The Student's Elements of Geology Part 43

Mr. Conybeare was enabled, in 1824, after examining many skeletons nearly perfect, to give an ideal restoration of the osteology of this genus, and of that of the Plesiosaurus (Geological Society Transactions Second Series volume 1 page 49.). (See Figures 380, 381.) The latter animal had an extremely long neck and small head, with teeth like those of the crocodile, and paddles a.n.a.logous to those of the Ichthyosaurus, but larger. It is supposed to have lived in shallow seas and estuaries, and to have breathed air like the Ichthyosaur and our modern cetacea. (Conybeare and De la Beche, Geological Transactions First Series volume 5 page 559; and Buckland Bridgewater Treatise page 203.) Some of the reptiles above mentioned were of formidable dimensions. One specimen of Ichthyosaurus platydon, from the Lias at Lyme, now in the British Museum, must have belonged to an animal more than 24 feet in length; and there are species of Plesiosaurus which measure from 18 to 20 feet in length. The form of the Ichthyosaurus may have fitted it to cut through the waves like the porpoise; as it was furnished besides its paddles with a tail-fin so constructed as to be a powerful organ of motion; but it is supposed that the Plesiosaurus, at least the long-necked species (Figure 381), was better suited to fish in shallow creeks and bays defended from heavy breakers.

It is now very generally agreed that these extinct saurians must have inhabited the sea; and it was urged that as there are now chelonians, like the tortoise, living in fresh water, and others, as the turtle, frequenting the ocean, so there may have been formerly some saurians proper to salt, others to fresh water. The common crocodile of the Ganges is well-known to frequent equally that river and the brackish and salt water near its mouth; and crocodiles are said in like manner to be abundant both in the rivers of the Isla de Pinos (Isle of Pines), south of Cuba, and in the open sea round the coast. In 1835 a curious lizard (Amblyrhynchus cristatus) was discovered by Mr. Darwin in the Galapagos Islands. (See Darwin Naturalist's Voyage page 385 Murray.) It was found to be exclusively marine, swimming easily by means of its flattened tail, and subsisting chiefly on seaweed. One of them was sunk from the s.h.i.+p by a heavy weight, and on being drawn up after an hour was quite unharmed.

The families of Dinosauria, crocodiles, and Pterosauria or winged reptiles, are also represented in the Lias.

SUDDEN DESTRUCTION OF SAURIANS.

It has been remarked, and truly, that many of the fish and saurians, found fossil in the Lias, must have met with sudden death and immediate burial; and that the destructive operation, whatever may have been its nature, was often repeated.

"Sometimes," says Dr. Buckland, "scarcely a single bone or scale has been removed from the place it occupied during life; which could not have happened had the uncovered bodies of these saurians been left, even for a few hours, exposed to putrefaction, and to the attacks of fishes and other smaller animals at the bottom of the sea." (Bridgewater Treatise page 115.) Not only are the skeletons of the Ichthyosaurs entire, but sometimes the contents of their stomachs still remain between their ribs, as before remarked, so that we can discover the particular species of fish on which they lived, and the form of their excrements. Not unfrequently there are layers of these coprolites, at different depths in the Lias, at a distance from any entire skeletons of the marine lizards from which they were derived; "as if," says Sir H. De la Beche, "the muddy bottom of the sea received small sudden accessions of matter from time to time, covering up the coprolites and other exuviae which had acc.u.mulated during the intervals." (Geological Researches page 334.) It is further stated that, at Lyme Regis, those surfaces only of the coprolites which lay uppermost at the bottom of the sea have suffered partial decay, from the action of water before they were covered and protected by the muddy sediment that has afterwards permanently enveloped them.

Numerous specimens of the Calamary or pen-and-ink fish, (Geoteuthis bollensis) have also been met with in the Lias at Lyme, with the ink-bags still distended, containing the ink in a dried state, chiefly composed of carbon, and but slightly impregnated with carbonate of lime. These Cephalopoda, therefore, must, like the saurians, have been soon buried in sediment; for, if long exposed after death, the membrane containing the ink would have decayed. (Buckland Bridgewater Treatise page 307.)

