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.
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