ce. 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 Kimmeridgien_ 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 limeston