[58] Ibid, vol. xvii., p. 483.

[59] Ibid, vol. xvi., p. 374.

[60] Ibid, vol. xx., p. 103.

JURa.s.sIC PERIOD.

This period, one of the most important in the physical history of the globe, has received its name from the Jura mountains in France, the Jura range being composed of the rocks deposited in the seas of the period.

In the term Jura.s.sic, the formations designated as the "Oolite" and "Lias" are included, both being found in the Jura mountains. The Jura.s.sic period presents a very striking a.s.semblage of characteristics, both in its vegetation and in the animal remains which belong to it; many genera of animals existing in the preceding age have disappeared, new genera have replaced them, comprising a very specially organised group, containing not less than 4,000 species.

The Jura.s.sic period is sub-divided into two sub-periods: those of the _Lias_ and the _Oolite_.

THE LIAS

is an English provincial name given to an argillaceous limestone, which, with marl and clay, forms the base of the Jura.s.sic formation, and pa.s.ses almost imperceptibly into the Lower Oolite in some places, where the Marlstone of the Lias partakes of the mineral character, as well as the fossil remains of the Lower Oolite; and it is sometimes treated of as belonging to that formation. "Nevertheless, the Lias may be traced throughout a great part of Europe as a separate and independent group, of considerable thickness, varying from 500 to 1,000 feet, containing many peculiar fossils, and having a very uniform lithological aspect."[61] The rocks which represent the Lia.s.sic period form the base of the Jura.s.sic system, and have a mean thickness of about 1,200 feet.

In the inferior part we find argillaceous sandstones, which are called the sandstones of the Lias, and comprehend the greater part of the _Quadersandstein_, or building-stone of the Germans, above which comes compact limestone, argillaceous, bluish, and yellowish; finally, the formation terminates in the marlstones which are sometimes sandy, and occasionally bituminous.

[61] Lyell, "Elements of Geology," p. 413.

The Lias, in England, is generally in three groups: 1, the upper, clays and shales, underlying sands; 2, the middle, lias or marlstone; and 3, the lower, clays and limestone; but these have been again sub-divided--the last into six zones, each marked by its own peculiar species of Ammonites; the second into three zones; the third consists of clay, shale, and argillaceous limestone. For the purposes of description we shall, therefore, divide the Lias into these three groups:--

1. _Upper Lias Clay_, consists of blue clay, or shale, containing nodular bands of claystones at the base, crowded with _Ammonites serpentinus_, _A. bifrons_, _Belemnites_, &c.

2. The _Middle Lias_, commonly known as the Marlstone, is surmounted by a bed of oolitic ironstone, largely worked in Leicestershire and in the north of England as a valuable ore of iron. The underlying marls and sands, the latter of which become somewhat argillaceous below, form beds from 200 to 300 feet thick in Dorsetshire and Gloucestershire; the fossils are _Ammonites margaritaceus_, _A. spinatus_, _Belemnites tripart.i.tus_. The upper rock-beds, especially the bed of ironstone on the top, is generally remarkably rich in fossils.

[Ill.u.s.tration: Fig. 88.--Gryphaea incurva.]

3. _Lower Lias_ (averaging from 600 to 900 feet in thickness) consists, in the lower part, of thin layers of bluish argillaceous limestone, alternating with shales and clays; the whole overlaid by the blue clay of which the lower member of the Lia.s.sic group usually consists. This member of the series is well developed in Yorkshire, at Lyme Regis and Charmouth in Dorsetshire, and generally over the South-West and Midland Counties of England. _Gryphaea incurva_ (Fig. 88), with sandy bands, occurs at the base, in addition to which we find _Ammonites planorbis Bucklandi_, _A. Ostrea lia.s.sica_, _Lima gigantea_, _Ammonites Bucklandi_, &c., in the lower limestones and shales.

Above the clay are yellow sands from 100 to 200 feet thick, underlying the limestone of the Inferior Oolite. These sands were, until lately, considered to belong to the latter formation--as they undoubtedly do physically--until they were shown, by Dr. Thomas Wright, of Cheltenham, to be more nearly allied, by their fossils, to the Lias below than to the Inferior Oolite above, into which they form the pa.s.sage-beds.

In France the Lias abounds in the Calvados, in Burgundy, Lorraine, Normandy, and the Lyonnais. In the Vosges and Luxembourg, M. Elie de Beaumont states that the Lias containing _Gryphaea incurva_ and _Lima gigantea_, and some other marine fossils, becomes arenaceous; and around the Harz mountains, in Westphalia and Bavaria, in its lower parts the formation is sandy, and is sometimes a good building-stone.

