The Student's Elements of Geology Part 44

In Wurtemberg there are two bone-beds, namely, that containing the Microlestes, which has just been described, which const.i.tutes, as we have seen, the uppermost member of the Trias, and another of still greater extent, and still more rich in the remains of fish and reptiles, which is of older date, intervening between the Keuper and Muschelkalk.

The genera Saurichthys, Hybodus, and Gyrolepis are found in both these breccias, and one of the species, Saurichthys Mongeoti, is common to both bone-beds, as is also a remarkable reptile called Nothosaurus mirabilis. The saurian called Belodon by H. von Meyer, of the Thecodont family, is another Tria.s.sic form, a.s.sociated at Diegerloch with Microlestes.

TRIAS OF ENGLAND.

Between the Lias and the Coal (or Carboniferous group) there is interposed, in the midland and western counties of England, a great series of red loams, shales, and sandstones, to which the name of the "New Red Sandstone formation"

was first given, to distinguish it from other shales and sandstones called the "Old Red," often identical in mineral character, which lie immediately beneath the coal. The name of "Red Marl" has been incorrectly applied to the red clays of this formation, as before explained (Chapter 2), for they are remarkably free from calcareous matter. The absence, indeed, of carbonate of lime, as well as the scarcity of organic remains, together with the bright red colour of most of the rocks of this group, causes a strong contrast between it and the Jura.s.sic formations before described.

The group in question is more fully developed in Germany than in England or France. It has been called the Trias by German writers, or the Triple Group, because it is separable into three distinct formations, called the "Keuper," the "Muschelkalk," and the "Bunter-sandstein." Of these the middle division, or the Muschelkalk, is wholly wanting in England, and the uppermost (Keuper) and lowest (Bunter) members of the series are not rich in fossils.

UPPER TRIAS OR KEUPER.

In certain grey indurated marls below the bone-bed Mr. Boyd Dawkins has found at Watchet, on the coast of Somersets.h.i.+re, a molar tooth of Microlestes, enabling him to refer to the Trias strata formerly supposed to be Lia.s.sic. Mr. Charles Moore had previously discovered many teeth of mammalia of the same family near Frome, in Somersets.h.i.+re, in the contents of a vertical fissure traversing a ma.s.s of carboniferous limestone. The top of this fissure must have communicated with the bed of the Tria.s.sic sea, and probably at a point not far from the ancient sh.o.r.e on which the small marsupials of that era abounded.

This upper division of the Trias called the Keuper is of great thickness in the central counties of England, attaining, according to Mr. Hull's estimate, no less than 3450 feet in Ches.h.i.+re, and it covers a large extent of country between Lancas.h.i.+re and Devons.h.i.+re.

(FIGURE 390. Estheria minuta, Bronn.)

In Worcesters.h.i.+re and Warwicks.h.i.+re in sandstone belonging to the uppermost part of the Keuper the bivalve crustacean Estheria minuta occurs. The member of the English "New Red" containing this sh.e.l.l, in those parts of England, is, according to Sir Roderick Murchison and Mr. Strickland, 600 feet thick, and consists chiefly of red marl or slate, with a band of sandstone.

Ichthyodorulites, or spines of Hybodus, teeth of fishes, and footprints of reptiles were observed by the same geologists in these strata.

(FIGURE 391. Hyperodapedon Gordoni. Left palate, maxillary. (Showing the two rows of palatal teeth on opposite sides of the jaw.) a. Under surface.

b. Exterior right side.)

In the Upper Trias or Keuper the remains of two saurians of the order Lacertilia have been found. The one called Rhynchosaurus occurred at Grinsell near Shrewsbury, and is characterised by having a small bird-like skull and jaws without teeth. The other Hyperodapedon (Figure 391) was first noticed in 1858, near Elgin, in strata now recognised as Upper Tria.s.sic, and afterwards in beds of about the same age in the neighbourhood of Warwick. Remains of the same genus have been found both in Central India and Southern Africa in rocks believed to be of Tria.s.sic age. The Hyperodapedon has been shown by Professor Huxley to be a terrestrial reptile having numerous palatal teeth, and closely allied to the living Sphenodon of New Zealand.

