II. _Lower Greensand_ (_Neocomien_ of D'Orbigny).--The Wealden
beds pass upward, often by insensible gradations, into the Lower
Greensand. The name Lower Greensand is not an appropriate one,
for green sands only occur sparingly and occasionally, and are
found in other formations. For this reason it has been proposed
to substitute for Lower Greensand the name _Neocomian_, derived
from the town of Neufchatel--anciently called _Neocomum_--in
Switzerland. If this name were adopted, as it ought to be, the
Wealden beds would be called the Lower Neocomian.
The Lower Greensand or Neocomian of Britain has a thickness of
about 850 feet, and consists of alternations of sands, sandstones,
and clays, with occasional calcareous bands. The general colour
of the series is dark brown, sometimes red; and the sands are
occasionally green, from the presence of silicate of iron.
The fossils of the Lower Greensand are purely marine, and among
the most characteristic are the shells of _Cephalopods_.
The most remarkable point, however, about the fossils of the
Lower Cretaceous series, is their marked divergence from the
fossils of the Upper Cretaceous rocks. Of 280 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.
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