0 species of fossils
in the Lower Cretaceous series, only 51, or about 18 per cent, pass
on into the Upper Cretaceous. This break in the life of the two
periods is accompanied by a decided physical break as well; for the
Gault is often, if not always, unconformably superimposed on the
Lower Greensand. 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. The group consists, in Britain,
of sands and clays, sometimes with bands of calcareous grit or
siliceous limestone, and occasionally containing concretions of
phosphate of lime, which are largely worked for agricultural
purposes.

V. _White Chalk_.--The top of the Upper Greensand becomes
argillaceous, and passes up gradually into the base of the great
formation known as the true _Chalk_, divided into the three
subdivisions of the chalk-marl, white chalk without flints, and
white chalk with flints. The first of these is simply argillaceous
chalk, and passes up into a great mass of obscurely-stratified
white chalk in which there are no flints (_Turonien_ of D'Orbigny;
_Mittelquader_ of Germany). This, in turn, pa