belts, permitted them to re-form themselves, regenerate their species, and prepare for a succeeding invasion of the continental areas. On the American continent, the St. Lawrence embayment had done repeated duty in this line; but there is no specific evidence that it participated notably in the Permo-Triassic transition. The border of the Gulf of Mexico, the Mediterranean tract, notably in the region of Sicily and southeast Europe, and the Ganges-Indus tract of southern Asia. seem to have been special areas of refuge and regeneration. Here and on the continental borders generally, the shallow-water marine faunas passed from the Paleozoic to the Mesozoic phases. The restriction, compared with the expansional stage of the Mississippian period, was great; but the faunas emerged with new species born in adversity, ready for conquest when the rB-advancing seas should give them an expanding realm. Unfortunately, the sediments in which this transition of faunas should be recorded, are, for the most part, buried and inaccessible.
[p. 677]
Between the marvelous deployment of glaciation, a strangely dispersed deposition of salt and gypsum, an extraordinary development of red beds, a decided change in terrestrial vegetation, a great depletion of marine life, a remarkable shifting of geographic outlines, and a pronounced stage of crustal folding, the events of the Permian period constitute a climacteric combination. Each of these phenomena brings its own unsolved questions, while their combination presents a series of problems of great difficulty. More than any other period since the Cambrian, the Permian is the period of problems. These marked phenomena were probably related to one another, and their explanation is quite sure to be found in a common group of co-operative factors. While it is too much to hope for a full explanation at once, there is no occasion to blink the facts or evade the issues they raise.
It is to be noted that none of the factors in this combination were wholly new to geological history. There had been glaciations almost as strange in early Cambrian or pre-Cambrian times (Norway, China, Australia), and perhaps in the Devonian (South Africa); there had been signs of unusual aridity in the salt and gypsum deposits of Silurian and other early times; there had been red beds in the Devonian period, and in the Keweenawan; there had been marked restrictions of life, as at the close of the Ordovician; there had been extensive geographic changes in earlier Paleozoic periods; and there had been foldings of surpassing intensity in Archean and Proterozoic times. The peculiarity of the Permian was the complexity of the combination, and the extent of the glaciation and aridity.
The chronological setting of the combination lends some advantages to its study. It lies in the midst of geologic history, with periods of climatic uniformity and polar geniality both before and after it. No appeal can be taken to a supposed final cooling of the earth, or to any senile condition. It was an episode in the midst of a long history, and its problems must be faced with this setting in mind.[13]
Chapter XXI — THE PENNSYLVANIAN (UPPER CARBONIFEROUS) PERIOD | Index | Chapter XXIII — THE TRIASSIC PERIOD |
For an account of the Permian of this region, see Prosser, Geol. Surv. of Kansas, Vol. II, 1897, pp. 55-97; Knight, Jour. Geol., Vol. VII, pp. 35-74, and Bull. 45, Wyo. Experiment Station, and Jour. Geol., Vol. X, pp. 413^22; Barbour, Nebraska Geol. Surv., Vol. I, p. 129, and Darton, 19th Ann. Rept., U. S. Geol. Surv., Pt. IV. ↩︎
Cross, Jour. Geol., Vol. XV, p. 633. ↩︎
For Permian of Texas, see Cummins, Geol. Surv. of Texas, 2d Ann. Rept., pp. 394-424. Ibid., 4th Ann. Rept., 212-232. ↩︎
Cross, and Cross and Howe, Bull. G. S. A., XVI, 447; Silverton Folio; U. S. Geol. Surv., and Jour, of Geol., Vol. XV, p. 634. ↩︎
Herrick, Jour. Geol., Vol. VIII, pp. 112-125; and Am. ( SeoL, Vol. XX X I, p. 76. ↩︎
Walcott, Am. Jour. Sci., Vol. XX, 1880, p. 221. ↩︎
Boutwell, Jour. Geol. XV, p. 434. ↩︎
Ramsay, Q. J. G. S., 1855; and Geikie, op. cit., p. 1064. There seems to be some question as to whether these formations should be referred to the Carboniferous or the Permian. ↩︎
Q. J. G. S… Vol. I. II. 1806. p. 289; Am.Geol., Vol. XIII. L889,p.299; and Scot. Geog. Mag., Vol. XVII, 1901, p. 57. ↩︎
In Brazil, this flora, closely allied to that of the Talchir-Karharbari series of India, is found in the coal-bearing formations. I. C. White and David White, Commissao da Costudos das Minas de Carvao de Pedra do Brazil, 1908. ↩︎
Williston, The Faunal Relations of the Early Vertebrates, Jour. Geol., Vol. XVII, 1909. ↩︎
The Karoo beds, so wonderfully rich in significant vertebrate remains, are regarded as Permian in part, and Triassic in part. Broom, Geol. of Cape Colony, 1905, pp. 228-249. ↩︎
It is impracticable to discuss these problems here, but they are considered in the authors’ larger work, Vol. II, pp. 655-677. ↩︎