CHAF. XI.] HANTONIAN PERIOD. 223 



this time is a very difficult point to determine. We know, 

 however, that in Wiltshire the London Clay thins out and 

 is replaced by sands (see Tig. 5), so that probably the sea 

 did not extend very far west of Marlborough, perhaps no 

 farther than the present position of the Chalk escarpment ; 

 thence it must have trended south-west to the western 

 border of Dorset, and north-east outside the present limits 

 of the Chalk, to the western border of Norfolk. The great 

 plain of the Fenland was then, of course, covered by a 

 continuous sheet of chalk, and there is no reason to sup- 

 pose that the Eocene strata overlapped this either west- 

 ward or northward. The probable limits of the sea at this 

 time are indicated on Plate XI. 



The London Clay is a purely marine deposit, and does 

 not exhibit any traces of fluviatile action till we reach its 

 highest beds, but the material of which it consists is 

 usually supposed to have been brought down by some 

 large river. Sir Charles Lyell thought that this river 

 might have drained a continent that lay to the west or 

 south-west of Britain, and Mr. J. S. Gardner l has accepted 

 this view, though he does not urge any positive evidence in 

 support of it. The London Clay does not assume a more 

 estuarine facies either to the west or the south-west, but 

 seems merely to thin out and to be partially replaced by 

 sands. When we remember, indeed, that the locality 

 which has yielded the most distinct evidence of fluviatile 

 action lies to the east of London, there seems to be rather 

 a greater probability that the river which carried the fruits 

 to Sheppey came from an eastern quarter and drained 

 some portion of the European continent. 



Here, however, we must ask whether the phenomena of 



the London Clay really require the agency of a single large 



river, and whether they cannot be equally well accounted 



for on the supposition that several, if not many, rivers 



1 " Proc. Geol. Assoc.," vol. ri. p. 92. 



