A Manual of Elementary Geology Part 42

[Ill.u.s.tration: Fig. 309. _Chimaera monstrosa._[276-B]

_a._ Spine forming anterior part of the dorsal fin.]

_Reptiles of the Lias._--It is not, however, the fossil fish which form the most striking feature in the organic remains of the Lias; but the reptiles, which are extraordinary for their number, size, and structure. Among the most singular of these are several species of _Ichthyosaurus_ and _Plesiosaurus_. The genus _Ichthyosaurus_, or fish-lizard, is not confined to this formation, but has been found in strata as high as the chalk-marl and gault of England, and as low as the muschelkalk of Germany, a formation which immediately succeeds the lias in the descending order.[276-C] It is evident from their fish-like vertebrae, their paddles, resembling those of a porpoise or whale, the length of their tail, and other parts of their structure, that the habits of the Ichthyosaurs were aquatic. Their jaws and teeth show that they were carnivorous; and the half-digested remains of fishes and reptiles, found within their skeletons, indicate the precise nature of their food.[276-D]

A specimen of the hinder fin or paddle of _Ichthyosaurus communis_ was discovered in 1840 at Barrow-on-Soar, by Sir P. Egerton, which distinctly exhibits on its posterior margin the remains of cartilaginous rays that bifurcate as they approach the edge, like those in the fin of a fish (see _a_, fig. 312.). It had previously been supposed, says Mr.

Owen, that the locomotive organs of the Ichthyosaurus were enveloped, while living, in a smooth integument, like that of the turtle and porpoise, which has no other support than is afforded by the bones and ligaments within; but it now appears that the fin was much larger, expanding far beyond its osseous framework, and deviating widely in its fish-like rays from the ordinary reptilian type. In fig. 312. the posterior bones, or digital ossicles of the paddle, are seen near _b_; and beyond these is the dark carbonized integument of the terminal half of the fin, the outline of which is beautifully defined.[277-A] Mr. Owen believes that, besides the fore-paddles, these short-and stiff-necked saurians were furnished with a tail-fin without bones and purely tegumentary, expanding in a vertical direction; an organ of motion which enabled them to turn their heads rapidly.[277-B]

[Ill.u.s.tration: Fig. 310. _Ichthyosaurus communis_, restored by Conybeare and Cuvier.

_a._ costal vertebrae.]

[Ill.u.s.tration: Fig. 311. _Plesiosaurus dolichodeirus_, restored by Rev. W. D. Conybeare.

_a._ cervical vertebra.]

[Ill.u.s.tration: Fig. 312. Posterior part of hind fin or paddle of _Ichthyosaurus communis_.]

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_.[278-A] (See figs. 310, 311.) 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.[278-B] Some of the reptiles above mentioned were of formidable dimensions. One specimen of _Ichthyosaurus platyodon_, from the lias at Lyme, now in the British Museum, must have belonged to an animal more than 24 feet in length; and another of the _Plesiosaurus_, in the same collection, is 11 feet long. The form of the _Ichthyosaurus_ may have fitted it to cut through the waves like the porpoise; but it is supposed that the _Plesiosaurus_, at least the long-necked species (fig. 311.), was better suited to fish in shallow creeks and bays defended from heavy breakers.

In many specimens both of Ichthyosaur and Plesiosaur the bones of the head, neck, and tail, are in their natural position, while those of the rest of the skeleton are detached and in confusion. Mr. Stutchburg has suggested that their bodies after death became inflated with gases, and, while the abdominal viscera were decomposing, the bones, though disunited, were retained within the tough dermal covering as in a bag, until the whole, becoming water-logged, sank to the bottom.[278-C] As they belonged to individuals of all ages they are supposed, by Dr. Buckland, to have experienced a violent death; and the same conclusion might also be drawn from their having escaped the attacks of their own predaceous race, or of fishes, found fossil in the same beds.

