36 HAMPSTEAD HILL. 



ment of organism that pickaxe and spade revealed. Thus have 

 been preserved for the instruction of future generations (the 

 collection is now safe practically for all time in the British 

 Museum) the remains of the animals that lived where London 

 now stands, long before those Alpine peaks we call " the ever- 

 lasting hills " were in existence. 



These animals were all marine ; they all lived in a salt-water 

 sea such as washes the London Clay shores on the coast of 

 Essex to-day. And that sea must have continued receiving 

 muddy sediment during an immense period of time, for the 

 accumulated mass was 500 feet thick. 



The present thickness of the London Clay varies very much, 

 local thinning being due to eroding or denuding agencies that 

 have taken away to the sea by means of rain, streams, and rivers, 

 from areas which are now comparatively low-lying, the greater 

 portion of the original thickness. Besides, however, local 

 thinnings, there is a general diminution of the thickness of the 

 deposit towards the west. The greatest thickness occurs in the 

 Isle of Sheppy, where it is found to be about 470 feet. At 

 Hampstead, where the London Clay is overlain by the next 

 succeeding formation, the Bagshot Sands, there is presumably the 

 full original thickness of the deposit in the Middlesex area, and 

 it is here about 400 feet thick. 



The data given by the details of the sections exposed by 

 wells that have been sunk in various parts of the district show 

 clearly that the thickness depends upon the elevation above sea- 

 level, and that therefore the thinning process has taken place on 

 the top, and, consequently, has been due to denudation, or the 

 wearing away of the surface by weathering action, which at a few 

 points has spared portions of the next newer deposit, and so the 

 original thickness of the London Clay at those places is shown. 

 Thus at Whitaker's Brewery, Camden Town, the London Clay 



