258 



FOSSILS OF LOWER GREENSAND. [Ch. XVIIL 



Fig. 296. 



Perna Mulleti. Desh. in Leym. 

 a. Exterior. &. Part of hinge of upper valve. 



In the south of England, during the accumulation of the Lower Green- 

 sand above described, the bed of the sea appears to have been continually 

 sinking, from the commencement of the period, when the freshwater 

 Wealden beds were submerged, to the deposition of those strata on which 

 the gault immediately reposes. 



Pebbles of quartzose sandstone, jasper, and flinty slate, together with 

 gTains of chlorite and mica, speak plainly of the nature of the pre-existing 

 rocks, from the wearing down of which the Greensand beds were derived. 

 The land, consisting of such rocks, was doubtless submerged before the 

 origin of the white chalk, a deposit which originated in a more open sea, 

 and in clearer waters. 



The fossils of the Lower Cretaceous are for the most part specifically 

 distinct from those of the Upper Cretaceous strata. 



Among the former we often meet with the genus Scaphites (fig. 297) 



Fig. 297. 



Fig. 29S. 



Scaphites gigas, Sow. Syn. Ancyloceras gigas, D'Orb. 



JS^autihis plicatus. Sow., in 

 Fitton's Monog. 



