256 PLASTIC CLAY FORMATION. 



XVI. 



§ III. 10. PLASTIC CLAY FORMATION. 



In conformity with the nomenclature of M. M. Cuvier, and Brong- 

 niart, a principal division of the tertiary formations, consisting of various 

 beds of sand, clay, marl, and gravel, is distinguished by the name of Plastic 

 Clay. (Argille Plastique). An attentive examination of the general 

 points of resemblance in the physical characters, and organic remains, of 

 these irregular alternations above the chalk, leaves no doubt of their being 

 members of a series of nearly contemporaneous depositions, intermediate 

 between that formation and the London clay*. 



A fine series of beds belonging to this division, occurs at Castle Hill, on 

 the western side of Newhaven harbour, lying upon the chalk chfFs, which 

 are there about fifty feet high. The summit of the hill is broken and 

 rugged, and its appearance differs so remarkably from the smooth rounded 

 surface of the surrounding downs, that the geologist, even at a distance, 

 would suspect the existence of strata of a very dissimilar character to any 

 that exist in its vicinity. Towards the sea it presents a steep declivity, 

 having numerous shelvings, and fissures, occasioned by repeated slips 

 of the strata. On the eastern brow, an ancient circular entrenchment still 

 remains; and from this point the beds extend about one mile westward, 

 where the chalk chfFs are covered only by a thin layer of ochraceous clay, 

 and vegetable mould. These deposits were first noticed by Henry War- 

 burton, Esq. F.R.S., secretary to the Geological Society, &c., and have 



* Geological Transactions, Vol. iv. p. 209. 