The recent discoveries of a living saurian in New Zealand so closely allied to this supposed extinct division of the Lacertilia seems to afford an ill.u.s.tration of a principle pointed out by Mr. Darwin of the survival in insulated tracts, after many changes in physical geography, of orders of which the congeners have become extinct on continents where they have been exposed to the severer compet.i.tion of a larger progressive fauna.

(FIGURE 392. Tooth of Labyrinthodon; natural size. Warwick sandstone.)

(FIGURE 393. Transverse section of upper part of tooth of Labyrinthodon Jaegeri, Owen (Mastodonsaurus Jaegeri, Meyer); natural size, and a segment magnified.

a. Pulp cavity, from which the processes of pulp and dentine radiate.)

Teeth of Labyrinthodon (Figure 392) found in the Keuper in Warwicks.h.i.+re were examined microscopically by Professor Owen, and compared with other teeth from the German Keuper. He found after careful investigation that neither of them could be referred to true saurians, although they had been named Mastodonsaurus and Phytosaurus by Jager. It appeared that they were of the Batrachian order, and of gigantic dimensions in comparison with any representatives of that order now living. Both the Continental and English fossil teeth exhibited a most complicated texture, differing from that previously observed in any reptile, whether recent or extinct, but most nearly a.n.a.logous to the Ichthyosaurus. A section of one of these teeth exhibits a series of irregular folds, resembling the labyrinthic windings of the surface of the brain; and from this character Professor Owen has proposed the name Labyrinthodon for the new genus. Figure 393 of part of one is given from his "Odontography," plate 64, a. The entire length of this tooth is supposed to have been about three inches and a half, and the breadth at the base one inch and a half.

ROCK-SALT.

In Ches.h.i.+re and Lancas.h.i.+re there are red clays containing gypsum and salt of the age of the Trias which are between 1000 and 1500 feet thick. In some places lenticular ma.s.ses of pure rock-salt nearly 100 feet thick are interpolated between the argillaceous beds. At the base of the formation beneath the rock- salt occur the Lower Sandstones and Marl, called provincially in Ches.h.i.+re "water-stones," which are largely quarried for building. They are often ripple- marked, and are impressed with numerous footprints of reptiles.

The bas.e.m.e.nt beds of the Keuper rest with a slight unconformability upon an eroded surface of the "Bunter" next to be described.

LOWER TRIAS OR BUNTER.

(FIGURE 394. Single footstep of Cheirotherium. Bunter-sandstein, Saxony, one- eighth of natural size.)

(FIGURE 395. Line of footsteps on slab of sandstone. Hildburghausen, in Saxony.)

The lower division or English representative of the "Bunter" attains a thickness of 1500 feet in the counties last mentioned, according to Professor Ramsay.

Besides red and green shales and red sandstones, it comprises much soft white quartzose sandstone, in which the trunks of silicified trees have been met with at Allesley Hill, near Coventry. Several of them were a foot and a half in diameter, and some yards in length, decidedly of coniferous wood, and showing rings of annual growth. (Buckland Proceedings of the Geological Society volume 2 page 439 and Murchison and Strickland Geological Transactions Second Series volume 5 page 347.) Impressions, also, of the footsteps of animals have been detected in Lancas.h.i.+re and Ches.h.i.+re in this formation. Some of the most remarkable occur a few miles from Liverpool, in the whitish quartzose sandstone of Storton Hill, on the west side of the Mersey. They bear a close resemblance to tracks first observed in this member of the Upper New Red Sandstone, at the village of Hesseberg, near Hildburghausen, in Saxony. For many years these footprints have been referred to a large unknown quadruped, provisionally named Cheirotherium by Professor Kaup, because the marks both of the fore and hind feet resembled impressions made by a human hand. (See Figure 394.) The foot- marks at Hesseberg are partly concave, and partly in relief, the former, or the depressions, are seen upon the upper surface of the sandstone slabs, but those in relief are only upon the lower surfaces, being, in fact, natural casts, formed in the subjacent footprints as in moulds. The larger impressions, which seem to be those of the hind foot, are generally eight inches in length, and five in width, and one was twelve inches long. Near each large footstep, and at a regular distance (about an inch and a half) before it, a smaller print of a fore foot, four inches long and three inches wide, occurs. The footsteps follow each other in pairs, each pair in the same line, at intervals of fourteen inches from pair to pair. The large as well as the small steps show the great toes alternately on the right and left side; each step makes the print of five toes, the first, or great toe, being bent inward like a thumb. Though the fore and hind foot differ so much in size, they are nearly similar in form.

