CH. XVII.] GEOLOGY OF THE THAMES BASIN. 273 



solidify; and lastly in living matter; which, on the whole, 

 tends constantly to increase the solids of the globe, at the 

 expense of its fluid and gaseous components. 



With these conceptions of the general nature of the 

 agents which are now at work in modifying the crust of the 

 earth, it will be possible to start with profit upon a new 

 series of considerations. 



The Thames basin presents, as has been seen, a surface 

 diversified with hills and valleys ; this surface is everywhere 

 covered with a comparatively thin layer of soil, which, in 

 most places has been more or less disturbed by the opera- 

 tions of agriculture, and is then known as "made ground." 

 But beneath this lies the sub-soil, which forms the upper- 

 most part of the solid floor of the basin. It has been seen 

 that this sub-soil varies very much at different places, being 

 here gravel or sand, there clay, in another place chalk, in 

 another a different kind of calcareous rock. Moreover, it has 

 been incidentally mentioned, that these materials are arranged 

 in layers or strata ; so that, if the floor of the Thames basin 

 could be cut down vertically, the faces of the section would 

 present a succession of layers, one above the other. It has 

 been mentioned that quarries and railway cuttings, here and 

 there, afford the opportunity of examining the strata in their 

 natural relations and order of superposition. Sections of 

 this kind afford direct evidence of the structure of the earth 

 for only a very little way below the surface ; but more is to 

 be learned from the deep borings for wells, artesian and 

 other, which have been referred to in Chapter II. 



Such borings have been carried to a depth of 1,300 feet,^ 

 and they show that the subsoil of the Thames basin, in and 



1 A famous boring for water at Kentish Town, in 1854, was carried 

 down to a depth of 1,302 feet. The boring passed through 236 feet of 

 London clay ; 88^ feet of Lower London Tertiaries ; 645 feet of chalk ; 



T 