"In England the Lias const.i.tutes," says Professor Ramsay, "a well-defined belt of strata, running continuously from Lyme Regis, on the south-west, through the whole of England, to Yorkshire on the north-east, and is an extensive series of alternating beds of clay, shale, and limestone, with occasional layers of jet in the upper part.

The unequal hardness of the clays and limestones of the Lia.s.sic strata causes some of its members to stand out in the distinct minor escarpments, often facing the west and north-west. The Marlstone forms the most prominent of these, and overlooks the broad meadows of the lower Lias-clay, that form much of the centre of England." In Scotland there are few traces of the Lias. Zoophytes, Mollusca, and Fishes of a peculiar organisation, but, above all, Reptiles of extraordinary size and structure gave to the sea of the Lia.s.sic period an interest and features quite peculiar. Well might Cuvier exclaim, when the drawings of the Plesiosaurus were sent to him: "Truly this is altogether the most monstrous animal that has yet been dug out of the ruins of a former world!" In the whole of the English Lias there are about 243 genera, and 467 species of fossils. The whole series has been divided into zones characterised by particular Ammonites, which are found to be limited to them, at least locally.

Among the Echinodermata belonging to the Lias we may cite _Asterias lumbricalis_ and _Palaeocoma Furstembergii_, which const.i.tutes a genus not dissimilar to the star-fishes, of which its radiated form reminds us. The Pentacrinites, of which _Pentacrinites Briareus_ is a type, ornaments many collections by its elegant form, and is represented in Figs. 79 and 89. It belongs to the order of Crinoidea, which is represented at the present time by a single living species, _Pentacrinus caput-Medusae_, one of the rare and delicate Zoophytes of the Caribbean sea.

Oysters (_Ostrea_) made their appearance in the Muschelkalk of the last period, but only in a small number of species; they increased greatly in importance in the Lia.s.sic seas.

[Ill.u.s.tration: Fig. 89.--Pentacrinites Briareus. Half natural size.]

The _Ammonites_, a curious genus of Cephalopoda, which made their first appearance in small numbers towards the close of the preceding Tria.s.sic period, become quite special in the Secondary epoch, with the close of which they disappear altogether. They were very abundant in the Jura.s.sic period, and, as we have already said, each zone is characterised by its peculiar species. The name is taken from the resemblance of the sh.e.l.l to the ram's-horn ornaments which decorated the front of the temple of Jupiter Ammon and the bas-reliefs and statues of that pagan deity. They were Cephalopodous Mollusca with circular sh.e.l.ls, rolled upon themselves symmetrically in the same plane, and divided into a series of chambers.

The animal only occupied the outer chamber of the sh.e.l.l; all the others were empty. A siphon or tube issuing from the first chamber traversed all the others in succession, as is seen in all the Ammonites and Nautili. This tube enabled the animal to rise to the surface, or to sink to the bottom, for the Ammonite could fill the chambers with water at pleasure, or empty them, thus rendering itself lighter or heavier as occasion required. The Nautilus of our seas is provided with the same curious organisation, and reminds us forcibly of the Ammonites of geological times.

Sh.e.l.ls are the only traces which remain of the Ammonites. We have no exact knowledge of the animal which occupied and built them. The attempt at restoration, as exhibited in Fig. 91, will probably convey a fair idea of the Ammonite when living. We a.s.sume that it resembled the Nautilus of modern times. What a curious aspect these early seas must have presented, covered by myriads of these Molluscs of all sizes, swimming about in eager pursuit of their prey!

The Ammonites of the Jura.s.sic age present themselves in a great variety of forms and sizes; some of them of great beauty. _Ammonites bifrons_, _A. Noditia.n.u.s_, _A. bisulcatus_, _A. Turneri_ (Fig. 90), and _A.

margaritatus_, are forms characteristic of the Lias.

[Ill.u.s.tration: Fig. 90.--Ammonites Turneri, from the Lower Lias.]

The _Belemnites_, molluscous Cephalopods of a very curious organisation, appeared in great numbers, and for the first time, in the Jura.s.sic seas.

