266 HISTORICAL PALEONTOLOGY. 



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 Cephalopoda. 



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 Cretaceous rocks, 

 almost every one 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 southeast 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 Planerkalk 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 occa- 

 sionally containing concretions of phosphate of lime, which are 

 largely worked for agricultural purposes. 



V. White Chalk. The top of the Upper Greensand be- 

 comes argillaceous, and passes up gradually into the base of 



