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.