As we know that river-fish are sometimes stifled, even in their own element, by muddy water during floods, it can not be doubted that the periodical discharge of large bodies of turbid fresh water in the sea may be still more fatal to marine tribes. In the "Principles of Geology" I have shown that large quant.i.ties of mud and drowned animals have been swept down into the sea by rivers during earthquakes, as in Java in 1699; and that indescribable mult.i.tudes of dead fishes have been seen floating on the sea after a discharge of noxious vapours during similar convulsions. But in the intervals between such catastrophes, strata may have acc.u.mulated slowly in the sea of the Lias, some being formed chiefly of one description of sh.e.l.l, such as ammonites, others of gryphites.

FRESH-WATER DEPOSITS.-- INSECT BEDS.

(FIGURE 382. Wing of a neuropterous insect, from the Lower Lias, Gloucesters.h.i.+re. (Reverend P.B. Brodie.))

From the above remarks the reader will infer that the Lias is for the most part a marine deposit. Some members, however, of the series have an estuarine character, and must have been formed within the influence of rivers. At the base of the Upper and Lower Lias respectively, insect-beds appear to be almost everywhere present throughout the Midland and South-western districts of England. These beds are crowded with the remains of insects, small fish, and crustaceans, with occasional marine sh.e.l.ls. One band in Gloucesters.h.i.+re, rarely exceeding a foot in thickness, has been named the "insect limestone." It pa.s.ses upward, says the Reverend P.B. Brodie, into a shale containing Cypris and Estheria, and is full of the wing-cases of several genera of Coleoptera, with some nearly entire beetles, of which the eyes are preserved. (A History of Fossil Insects etc 1846 London.) The nervures of the wings of neuropterous insects (Figure 382) are beautifully perfect in this bed. Ferns, with Cycads and leaves of monocotyledonous plants, and some apparently brackish and fresh-water sh.e.l.ls, accompany the insects in several places, while in others marine sh.e.l.ls predominate, the fossils varying apparently as we examine the bed nearer or farther from the ancient land, or the source whence the fresh water was derived.

After studying 300 specimens of these insects from the Lias, Mr. Westwood declares that they comprise both wood-eating and herb-devouring beetles, of the Linnean genera Elater, Carabus, etc., besides gra.s.shoppers (Gryllus), and detached wings of dragon-flies and may-flies, or insects referable to the Linnean genera Libellula, Ephemera, Hemerobius, and Panorpa, in all belonging to no less than twenty-four families. The size of the species is usually small, and such as taken alone would imply a temperate climate; but many of the a.s.sociated organic remains of other cla.s.ses must lead to a different conclusion.

FOSSIL PLANTS.

Among the vegetable remains of the Lias, several species of Zamia have been found at Lyme Regis, and the remains of coniferous plants at Whitby. M. Ad.

Brongniart enumerates forty-seven lia.s.sic acrogens, most of them ferns; and fifty gymnosperms, of which thirty-nine are cycads, and eleven conifers. Among the cycads the predominance of Zamites, and among the ferns the numerous genera with leaves having reticulated veins (as in Figure 349), are mentioned as botanical characteristics of this era. (Tableau des Veg. Foss. 1849 page 105.) The absence as yet from the Lias and Oolite of all signs of dicotyledonous angiosperms is worthy of notice. The leaves of such plants are frequent in tertiary strata, and occur in the Cretaceous, though less plentifully (see Chapter 17). The angiosperms seem, therefore, to have been at the least comparatively rare in these older secondary periods, when more s.p.a.ce was occupied by the Cycads and Conifers.

ORIGIN OF THE OOLITE AND LIAS.

The entire group of Oolite and Lias consists of repeated alternations of clay, sandstone, and limestone, following each other in the same order. Thus the clays of the Lias are followed by the sands now considered (see Chapter 20) as belonging to the same formation, though formerly referred to the Inferior Oolite, and these sands again by the sh.e.l.ly and coralline limestone called the Great or Bath Oolite. So, in the Middle Oolite, the Oxford Clay is followed by calcareous grit and coral rag; lastly, in the Upper Oolite, the Kimmeridge Clay is followed by the Portland Sand and limestone (see Figure 298). (Conybeare and Philips's Outlines etc. page 166.) The clay beds, however, as Sir H. de la Beche remarks, can be followed over larger areas than the sand or sandstones.

