The Student's Elements of Geology Part 46

The footprints of Labyrinthodon observed in the clays of this formation at Hildburghausen, in Saxony, have already been mentioned. Some idea of the variety and importance of the terrestrial vertebrate fauna of the three members of the Trias in Northern Germany may be derived from the fact that in the great monograph by the late Hermann von Meyer on the reptiles of the Trias, the remains of no less than eighty distinct species are described and figured.

TRIAS OF THE UNITED STATES.

NEW RED SANDSTONE OF THE VALLEY OF THE CONNECTICUT RIVER.

(FIGURE 408. Footprints of a bird, Turner's Falls, Valley of the Connecticut.)

In a depression of the granitic or hypogene rocks in the States of Ma.s.sachusetts and Connecticut strata of red sandstone, shale, and conglomerate are found, occupying an area more than 150 miles in length from north to south, and about five to ten miles in breadth, the beds dipping to the eastward at angles varying from 5 to 50 degrees. The extreme inclination of 50 degrees is rare, and only observed in the neighbourhood of ma.s.ses of trap which have been intruded into the red sandstone while it was forming, or before the newer parts of the deposit had been completed. Having examined this series of rocks in many places, I feel satisfied that they were formed in shallow water, and for the most part near the sh.o.r.e, and that some of the beds were from time to time raised above the level of the water, and laid dry, while a newer series, composed of similar sediment, was forming.

According to Professor Hitchc.o.c.k, the footprints of no less than thirty-two species of bipeds, and twelve of quadrupeds, have been already detected in these rocks. Thirty of these are believed to be those of birds, four of lizards, two of chelonians, and six of batrachians. The tracks have been found in more than twenty places, scattered through an extent of nearly 80 miles from north to south, and they are repeated through a succession of beds attaining at some points a thickness of more than 1000 feet. (Hitchc.o.c.k Mem. of the American Academy New Series volume 3 page 129 1848.)

The bipedal impressions are, for the most part, trifid, and show the same number of joints as exist in the feet of living tridactylous birds. Now, such birds have three phalangeal bones for the inner toe, four for the middle, and five for the outer one (see Figure 408); but the impression of the terminal joint is that of the nail only. The fossil footprints exhibit regularly, where the joints are seen, the same number; and we see in each continuous line of tracks the three- jointed and five-jointed toes placed alternately outward, first on the one side, and then on the other. In some specimens, besides impressions of the three toes in front, the rudiment is seen of the fourth toe behind. It is not often that the matrix has been fine enough to retain impressions of the integument or skin of the foot; but in one fine specimen found at Turner's Falls, on the Connecticut, by Dr. Deane, these markings are well preserved, and have been recognised by Professor Owen as resembling the skin of the ostrich, and not that of reptiles.

The casts of the footprints show that some of the fossil bipeds of the red sandstone of Connecticut had feet four times as large as the living ostrich, but scarcely, perhaps, larger than the Dinornis of New Zealand, a lost genus of feathered giants related to the Apteryx, of which there were many species which have left their bones and almost entire skeletons in the superficial alluvium of that island. By referring to what was said of the Iguanodon of the Wealden, the reader will perceive that the Dinosaur was somewhat intermediate between reptiles and birds, and left a series of tridactylous impressions on the sand.

To determine the exact age of the red sandstone and shale containing these ancient footprints, in the United States, is not possible at present. No fossil sh.e.l.ls have yet been found in the deposit, nor plants in a determinable state.

The fossil fish are numerous and very perfect; but they are of a peculiar type, called Ischypterus, by Sir Philip Egerton, from the great size and strength of the fulcral rays of the dorsal fin, from ischus, strength, and pteron, a fin.

The age of the Connecticut beds can not be proved by direct superposition, but may be presumed from the general structure of the country. That structure proves them to be newer than the movements to which the Appalachian or Allegheny chain owes its flexures, and this chain includes the ancient or palaeozoic coal- formation among its contorted rocks.

COAL-FIELD OF RICHMOND, VIRGINIA.

In the State of Virginia, at the distance of about 13 miles eastward of Richmond, the capital of that State, there is a coal-field occurring in a depression of the granite rocks, and occupying a geological position a.n.a.logous to that of the New Red Sandstone, above-mentioned, of the Connecticut valley. It extends 26 miles from north to south, and from four to twelve from east to west.