As neither in Germany nor in England had any bones or teeth been met with in the same identical strata as the footsteps, anatomists indulged, for several years, in various conjectures respecting the mysterious animals from which they might have been derived. Professor Kaup suggested that the unknown quadruped might have been allied to the Marsupialia; for in the kangaroo the first toe of the fore foot is in a similar manner set obliquely to the others, like a thumb, and the disproportion between the fore and hind feet is also very great. But M. Link conceived that some of the four species of animals of which the tracks had been found in Saxony might have been gigantic Batrachians, and when it was afterwards inferred that the Labyrinthodon was an air-breathing reptile, it was conjectured by Professor Owen that it might be one and the same as the Cheirotherium.

DOLOMITIC CONGLOMERATE OF BRISTOL.

(FIGURE 396. Tooth of Thecodontosaurus; three times magnified. Riley and Stutchbury. Dolomitic conglomerate. Redland, near Bristol.)

Near Bristol, in Somersets.h.i.+re, and in other counties bordering the Severn, the lowest strata belonging to the Tria.s.sic series consist of a conglomerate or breccia resting unconformably upon the Old Red Sandstone, and on different members of the Carboniferous rocks, such as the Coal Measures, Millstone Grit, and Mountain Limestone. This mode of superposition will be understood by reference to the section below Dundry Hill (Figure 85), where No. 4 is the dolomitic conglomerate. Such breccias may have been partly the result of the subaerial waste of an old land-surface which gradually sank down and suffered littoral denudation in proportion as it became submerged. The pebbles and fragments of older rocks which const.i.tute the conglomerate are cemented together by a red or yellow base of dolomite, and in some places the encrinites and other fossils derived from the Mountain Limestone are so detached from the parent rocks that they have the deceptive appearance of belonging to a fauna contemporaneous with the dolomitic beds in which they occur. The imbedded fragments are both rounded and angular, some consisting of sandstone from the coal-measures, being of vast size, and weighing nearly a ton. Fractured bones and teeth of saurians which are truly of contemporaneous origin are dispersed through some parts of the breccia, and two of these reptiles called Thecodont saurians, named from the manner in which the teeth were implanted in the jawbone, obtained great celebrity because the patches of red conglomerate in which they were found, near Bristol, were originally supposed to be of Permian or Palaeozoic age, and therefore the only representatives in England of vertebrate animals of so high a grade in rocks of such antiquity. The teeth of these saurians are conical, compressed, and with finely serrated edges (see Figure 396); they are referred by Professor Huxley to the Dinosaurian order.

ORIGIN OF RED SANDSTONE AND ROCK-SALT.

In various parts of the world, red and mottled clays and sandstones, of several distinct geological epochs, are found a.s.sociated with salt, gypsum, and magnesian limestone, or with one or all of these substances. There is, therefore, in all likelihood, a general cause for such a coincidence.

Nevertheless, we must not forget that there are dense ma.s.ses of red and variegated sandstones and clays, thousands of feet in thickness, and of vast horizontal extent, wholly devoid of saliferous or gypseous matter. There are also deposits of gypsum and of common salt, as in the blue-clay formation of Sicily, without any accompanying red sandstone or red clay.

These red deposits may be accounted for by the decomposition of gneiss and mica schist, which in the eastern Grampians of Scotland has produced a ma.s.s of detritus of precisely the same colour as the Old Red Sandstone.