[Ill.u.s.tration: Fig 313. _Amblyrhynchus cristatus_, Bell. Length varying from 3 to 4 feet. The only existing marine lizard now known.

_a._ Tooth, natural size and magnified.]

For the last twenty years, anatomists have agreed that these extinct saurians must have inhabited the sea; and it was argued 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 (or Isle of Pines), south of Cuba, and in the open sea round the coast. More recently a saurian has been discovered of aquatic habits and exclusively marine. This creature was found in the Galapagos Islands, during the visit of H. M. S. Beagle to that archipelago, in 1835, and its habits were then observed by Mr. Darwin. The islands alluded to are situated under the equator, nearly 600 miles to the westward of the coast of South America. They are volcanic, some of them being 3000 or 4000 feet high; and one of them, Albemarle Island, 75 miles long. The climate is mild; very little rain falls; and, in the whole archipelago, there is only one rill of fresh water that reaches the coast. The soil is for the most part dry and harsh, and the vegetation scanty. The birds, reptiles, plants, and insects are, with very few exceptions, of species found no where else in the world, although all partake, in their general form, of a South American type. Of the mammalia, says Mr. Darwin, one species alone appears to be indigenous, namely, a large and peculiar kind of mouse; but the number of lizards, tortoises, and snakes is so great, that it may be called a land of reptiles. The variety, indeed, of species is small; but the individuals of each are in wonderful abundance. There is a turtle, a large tortoise (_Testudo Indicus_), four lizards, and about the same number of snakes, but no frogs or toads. Two of the lizards belong to the family _Iguanidae_ of Bell, and to a peculiar genus (_Amblyrhynchus_) established by that naturalist, and so named from their obtusely truncated head and short snout.[279-A] Of these lizards one is terrestrial in its habits, and burrows in the ground, swarming everywhere on the land, having a round tail, and a mouth somewhat resembling in form that of the tortoise. The other is aquatic, and has its tail flattened laterally for swimming (see fig.

313.). "This marine saurian," says Mr. Darwin, "is extremely common on all the islands throughout the archipelago. It lives exclusively on the rocky sea-beaches, and I never saw one even ten yards insh.o.r.e. The usual length is about a yard, but there are some even 4 feet long. It is of a dirty black colour, sluggish in its movements on the land; but, when in the water, it swims with perfect ease and quickness by a serpentine movement of its body and flattened tail, the legs during this time being motionless, and closely collapsed on its sides. Their limbs and strong claws are admirably adapted for crawling over the rugged and fissured ma.s.ses of lava which everywhere form the coast. In such situations, a group of six or seven of these hideous reptiles may oftentimes be seen on the black rocks, a few feet above the surf, basking in the sun with outstretched legs. Their stomachs, on being opened, were found to be largely distended with minced sea-weed, of a kind which grows at the bottom of the sea at some little distance from the coast. To obtain this, the lizards go out to sea in shoals. One of these animals was sunk in salt water, from the s.h.i.+p, with a heavy weight attached to it, and on being drawn up again after an hour it was quite active and unharmed.

It is not yet known by the inhabitants where this animal lays its eggs; a singular fact, considering its abundance, and that the natives are well acquainted with the eggs of the terrestrial _Amblyrhynchus_, which is also herbivorous."[280-A]

In those deposits now forming by the sediment washed away from the wasting sh.o.r.es of the Galapagos Islands the remains of saurians, both of the land and sea, as well as of chelonians and fish, may be mingled with marine sh.e.l.ls, without any bones of land quadrupeds or batrachian reptiles; yet even here we should expect the remains of marine mammalia to be imbedded in the new strata, for there are seals, besides several kinds of cetacea, on the Galapagian sh.o.r.es; and, in this respect, the parallel between the modern fauna, above described, and the ancient one of the lias, would not hold good.