(Geological Researches page 337.) It should also be remembered that while the Oolite system becomes arenaceous and resembles a coal-field in Yorks.h.i.+re, it a.s.sumes in the Alps an almost purely calcareous form, the sands and clays being omitted; and even in the intervening tracts it is more complicated and variable than appears in ordinary descriptions. Nevertheless, some of the clays and intervening limestones do retain, in reality, a pretty uniform character for distances of from 400 to 600 miles from east to west and north to south.

In order to account for such a succession of events, we may imagine, first, the bed of the ocean to be the receptacle for ages of fine argillaceous sediment, brought by oceanic currents, which may have communicated with rivers, or with part of the sea near a wasting coast. This mud ceases, at length, to be conveyed to the same region, either because the land which had previously suffered denudation is depressed and submerged, or because the current is deflected in another direction by the altered shape of the bed of the ocean and neighbouring dry land. By such changes the water becomes once more clear and fit for the growth of stony zoophytes. Calcareous sand is then formed from comminuted sh.e.l.l and coral, or, in some cases, arenaceous matter replaces the clay; because it commonly happens that the finer sediment, being first drifted farthest from coasts, is subsequently overspread by coa.r.s.e sand, after the sea has grown shallower, or when the land, increasing in extent, whether by upheaval or by sediment filling up parts of the sea, has approached nearer to the spots first occupied by fine mud.

The increased thickness of the limestones in those regions, as in the Alps and Jura, where the clays are comparatively thin, arises from the calcareous matter having been derived from species of corals and other organic beings which live in clear water, far from land, to the growth of which the influx of mud would be unfavourable. Portions therefore of these clays and limestones have probably been formed contemporaneously to a greater extent than we can generally prove, for the distinctness of the species of organic beings would be caused by the difference of conditions between the more littoral and the more pelagic areas and the different depths and nature of the sea-bottom. Independently of those ascending and descending movements which have given rise to the superposition of the limestones and clays, and by which the position of land and sea are made in the course of ages to vary, the geologist has the difficult task of allowing for the contemporaneous thinning out in one direction and thickening in another, of the successive organic and inorganic deposits of the same era.

CHAPTER XXI.

TRIAS, OR NEW RED SANDSTONE GROUP.

Beds of Pa.s.sage between the Lias and Trias, Rhaetic Beds.

Tria.s.sic Mammifer.

Triple Division of the Trias.

Keuper, or Upper Trias of England.

Reptiles of the Upper Trias.

Foot-prints in the Bunter formation in England.

Dolomitic Conglomerate of Bristol.

Origin of Red Sandstone and Rock-salt.

Precipitation of Salt from inland Lakes and Lagoons.

Trias of Germany.

Keuper.

St. Ca.s.sian and Hallstadt Beds.

Peculiarity of their Fauna.

Muschelkalk and its Fossils.

Trias of the United States.

Fossil Foot-prints of Birds and Reptiles in the Valley of the Connecticut.

Tria.s.sic Mammifer of North Carolina.

Tria.s.sic Coal-field of Richmond, Virginia.

Low Grade of early Mammals favourable to the Theory of Progressive Development.

BEDS OF Pa.s.sAGE BETWEEN THE LIAS AND TRIAS-- RHAETIC BEDS.

We have mentioned in the last chapter that the base of the Lower Lias is characterised, both in England and Germany, by beds containing distinct species of Ammonites, the lowest subdivision having been called the zone of Ammonites planorbis. Below this zone, on the boundary line between the Lias and the strata of which we are about to treat, called "Trias," certain cream-coloured limestones devoid of fossils are usually found. These white beds were called by William Smith the White Lias, and they have been shown by Mr. Charles Moore to belong to a formation similar to one in the Rhaetian Alps of Bavaria, to which Mr. Gumbel has applied the name of Rhaetic. They have also long been known as the Koessen beds in Germany, and may be regarded as beds of pa.s.sage between the Lias and Trias. They are named the Penarth beds by the Government surveyors of Great Britain, from Penarth, near Cardiff, in Glamorgans.h.i.+re, where they sometimes attain a thickness of fifty feet.