It is a general fact, and one not yet accounted for, that scarcely any fossil remains are ever preserved in stratified rocks in which this oxide of iron abounds; and when we find fossils in the New or Old Red Sandstone in England, it is in the grey, and usually calcareous beds, that they occur. The saline or gypseous interstratified beds may have been produced by submarine gaseous emanations, or hot mineral springs, which often continue to flow in the same spots for ages. Beds of rock-salt are, however, more generally attributed to the evaporation of lakes or lagoons communicating at intervals with the ocean. In Ches.h.i.+re two beds of salt occur of the extraordinary thickness of 90 or even 100 feet, and extending over an area supposed to be 150 miles in diameter. The adjacent beds present ripple-marked sandstones and footprints of animals at so many levels as to imply that the whole area underwent a slow and gradual depression during the formation of the red sandstone.

Major Harris, in his "Highlands of Ethiopia," describes a salt lake, called the Bahr a.s.sal, near the Abyssinian frontier, which once formed the prolongation of the Gulf of Tadjara, but was afterwards cut off from the gulf by a broad bar of lava or of land upraised by an earthquake. "Fed by no rivers, and exposed in a burning climate to the unmitigated rays of the sun, it has shrunk into an elliptical basin, seven miles in its transverse axis, half filled with smooth water of the deepest caerulean hue, and half with a solid sheet of glittering snow-white salt, the offspring of evaporation." "If," says Mr. Hugh Miller, "we suppose, instead of a barrier of lava, that sand-bars were raised by the surf on a flat arenaceous coast during a slow and equable sinking of the surface, the waters of the outer gulf might occasionally topple over the bar, and supply fresh brine when the first stock had been exhausted by evaporation."

The Runn of Cutch, as I have shown elsewhere (Principles of Geology chapter 27.), is a low region near the delta of the Indus, equal in extent to about a quarter of Ireland, which is neither land nor sea, being dry during part of every year, and covered by salt water during the monsoons. Here and there its surface is incrusted over with a layer of salt caused by the evaporation of sea- water. A subsiding movement has been witnessed in this country during earthquakes, so that a great thickness of pure salt might result from a continuation of such sinking.

TRIAS OF GERMANY.

In Germany, as before hinted, chapter 21, the Trias first received its name as a Triple Group, consisting of two sandstones with an intermediate marine calcareous formation, which last is wanting in England.

NOMENCLATURE OF TRIAS.

COLUMN 1: GERMAN.

COLUMN 2: FRENCH.

COLUMN 3: ENGLISH.

Keuper: Marnes irisees: Saliferous and gypseous shales and sandstone.

Muschelkalk: Muschelkalk, ou calcaire coquillier: Wanting in England.

Bunter-sandstein: Gres bigarre: Sandstone and quartzose conglomerate.

KEUPER.

(FIGURE 397. Equiset.i.tes columnaris. (Syn. Equisetum columnare.) Fragment of stem, and a small portion of same magnified. Keuper.)

The first of these, or the Keuper, underlying the beds before described as Rhaetic, attains in Wurtemberg a thickness of about 1000 feet. It is divided by Alberti into sandstone, gypsum, and carbonaceous clay-slate. (Monog. des Bunter- Sandsteins.) Remains of reptiles called Nothosaurus and Phytosaurus, have been found in it with Labyrinthodon; the detached teeth, also, of placoid fish and of Rays, and of the genera Saurichthys and Gyrolepis (Figures 387, 388). The plants of the Keuper are generically very a.n.a.logous to those of the oolite and lias, consisting of ferns, equisetaceous plants, cycads, and conifers, with a few doubtful monocotyledons. A few species such as Equiset.i.tes columnaris, are common to this group and the oolite.

ST. Ca.s.sIAN AND HALLSTADT BEDS (SEE MAP, FIGURE 398).

(FIGURE 398. Map of Tyrol and Styria showing St. Ca.s.sian and Hallstadt Beds.)