CRETACEOUS PERIOD. 541 



The fossils of the Wealden series are, naturally, mostly the 

 remains of such animals as we know at the present day as in- 

 habiting rivers. We have, namely, fresh-water mussels ( Unio], 

 river-snails (Paludina), and other fresh-water shells, with nume- 

 rous little bivalved Crustaceans, and some fishes. 



II. 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 for- 

 mations. 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 Switzer- 

 land. 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, uncon- 

 formably 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. 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 Ba- 