(FIGURE 383. Cardium rhaetic.u.m, Merrian. Natural size. Rhaetic Beds.)

(FIGURE 384. Pecten Valoniensis. Dfr. 1/2 natural size. Portrush, Ireland, etc.

Rhaetic Beds.)

(FIGURE 385. Avicula contorta. Portlock. Portrush, Ireland, etc. Natural size.

Rhaetic Beds.)

The princ.i.p.al member of this group has been called by Dr. Wright the Avicula contorta bed, as this sh.e.l.l is very abundant, and has a wide range in Europe.

(Dr. Wright, on Lias and Bone Bed, Quarterly Geological Journal 1860 volume 16.) General Portlock first described the formation as it occurs at Portrush, in Antrim, where the Avicula contorta is accompanied by Pecten Valoniensis, as in Germany.

The best known member of the group, a thin band or bone-breccia, is conspicuous among the black shales in the neighbourhood of Axmouth in Devons.h.i.+re, and in the cliffs of Westbury-on-Severn, as well as at Aust and other places on the borders of the Bristol Channel. It abounds in the remains of saurians and fish, and was formerly cla.s.sed as the lowest bed of the Lias; but Sir P. Egerton first pointed out, in 1841, that it should be referred to the Upper New Red Sandstone, because it contained an a.s.semblage of fossil fish which are either peculiar to this stratum, or belong to species well-known in the Muschelkalk of Germany. These fish belong to the genera Acrodus, Hybodus, Gyrolepis, and Saurichthys.

(FIGURE 386. Hybodus plicatilis, Aga.s.siz. Teeth. Bone-bed, Aust and Axmouth.)

(FIGURE 387. Saurichthys apicalis, Aga.s.siz. Tooth; natural size and magnified.

Axmouth.)

(FIGURE 388. Gyrolepis tenuistriatus, Aga.s.siz. Scale; natural size and magnified. Axmouth.)

Among those common to the English bone-bed and the Muschelkalk of Germany are Hybodus plicatilis (Figure 386), Saurychthys apicalis (Figure 387), Gyrolepis tenuistriatus (Figure 388), and G. Albertii. Remains of saurians, Plesiosaurus among others, have also been found in the bone-bed, and plates of an Encrinus.

It may be questioned whether some of those fossils which have the most Tria.s.sic character may not have been derived from the destruction of older strata, since in bone-beds, in general, many of the organic remains are undoubtedly derivative.

TRIa.s.sIC MAMMIFER.

(FIGURE 389. Microlestes antiquus, Plieninger. Molar tooth, magnified. Upper Trias. Diegerloch, near Stuttgart, Wurtemberg.

a. View of inner side?

b. Same, outer side?

c. Same in profile.

d. Crown of same.)

In North-western Germany, as in England, there occurs beneath the Lias a remarkable bone breccia. It is filled with sh.e.l.ls and with the remains of fishes and reptiles, almost all the genera of which, and some even of the species, agree with those of the subjacent Trias. This breccia has accordingly been considered by Professor Quenstedt, and other German geologists of high authority, as the newest or uppermost part of the Trias. Professor Plieninger found in it, in 1847, the molar tooth of a small Tria.s.sic mammifer, called by him Microlestes antiquus. He inferred its true nature from its double fangs, and from the form and number of the protuberances or cusps on the flat crown; and considering it as predaceous, probably insectivorous, he called it Microlestes from micros, little, and lestes, a beast of prey. Soon afterwards he found a second tooth, also at the same locality, Diegerloch, about two miles to the south-east of Stuttgart.

No anatomist had been able to give any feasible conjecture as to the affinities of this minute quadruped until Dr. Falconer, in 1857, recognised an unmistakable resemblance between its teeth and the two back molars of his new genus Plagiaulax (Figure 306), from the Purbeck strata. This would lead us to the conclusion that Microlestes was marsupial and plant-eating.