At the same time, the Lower and Upper Cretaceous
groups form a closely-connected and inseparable series, as shown
by a comparison of their fossils with those of the underlying
Jurassic rocks and the overlying Tertiary beds. Thus, in Britain
no marine fossil is known to be common to the marine beds of
the Upper Oolites and the Lower Greensand; and of more than 500
species of fossils in the Upper Cretaceous rocks, almost everyone
died out before the formation of the lowest Tertiary strata, the
only survivors being one Brachiopod and a few _Foraminifera_.
III. _Gault_ (_Aptien_ of D'Orbigny).--The lowest member of the
Upper Cretaceous series is a stiff, dark-grey, blue, or brown
clay, often worked for brick-making, and known as the _Gault_,
from a provincial English term. It occurs chiefly in the south-east
of England, but can be traced through France to the flanks of
the Alps and Bavaria. It never exceeds 100 feet in thickness;
but it contains many fossils, usually in a state of beautiful
preservation.
IV. _Upper Greensand_ (_Albien_ of D'Orbigny; _Unterquader_ and
_Lower Plaenerkalk_ of Germany).--The Gault is succeeded upward by
the _Upper Greensand_, which varies in thickness from 3 up to 100
feet, and which derives its name from the occasional occurrence
in it of green sands. These, however, are local and sometimes
wanting, and the name "Upper Greensand" is to be regarded as a
_name_ and not a description.
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