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 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.
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