The plants consist chiefly of zamites, calamites, equiseta, and ferns, and, upon the whole, are considered by Professor Heer to have the nearest affinity to those of the European Keuper.

The equiseta are very commonly met with in a vertical position more or less compressed perpendicularly. It is clear that they grew in the places where they are now buried in strata of hardened sand and mud. I found them maintaining their erect att.i.tude, at points many miles apart, in beds both above and between the seams of coal. In order to explain this fact, we must suppose such shales and sandstones to have been gradually acc.u.mulated during the slow and repeated subsidence of the whole region.

(FIGURE 409. Tria.s.sic coal-shale, Richmond, Virginia.

a. Estheria ovata.

b. Young of same.

c. Natural size of a.

d. Natural size of b.)

The fossil fish are Ganoids, some of them of the genus Catopterus, others belonging to the lia.s.sic genus Tetragonolepis (Aechmodus), see Figure 376. Two species of Entomostraca called Estheria are in such profusion in some shaly beds as to divide them like the plates of mica in micaceous shales (see Figure 409).

These Virginian coal-measures are composed of grits, sandstones, and shales, exactly resembling those of older or primary date in America and Europe, and they rival, or even surpa.s.s, the latter in the richness and thickness of the coal-seams. One of these, the main seam, is in some places from 30 to 40 feet thick, composed of pure bituminous coal. The coal is like the finest kinds s.h.i.+pped at Newcastle, and when a.n.a.lysed yields the same proportions of carbon and hydrogen-- a fact worthy of notice, when we consider that this fuel has been derived from an a.s.semblage of plants very distinct specifically, and in part generically, from those which have contributed to the formation of the ancient or palaeozoic coal.

TRIa.s.sIC MAMMIFER.

In North Carolina, the late Professor Emmons has described the strata of the Chatham coal-field, which correspond in age to those near Richmond, in Virginia.

In beds underlying them he has met with three jaws of a small insectivorous mammal which he has called Dromatherium sylvestre, closely allied to Spalacotherium. Its nearest living a.n.a.logue, says Professor Owen, "is found in Myrmecobius; for each ramus of the lower jaw contained ten small molars in a continuous series, one canine, and three conical incisors-- the latter being divided by short intervals."

LOW GRADE OF EARLY MAMMALS FAVOURABLE TO THE THEORY OF PROGRESSIVE DEVELOPMENT.

There is every reason to believe that this fossil quadruped is at least as ancient as the Microlestes of the European Trias described in Chapter 21; and the fact is highly important, as proving that a certain low grade of marsupials had not only a wide range in time, from the Trias to the Purbeck, or uppermost oolitic strata of Europe, but had also a wide range in s.p.a.ce, namely, from Europe to North America, in an east and west direction, and, in regard to lat.i.tude, from Stonesfield, in 52 degrees N., to that of North Carolina, 35 degrees N.

If the three localities in Europe where the most ancient mammalia have been found-- Purbeck, Stonesfield, and Stuttgart-- had belonged all of them to formations of the same age, we might well have imagined so limited an area to have been peopled exclusively with pouched quadrupeds, just as Australia now is, while other parts of the globe were inhabited by placentals; for Australia now supports one hundred and sixty species of marsupials, while the rest of the continents and islands are tenanted by about seventeen hundred species of mammalia, of which only forty-six are marsupial, namely, the opossums of North and South America. But the great difference of age of the strata in each of these three localities seems to indicate the predominance throughout a vast lapse of time (from the era of the Upper Trias to that of the Purbeck beds) of a low grade of quadrupeds; and this persistency of similar generic and ordinal types in Europe while the species were changing, and while the fish, reptiles, and mollusca were undergoing great modifications, would naturally lead us to suspect that there must also have been a vast extension in s.p.a.ce of the same marsupial forms during that portion of the Secondary or Mesozoic epoch which has been termed "the age of reptiles." Such an inference as to the wide geographical range of the ancient marsupials has been confirmed by the discovery in the Trias of North America of the above-mentioned Dromatherium. The predominance in earlier ages of these mammalia of a low grade, and the absence, so far as our investigations have yet gone, of species of higher organisation, whether aquatic or terrestrial, is certainly in favour of the theory of progressive development.

PRIMARY OR PALAEOZOIC SERIES.

CHAPTER XXII.

PERMIAN OR MAGNESIAN LIMESTONE GROUP.

