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.  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.