Part 17 (1/2)

[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 Leicesters.h.i.+re 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 Dorsets.h.i.+re and Gloucesters.h.i.+re; 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 Yorks.h.i.+re, at Lyme Regis and Charmouth in Dorsets.h.i.+re, 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 Yorks.h.i.+re 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.