Line of Separation between Mesozoic and Palaeozoic Rocks.

Distinctness of Tria.s.sic and Permian Fossils.

Term Permian.

Thickness of calcareous and sedimentary Rocks in North of England.

Upper, Middle, and Lower Permian.

Marine Sh.e.l.ls and Corals of the English Magnesian Limestone.

Reptiles and Fish of Permian Marl-slate.

Foot-prints of Reptiles.

Angular Breccias in Lower Permian.

Permian Rocks of the Continent.

Zechstein and Rothliegendes of Thuringia.

Permian Flora.

Its generic Affinity to the Carboniferous.

In pursuing our examination of the strata in descending order, we have next to pa.s.s from the base of the Secondary or Mesozoic to the uppermost or newest of the Primary or Palaeozoic formations. As this point has been selected as a line of demarkation for one of the three great divisions of the fossiliferous series, the student might naturally expect that by aid of lithological and palaeontological characters he would be able to recognise without difficulty a distinct break between the newer and older group. But so far is this from being the case in Great Britain, that nowhere have geologists found more difficulty in drawing the line of separation than between the Secondary and Primary series.

The obscurity has arisen from the great resemblance in colour and mineral character of the Tria.s.sic and Permian red marls and sandstones, and the scarcity and often total absence in them of organic remains. The thickness of the strata belonging to each group amounts in some places to several thousand feet; and by dint of a careful examination of their geological position, and of those fossil, animal, and vegetable forms which are occasionally met with in some members of each series, it has at length been made clear that the older or Permian rocks are more connected with the Primary or Palaeozoic than with the Secondary or Mesozoic strata already described.

The term Permian has been proposed for this group by Sir R. Murchison, from Perm, a Russian province, where it occupies an area twice the size of France, and contains a great abundance and variety of fossils, both vertebrate and invertebrate. Professor Sedgwick in 1832 described what is now recognised as the central member of this group, the Magnesian limestone, showing that it attained a thickness of 600 feet along the north-east of England, in the counties of Durham, Yorks.h.i.+re, and Nottinghams.h.i.+re, its lower part often pa.s.sing into a fossiliferous marl-slate and resting on an inferior Red Sandstone, the equivalent of the Rothliegendes of Germany. (Transactions of the Geological Society London Second Series volume 3 page 37.) It has since been shown that some of the Red Sandstones of newer date also belong to the Permian group; and it appears from the observations of Mr. Binney, Sir R. Murchison, Mr. Harkness, and others, that it is in the region where the limestone is most largely developed, as, for example, in the county of Durham, that the a.s.sociated red sandstones or sedimentary rocks are thinnest, whereas in the country where the latter are thickest the calcareous member is reduced to thirty, or even sometimes to ten feet. It is clear, therefore, says Mr. Hull, that the sedimentary region in the north of England area has been to the westward, and the calcareous area to the eastward; and that in this group there has been a development from opposite directions of the two types of strata.

In ill.u.s.tration of this he has given us the following table:

TABLE 22.1. THICKNESS OF PERMIAN STRATA IN NORTH OF ENGLAND.

COLUMN 1: NAME OF STRATA.

COLUMN 2: THICKNESS IN FEET IN N.W. OF ENGLAND.

COLUMN 3: THICKNESS IN FEET IN N.E. OF ENGLAND.

Upper Permian (Sedimentary): 600 : 50-100.

Middle Permian (Calcareous): 10-30 : 600.

Lower Permian (Sedimentary): 3000 : 100-250. (Edward Hull Ternary Cla.s.sification Quarterly Journal of Science No. 23 1869.)

UPPER PERMIAN.

What is called in this table the Upper Permian will be seen to attain its chief thickness in the north-west, or on the coast of c.u.mberland, as at St. Bee's Head, where it is described by Sir Roderick Murchison as consisting of ma.s.sive red sandstones with gypsum resting on a thin course of Magnesian Limestone with fossils, which again is connected with the Lower Red Sandstone, resembling the upper one in such a manner that the whole forms a continuous series. No fossil footprints have been found in this Upper as in the Lower Red Sandstone.

MIDDLE PERMIAN-- MAGNESIAN LIMESTONE AND MARL-SLATE.

(FIGURE 410. Schizodus Schlotheimi, Geinitz. Permian crystalline limestone.)

(FIGURE 411. The hinge of Schizodus truncatus, King. Permian.)