Of this Mollusc we only possess the fossilised internal "bone,"

a.n.a.logous to that of the modern cuttle-fish and the calamary of the present seas. This simple relic is very far from giving us an exact idea of what the animal was to which the name of Belemnite has been given (from ?e?e???, _a dart_) from their supposed resemblance to the head of a javelin. The slender cylindrical bone, the only vestige remaining to us, was merely the internal skeleton of the animal. When first discovered they were called, by the vulgar, "Thunder-stones" and "Ladies' fingers." They were, at last, inferred to be the sh.e.l.ly processes of some sort of ancient cuttle-fish. Unlike the Ammonite, which floated on the surface and sunk to the bottom at pleasure, the Belemnite, it has been thought, swam nearer the bottom of the sea, and seized its prey from below.

[Ill.u.s.tration: Fig. 91.--Ammonite restored.]

[Ill.u.s.tration: Fig. 92.--Belemnite restored.]

In Fig. 92 is given a restoration of the living Belemnite, by Dr.

Buckland and Professor Owen, in which the terminal part of the animal is marked in a slightly darker tint, to indicate the place of the bone which alone represents in our days this fossilised being. A sufficiently exact idea of this Mollusc may be arrived at from the existing cuttle-fish. Like the cuttle-fish, the Belemnite secreted a black liquid, a sort of ink or sepia; and the bag containing the ink has frequently been found in a fossilised state, with the ink dried up, and elaborate drawings have been made with this fossil pigment.

The beaks, or h.o.r.n.y mandibles of the mouth, which the Belemnite possessed in common with the other naked Cephalopoda, are represented in Fig. 78, p. 181.

As Sir H. De la Beche has pointed out, the destruction of the animals whose remains are known to us by the name of Belemnites was exceedingly great when the upper part of the Lias of Lyme Regis was deposited.

Mult.i.tudes seem to have perished almost simultaneously, and millions are entombed in a bed beneath Golden Cap, a lofty cliff between Lyme Regis and Bridport Harbour, as well as in the upper Lias generally.[62]

[62] De la Beche's "Geological Manual," 3rd ed., p. 447.

Among the Belemnites characteristic of the Lia.s.sic period may be cited _B. acutus_ (Fig. 93), _B. pistiliformis_, and _B. sulcatus_.

[Ill.u.s.tration: Fig. 93.--Belemnites acutus.]

The seas of the period contained a great number of the fishes called _Ganoids_; which are so called from the splendour of the hard and enamelled scales, which formed a sort of defensive armour to protect their bodies. _Lepidotus gigas_ was a fish of great size belonging to this age. A smaller fish was the _Tetragonolepis_, or _aechmodus Buchii_.

The _Acrodus n.o.bilis_, of which the teeth are still preserved, and popularly known by the name of _fossil leeches_, was a fish of which an entire skeleton has never been met with. Neither are we better informed as to the _Hybodus reticulatus_. The bony spines, which form the anterior part of the dorsal fin of this fish, had long been an object of curiosity to geologists, under the general name of _Ichthyodorulites_, before they were known to be fragments of the fin of the _Hybodus_. The Ichthyodorulites were supposed by some naturalists to be the jaw of some animal--by others, weapons like those of the living _Balistes_ or _Silurus_; but Aga.s.siz has shown them to be neither the one nor the other, but bony spines on the fin, like those of the living genera of _Cestracions_ and _Chimaeras_, in both of which the concave face is armed with small spines like those of the _Hybodus_. The spines were simply imbedded in the flesh, and attached to it by strong muscles. "They served," says Dr. Buckland, "as in the _Chimaera_, to raise and depress the fin, their action resembling that of a movable mast lowering backward."

[Ill.u.s.tration: Fig. 94.--Ichthyosaurus communis.]

Let us hasten to say, however, that these are not the beings that characterised the age, and were the salient features of the generation of animals which existed during the Jura.s.sic period. These distinguishing features are found in the enormous reptiles with lizard's head, crocodile's conical teeth, the trunk and tail of a quadruped, whale-like paddles, and the double-concave vertebrae of fishes; and this strange form, on such a gigantic scale that even their inanimate remains are examined with a curiosity not unmixed with awe. The country round Lyme Regis, in Dorsetshire, has long been celebrated for the curious fossils discovered in its quarries, and preserved in the muddy acc.u.mulations of the sea of the Lia.s.sic period. The country is hilly--"up one hill and down another," is a pretty correct provincial description of the walk from Bridport to Lyme Regis--where some of the most frightful creatures the living world has probably ever beheld, sleep the sleep of stones. The quarries of Lyme Regis form the cemetery of the Ichthyosauri; the sepulchre where lie interred these dragons of the ancient seas.

