CHAPTER IV. 



THE LONDON CLAY ITS ORIGIN, EXTENT, AND GEOLOGICAL 



POSITION. 



The London Clay, as a rule, is too stiff for ordinary brick- 

 makino- operations, but where it has been exposed to weathering 

 action for a long time it may be used for brick and especially 

 for tile-making. The Upper Sandy London Clay is, however, 

 extensively employed, as at the large brickyard below Caen 

 Wood. The ordinary clay is used at West Hampstead and at 

 Willcsden Green, and a brickyard some years since was worked 

 at Child's Hill. Brick Earth of the Thames Valley, a much newer 

 deposit than the London Clay, is, however, the usual material 

 employed at the brick-works near London. 



The primary origin of clay is to be found in the decomposition 

 of the felspar composing in part granitic and other felspathic 

 rocks, by the action of the carbonic acid gas of the atmosphere, 

 but a knowledge of the conditions of the accumulation of any 

 particular bed of clay forming, in whole or in part, what is known 

 as a " geological formation " can only be approached by attention 

 being given to the organic remains found embedded in it, and by 

 an acquaintance with its geographical extension and relative 

 position to other formations. Though the London Clay bears 

 that name on account of its constituting so much of the district 

 around the metropolis, it has a much greater extension than the 

 neighbourhood of London or even of the Thames Valley. 