_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."[280-B] 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."[281-A] It is farther 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.[281-B]

Numerous specimens of the pen-and-ink fish (_Sepia loligo_, Lin.; _Loligo vulgaris_, Lam.) 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.[281-C]

As we know that river fish are sometimes stifled, even in their own element, by muddy water during floods, it cannot be doubted that the periodical discharge of large bodies of turbid fresh water into 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 undescribable mult.i.tudes of dead fishes have been seen floating on the sea after a discharge of noxious vapours during similar convulsions.[281-D]

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.

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, especially in the lowest part of it, have an estuary character, and must have been formed within the influence of rivers. In Gloucesters.h.i.+re, where there is a good type of the lias of the West of England, it may be divided into an upper ma.s.s of shale with a base of marlstone, and a lower series of shales with underlying limestones and shales. We learn from the researches of the Rev.

P. B. Brodie[281-E], that in the superior of these two divisions numerous remains of insects and plants have been detected in several places, mingled with marine sh.e.l.ls; but in the inferior division similar fossils are still more plentiful. One band, rarely exceeding a foot in thickness, has been named the "insect limestone." It pa.s.ses upwards into a shale containing _Cypris_ and _Estheria_, and is charged with the wing-cases of several genera of coleoptera, and with some nearly entire beetles, of which the eyes are preserved. The nervures of the wings of neuropterous insects (fig. 314.) are beautifully perfect in this bed. Ferns, with leaves of monocotyledonous plants, and freshwater sh.e.l.ls, such as _Cyclas_ and _Unio_, accompany the insects in some 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. There are two, or even three, bands of "insect limestone" in several sections, and they have been ascertained by Mr. Brodie to retain the same lithological and zoological characters when traced from the centre of Warwicks.h.i.+re to the borders of the southern part of Wales. 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_, &c., 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.

[Ill.u.s.tration: Fig. 314. Wing of a neuropterous insect, from the Lower Lias, Gloucesters.h.i.+re. (Rev. B. Brodie.)]

_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. Fragments of wood are common, and often converted into limestone. That some of this wood, though now petrified, was soft when it first lay at the bottom of the sea, is shown by a specimen now in the museum of the Geological Society (see fig. 315.), which has the form of an _ammonite_ indented on its surface.

[Ill.u.s.tration: Fig. 315. Petrified wood.]

M. Ad. Brongniart enumerates forty-seven lia.s.sic acrogens, most of them ferns; and fifty gymnogens, of which thirty-nine are cycads, and eleven conifers. Among the cycads the predominance of _Zamites_ and _Nilsonia_, and among the ferns the numerous genera with leaves having reticulated veins (as in fig. 296. p. 272.), are mentioned as botanical characteristics of this era.[282-A]

_Origin of the Oolite and Lias._--If we now endeavour to restore, in imagination, the ancient condition of the European area at the period of the Oolite and Lias, we must conceive a sea in which the growth of coral reefs and sh.e.l.ly limestones, after proceeding without interruption for ages, was liable to be stopped suddenly by the deposition of clayey sediment. Then, again, the argillaceous matter, devoid of corals, was deposited for ages, and attained a thickness of hundreds of feet, until another period arrived when the same s.p.a.ce was again occupied by calcareous sand, or solid rocks of sh.e.l.l and coral, to be again succeeded by the recurrence of another period of argillaceous deposition. Mr.

Conybeare has remarked of the entire group of Oolite and Lias, that it 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 of the inferior oolite, and these again by sh.e.l.ly and coralline limestone (Bath oolite, &c.); 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.[283-A] The clay beds, however, as Sir H. De la Beche remarks, can be followed over larger areas than the sands or sandstones.[283-B] It should also be remembered that while the oolitic 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, in reality, retain a pretty uniform character, for distances of from 400 to 600 miles from east to west and north to south.

