368 SPECIAL GEOLOGY. 



defines its limits. This series measures about eiglit hundred 

 and forty feet in thickness, and constitutes what was long 

 known as the London clay. It is divisible into sixteen beds, 

 and is composed of the BracJclesham series, so called from 

 the identity of the lithological characters of the sands and 

 the species of shells contained therein, with the sands and 

 shells found at Bracklesham Bay; and the Barton series from 

 the clays and fossils of the beds resembling those found at 

 Barton. The next bed forms a massive cliff of fine light 

 yellow sand, striped with ochre and ashy bands : it is the 

 representative of the Headon Hill sands, and is two hun- 

 dred feet in thickness : it contains a seam of small black 

 fli.nt- pebbles, but no organic remains. 



The upper group consists of the lower freshwater and 

 estuary series, and measures about three hundred and fifty 

 feet. The fossils indicate the lacustrine and estuary con- 

 dition of the water in which they were deposited. 



The upper Marine series is an interesting group composed 

 of clays and marls containing an immense quantity of marine 

 shells, as Cytherea, Ostrea, Mactra, Psammobia, Natica, Fusus, 

 Ancillaria, Cancellaria, and Balanus. 



The upper Fresh-water series, which has been much 

 denuded, attains a thickness of 90 feet. The beds are made 

 up of the casts of Paludina, Melania, Melanopsis, and other 

 lacustrine species. 



The total thickness of the entire series is about 1975 feet. 



Feet, 

 Of this the Mottled clavs form 140 



The Bognor series 



The London clay 



Headon-hill sands 



Lower Freshwater and estuary 



Upper Marine series 



Upper Freshwater series 



340 

 840 

 200 

 350 



15 



90 



1975 feet. 



The section of Alum Bay (Fig. 250) is remarkable for the 

 750 feet of variously coloured sands succeeding the Bognor 

 beds, and which represent the London clay series. It is 

 impossible to describe the marvellous shading of these sandy 

 strata, as they are blended together and striped like the pen- 



