258 HISTORICAL PALAZONTOLOGY. 
reason it has been proposed to substitute for Lower Greensand 
the name /Veocomian, derived from the town of Neufchatel— 
anciently called Veocomum—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 thick- 
ness 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 phy- 
sical break as well; for the Gault is often, if not always, un- 
conformably 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 com- 
parison 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 Cretacecus rocks, 
almost every one died out before the formation of the lowest 
Tertiary strata, the only survivors being one Brachiopod and a 
few Foraminifera. 
Ill. 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; Unterguader 
and Lower Plinerkalk of Germany).—The Gault is succeeded 
upward by the Upper Greensand, which varies in thickness 
from 3 up to roo 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 
