Ch. XX.] 



LONDON CLAY. 279 



The arenaceous beds are chiefly laid open on the confines of 

 the basins of London and Hampshire, in following which we 

 discover at many places great beds of perfectly rounded flints. 

 Of this description, on the southern borders of the basin of 

 London, are the hills of Comb Hurst and the Addington hills, 

 which form a ridge stretching from Blackheath to Croydon. 

 Here they have much the appearance of banks of sand and 

 shingle formed near the shores of the tertiary sea ; but whether 

 they were really of littoral origin cannot be determined for 

 want of a sufficient number of sections which might enable us 

 to compare the tertiary strata at the edges with those in the 

 central parts of each basin. 



We have ample opportunities in the basin of Paris of exa- 

 mining steep cliffs of hard rock which bound many of the valleys, 

 and innumerable excavations have been made for building-stone, 

 limestone, and gypsum; but when we attempt to obtain a 

 connected view of any considerable part of the tertiary series 

 in the basin of London, we are almost entirely limited to a 

 single line of coast-section ; for in the interior the regular beds 

 are much concealed by an alluvial covering of flint gravel 

 spread alike over the summits and gentle slopes of the hills, 

 and over the bottoms of the valleys. 



Organic remains are extremely scarce in the Plastic clay 

 but when any shells occur they are of Eocene species. Vege- 

 table impressions and fossil wood sometimes occur, and even 

 beds of lignite, but none of the species of plants have, we be- 

 lieve, as yet been ascertained. 



London clay. This formation consists of a bluish or black- 

 ish clay, sometimes passing into a calcareous marl, rarely into a 

 solid rock. Its thickness is very great, sometimes exceeding 

 500 feet *. It contains many layers of ovate or flattish masses 

 of argillaceous limestone, which, in their interior, are generally 

 traversed in various directions by cracks, partially or wholly 

 filled by calcareous spar. These masses, called septaria, are 

 sometimes continued through a thickness of 200 feet f. 



* Con, and Phil. Outlines of Geol., p. 33. t Outlines of Geol., p. 27. 



