XVII.] GEOLOGY OF THE THAMES BASIN. 289 



confined at the present day to warmer seas ; Fig. 87, for 

 example, is a fossil Nautilus, a genus which is represented 

 by several species in the London clay. When it is re- 

 membered that, beneath London, this clay is about 400 

 feet thick, it will be readily admitted that such a deposit 

 of fine muddy matter must have required an enormous 

 period of time for its deposition. Moreover, it must be 

 borne in mind, that, in the case of this clay, as of so many 

 other sedimentary deposits, the present thickness does not 

 necessarily indicate its original thick- 

 ness, for much may have been re- 

 moved by denudation. 



Beneath the London clay of the 

 Thames basin, there is a series of 

 comparatively thin beds, known as 

 the Lower London Tertiaries. These 

 come to the surface along the edge 



P ,1 1 • • r .1 Fig. 87. — -Nautihis centralis. 



of the clay, as it rises from the from the London day. 



margin of the basin, but their 



width is too small to admit of representation on the map 

 in Plate V. Their position, however, is indicated in Fig. 11, 

 (p. 31). The uppermost members of the group consist of 

 rolled flint pebbles, which evidently represent an old beach 

 of shingle, associated with sands, which often contain marine 

 shells. These beds are known, from the localities in which 

 they are best exposed, as the Blackheath or Oldhaven beds. 

 They are succeeded, below, by clays, some of which are rich 

 in shells similar to those now living in brackish water; whence 

 it is concluded, that the beds must have been deposited in 

 an estuary. From the two localities in which these strata 

 are typically developed, they are known as the Woolwich 

 and Reading beds. Below these, come the Thanet beds, of 

 which good exposures may be seen between Heme Bay and 



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