According to M. Thirria, the entire oolitic group in the department of the Haute-Saone, in France, may be equal in thickness to that of England; but the importance of the argillaceous divisions is in the inverse ratio to that which they exhibit in England, where they are about equal to twice the thickness of the limestones, whereas, in the part of France alluded to, they reach only about a third of that thickness.[283-C] In the Jura the clays are still thinner; and in the Alps they thin out and almost vanish.

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.

In order to account for another great formation, like the Oxford clay, again covering one of coral limestone, we must suppose a sinking down like that which is now taking place in some existing regions of coral between Australia and South America. The occurrence of subsidences, on so vast a scale, may have caused the bed of the ocean and the adjoining land, throughout great parts of the European area, to a.s.sume a shape favourable to the deposition of another set of clayey strata; and this change may have been succeeded by a series of events a.n.a.logous to that already explained, and these again by a third series in similar order.

Both the ascending and descending movements may have been extremely slow, like those now going on in the Pacific; and the growth of every stratum of coral, a few feet of thickness, may have required centuries for its completion, during which certain species of organic beings disappeared from the earth, and others were introduced in their place; so that, in each set of strata, from the Upper Oolite to the Lias, some peculiar and characteristic fossils were embedded.

_Oolite and Lias of the United States._

[Ill.u.s.tration: Fig. 316. Section showing the geological position of the James River, or East Virginian Coal-field.

A. Granite, gneiss, &c.

B. Coal-measures.

C. Tertiary strata.

D. Drift or _ancient alluvium_.]

There are large tracts on the globe, as in Russia and the United States, where all the members of the oolitic series are unrepresented. In the state of Virginia, however, at the distance of about 13 miles eastward of Richmond, the capital of that State, there is a regular coal-field occurring in a depression of the granite rocks (see section, fig. 316.), which Professor W. B. Rogers first correctly referred to the age of the lower part of the Jura.s.sic group. This opinion I was enabled to confirm after collecting a large number of fossil plants, fish, and sh.e.l.ls, and examining the coal-field throughout its whole area. It extends 26 miles from north to south, and from 4 to 12, from east to west. The plants consist chiefly of zamites, calamites, and equisetums, and these last 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 now lie buried in strata of hardened sand and mud. I found them maintaining their erect att.i.tude, at points many miles distant from others, 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.

It is worthy of remark that the _Equisetum columnare_ of these Virginian rocks appears to be undistinguishable from the species found in the oolitic sandstones near Whitby in Yorks.h.i.+re, where it also is met with in an upright position. One of the American ferns, _Pecopteris Whitbyensis_, is also a species common to the Yorks.h.i.+re oolites.[285-A]

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 seams. One of these, the main seam, is in some places from 30 to 40 feet thick, composed of pure bituminous coal. On descending a shaft 800 feet deep, in the Blackheath mines in Chesterfield county, I found myself in a chamber more than 40 feet high, caused by the removal of this coal. Timber props of great strength supported the roof, but they were seen to bend under the inc.u.mbent weight. 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 paleozoic coal.

The fossil fish of these Richmond strata belong to the lia.s.sic genus _Tetragonolepis_, and to a new genus which I have called _Dictyopyge_.

Sh.e.l.ls are very rare, as usually in all coal-bearing deposits, but a species of _Posidonomya_ is in such profusion in some shaley beds as to divide them like the plates of mica in micaceous shales (see fig. 317.).

[Ill.u.s.tration: Fig. 317. Oolitic coal-shale, Richmond, Virginia.

_a._ _Posidonomya._ _b._ young of same.]

In India, especially in Cutch, a formation occurs clearly referable to the oolitic and lia.s.sic type, as shown by the sh.e.l.ls, corals, and plants; and there also coal has been procured from one member of the group.

FOOTNOTES:

[274-A] Conyb. and Phil. p. 261.

[275-A] Aga.s.siz, Pois. Fos. vol. ii. tab. 28, 29.

[276-A] Bridgewater Treatise, p. 290.

[276-B] Aga.s.siz, Poissons Fossiles, vol. iii. tab. C. fig. 1.