308 MINERALOGY OF THE SOUTH OP ENGLAND. 



diated calcarious spar. It contains the following fossil 

 shells: 



Planorbis, much resembling that which Brongniart says 



approaches to P. cornu. 

 Planorbis, two other species. 

 Planorbis, much resembling P. prevostinus. 

 Ampullaria. 

 Cyclostoma. 

 Limneus longiscatus. 



acuminatus. 



corneus. 

 Gyrogonites is the petrified seed of a species of chara* 



This formation is the latest of the flcetz rocks hitherto 

 observed in this island, and it agrees nearly with its cor- 

 responding formation in the Paris basin, with this differ- 

 ence, however, that it contains no siliceous beds. 



5. Alluvial Formations. 



The floetz rocks already described, are in many places 

 covered with various alluvial deposites. In the Isle of 

 Wight and London basins, the alluvium, besides the 

 vegetable earth, clays, marls and sands, pontains a vast 

 quantity of rounded quartpse pebbles, of various kinds 

 and sizes, which are irregularly distributed, in some 

 places forming thick beds, mixed with clay, sand, and 

 small fragments of flints; in others are mixed with shells 

 of various kinds, and sometimes almost without any other 

 substance. This compound is named Flint Gravel.^ 



* Some of these pebbles are evidently fragments of the flint origin- 

 ally belonging to the chalk formation ; but other varieties are of calce- 

 dony and hornstone. Another remarkable class of siliceous pebbles is 



