I. THE LIAS.—The base of the Jurassic series of Britain is formed by the great calcareo-argillaceous deposit of the "Lias," which usually rests conformably and almost inseparably upon the Rhætic beds (the so-called "White Lias"), and passes up, generally conformably, into the calcareous sandstones of the Inferior Oolite. The Lias is divisible into the three principal groups of the Lower, Middle, and Upper Lias, as under, and these in turn contain many well-marked "zones;" so that the Lias has some claims to be considered as an independent formation, equivalent to all the remaining Oolitic rocks. The Lower Lias (Terrain Sinemurien of D'Orbigny) sometimes attains a thickness of as much as 600 feet, and consists of a great series of bluish or greyish laminated clays, alternating with thin bands of blue or grey limestone—the whole, when seen in quarries or cliffs from a little distance, assuming a characteristically striped and banded appearance. By means of particular species of Ammonites, taken along with other fossils which are confined to particular zones, the Lower Lias may be subdivided into several well-marked horizons. The Middle Lias, or Marlstone Series (Terrain Liasien of D'Orbigny), may reach a thickness of 200 feet, and consists of sands, arenaceous marls, and argillaceous limestones, sometimes with ferruginous beds. The Upper Lias (Terrain Toarcien of D'Orbigny) attains a thickness of 300 feet, and consists principally of shales below, passing upwards into arenaceous strata.

II. THE LOWER OOLITES.—Above the Lias comes a complex series of partly arenaceous and argillaceous, but principally calcareous strata, of which the following are the more important groups: a, The Inferior Oolite (Terrain Bajocien of D'Orbigny), consisting of more than 200 feet of oolitic limestones, sometimes more or less sandy; b, The Fuller's Earth, a series of shales, clays, and marls, about 120 feet in thickness; c, The Great Oolite or Bath Oolite (Terrain Bathonien of D'Orbigny), consisting principally of oolitic limestones, and attaining a thickness of about 130 feet. The well-known "Stonesfield Slates" belong to this horizon; and the locally developed "Bradford Clay," "Corn brash," and "Forest-marble" may be regarded as constituting the summit of this group.

III. THE MIDDLE OOLITES.—The central portion of the Jurassic series of Britain is formed by a great argillaceous deposit, capped by calcareous strata, as follows: a, The Oxford Clay (Terrain Callovien and Terrain Oxfordien of D'Orbigny), consisting of dark-coloured laminated clays, sometimes reaching a thickness of 700 feet, and in places having its lower portion developed into a hard calcareous sandstone ("Kelloway Rock"); b, The Coral-Rag (Terrain Corallien of D'Orbigny, "Nerinean Limestone" of the Jura, "Diceras Limestone" of the Alps), consisting, when typically developed, of a central mass of oolitic limestone, underlaid and surmounted by calcareous grits.

IV. THE UPPER OOLITES.—a, The base of the Upper Oolites of Britain is constituted by a great thickness (600 feet or more) of laminated, sometimes carbonaceous or bituminous clays, which are known as the Kimmeridge Clay (Terrain Kimméridgien of D'Orbigny); b, The Portland Beds (Terrain Portlandien of D'Orbigny) succeed the Kimmeridge clay, and consist inferiorly of sandy beds surmounted by oolitic limestones ("Portland Stone"), the whole series attaining a thickness of 150 feet or more, and containing marine fossils; c, The Purbeck Beds are apparently peculiar to Great Britain, where they form the summit of the entire Oolitic series, attaining a total thickness of from 150 to 200 feet. The Purbeck beds consist of arenaceous, argillaceous, and calcareous strata, which can be shown by their fossils to consist of a most remarkable alternation of fresh-water, brackish-water, and purely marine sediments, together with old land-surfaces, or vegetable soils, which contain the upright stems of trees, and are locally known as "Dirt-beds."

One of the most important of the Jurassic deposits of the continent of Europe, which is believed to be on the horizon of the Coral-rag or of the lower part of the Upper Oolites, is the "Solenhofen Slate" of Bavaria, an exceedingly fine-grained limestone, which is largely used in lithography, and is celebrated for the number and beauty of its organic remains, and especially for those of Vertebrate animals.

The subjoined sketch-section (fig. 159) exhibits in a diagrammatic form the general succession of the Jurassic rocks of Britain.

Regarded as a whole, the Jurassic formation is essentially marine; and though remains of drifted plants, and of insects and other air-breathing animals, are not uncommon, the fossils of the formation are in the main marine. In the Purbeck series of Britain, anticipatory of the great river-deposit of the Wealden, there are fresh-water, brackish-water, and even terrestrial strata, indicating that the floor of the Oolitic ocean was undergoing upheaval, and that the marine conditions which had formerly prevailed were nearly at an end. In places also, as in Yorkshire and Sutherlandshire, are found actual beds of coal: but the great bulk of the formation is an indubitable sea-deposit; and its limestones, oolitic as they commonly are, nevertheless are composed largely of the comminuted skeletons of marine animals. Owing to the enormous number and variety of the organic remains which have been yielded by the richly fossiliferous strata of the Oolitic series, it will not be possible here to do more than to give an outline-sketch of the principal forms of life which characterise the Jurassic period as a whole. It is to be remembered, however, that every minor group of the Jurassic formation has its own peculiar fossils, and that by the labours of such eminent observers as Quenstedt, Oppel, D'Orbigny, Wright, De la Beche, Tate, and others, the entire series of Jurassic sediments admits of a more complete and more elaborate subdivision into zones characterised by special life-forms than has as yet been found practicable in the case of any other rock-series.

GENERALIZED SECTION OF THE JURASSIC ROCKS OF ENGLAND.
Fig. 159.

The plants of the Jurassic period consist principally of Ferns, Cycads, and Conifers—agreeing in this respect, therefore, with those of the preceding Triassic formation. The Ferns are very abundant, and belong partly to old and partly to new genera. The Cycads are also very abundant, and, on the whole, constitute the most marked feature of the Jurassic vegetation, many genera of this group being known (Pterophyllum, Otozamites, Zamites, Crossozamia, Williamsonia, Bucklandia, &c.) The so-called "dirt-bed" of the Purbeck series consists of an ancient soil, in which stand erect the trunks of Conifers and the silicified stools of Cycads of the genus Mantellia (fig.160). The Coniferœ of the Jurassic are represented by various