In 1811 a country girl, who made her precarious living by picking up fossils for which the neighbourhood was famous, was pursuing her avocation, hammer in hand, when she perceived some bones projecting a little out of the cliff. Finding, on examination, that it was part of a large skeleton, she cleared away the rubbish, and laid bare the whole creature imbedded in the block of stone. She hired workmen to dig out the block of Lias in which it was buried. In this manner was the first of these monsters brought to light: "a monster some thirty feet long, with jaws nearly a fathom in length, and huge saucer-eyes; which have since been found so perfect, that the petrified lenses have been split off and used as magnifiers," as a writer in _All the Year Round_ a.s.sures us.

[Ill.u.s.tration: Fig. 95.--Head of Ichthyosaurus platydon.]

In Fig. 95 the head of _I. platydon_ is represented. As in the Saurians, the openings of the nostrils are situated near the anterior angle of the orbits of the eyes, while those of the Crocodile are near the snout; but, on the other hand, in its osteology and its mode of dent.i.tion it nearly resembles the Crocodile; the teeth are pointed and conical--not, however, set in deep or separate sockets, but only implanted in a long and deep continuous groove hollowed in the bones of the jaw. These strong jaws have an enormous opening; for, in some instances, they have been found eight feet in length and armed with 160 teeth. Let us add that teeth lost through the voracity of the animal, or in contests with other animals, could be renewed many times; for, at the inner side of the base of every old tooth, there is always the bony germ of a new one.

The eyes of this marine monster were much larger than those of any animal now living; in volume they frequently exceed the human head, and their structure was one of their most remarkable peculiarities. In front of the sclerotic coat or capsule of the eye there is an annular series of thin bony plates, surrounding the pupil. This structure, which is now only met with in the eyes of certain turtles, tortoises, and lizards, and in those of many birds, could be used so as to increase or diminish the curvature of the transparent cornea, and thus increase or diminish the magnifying power, according to the requirements of the animal--performing the office, in short, of a telescope or microscope at pleasure. The eyes of the Ichthyosaurus were, then, an optical apparatus of wonderful power and of singular perfection, enabling the animal, by their power of adaptation and intensity of vision, to see its prey far and near, and to pursue it in the darkness and in the depths of the sea.

The curious arrangement of bony plates we have described furnished, besides, to its globular eye, the power necessary to bear the pressure of a considerable weight of water, as well as the violence of the waves, when the animal came to the surface to breathe, and raised its head above the waves. This magnificent specimen of the fish-lizard, or Ichthyosaurus, as it was named by Dr. Ure, now forms part of the treasures of the British Museum.

At no period in the earth's history have Reptiles occupied so important a place as they did in the Jura.s.sic period. Nature seems to have wished to bring this cla.s.s of animals to the highest state of development. The great Reptiles of the Lias are as complicated in their structure as the Mammals which appeared at a later period. They probably lived, for the most part, by fishing in shallow creeks and bays defended from heavy breakers, or in the open sea; but they seem to have sought the sh.o.r.e from time to time; they crawled along the beach, covered with a soft skin, perhaps not unlike some of our Cetaceae. The Ichthyosaurus, from its form and strength, may have braved the waves of the sea as the porpoise does now. Its destructiveness and voracity must have been prodigious, for Dr. Buckland describes a specimen which had between its ribs, in the place where the stomach might be supposed to have been placed, the skeleton of a smaller one--a proof that this monster, not content with preying on its weaker neighbours, was in the habit of devouring its own kind. In the same waters lived the Plesiosaurus, with long neck and form more strange than that of the Ichthyosaurus; and these potentates of the seas were warmed by the same sun and tenanted the same banks, in the midst of a vegetation not unlike that which the climate of Africa now produces.

The great Saurians in the Lias of Lyme Regis seem to have suffered a somewhat sudden death, partly in consequence of a series of small catastrophes suddenly destroying the animals then existing in particular spots. "In general the bones are not scattered about, and in a detached state, as would happen if the dead animal had descended to the bottom of the sea, to be decomposed, or devoured piecemeal, as, indeed, might also happen if the creature floated for a time on the surface, one animal devouring one part, and another carrying off a different portion; on the contrary, the bones of the skeleton, though frequently compressed, as must arise from the enormous pressure to which they have so long been subjected, are tolerably connected, frequently in perfect, or nearly perfect, order, as if prepared by the anatomist. The skin, moreover, may sometimes be traced, and the compressed contents of the intestines may at times be also observed--all tending to show that the animals were suddenly destroyed, and as suddenly preserved."[63]

[63] "Geological Manual," by H. T. De la Beche, 3rd ed., p. 346.