274 PHYSIOGRAPHY. [chap. 



about London, is everywhere made up of beds of gravel, 

 sand, and clay of varying thickness ; these rest upon a thick 

 bed of chalk ; and, beneath this, follow strata of sand- 

 stone, hardened clay, and calcareous rocks of a totally 

 different character from the chalk. Whether we travel to 

 the north, the south, and the west, or the east of London, 

 we find the bed of chalk, which underlies London at a depth 

 of upwards of 300 feet, coming up to the surface. In other 

 words, the layer of chalk, beneath the Thames basin, is bent 

 up, on all sides, in such a manner as to have the form of a 

 very shallow dish,^ the bottom of which is covered with 

 horizontal layers of sand and clay, while its eastern end 

 is notched by the estuary of the Thames. Passing over 

 the upturned edges of the layer of chalk on the north, the 

 west, and the south, other rocks, as we have seen, lie at 

 the surface ; and, some of these, such as the greensand 

 and the gault, are of the same nature as those which 

 follow on the chalk in the vertical borings. It is obvious, 

 therefore, that the stratum of chalk lies on the greensand 

 and gault strata, just as a basin fits inside one a size larger. 

 In the western part of the Thames basin, it has been seen 

 that the subsoil rocks consist of limestones, sandstones, and 

 clays. These beds are found beneath the chalk, greensand, 

 and gault, some distance to the eastward. Underneath 

 London, however, they are absent ; for the borings which 

 have been carried deep enough to traverse the chalk, green- 

 sand, and gault, enter rocks which are unlike any found at 



13^ feet of upper greensand ; 130^ feet of gault ; and 1 88 J feet of clays, 

 sandstones, and conglomerates, whicli were of doubtful age. 



1 The form into which the laytr of chalk is bent is altogether inde- 

 pendent of that of the Tliames basin itself, although the two happen, 

 to a certain extent, to correspond. Every area drained by a river has a 

 more or less dish-like form, whatever may be the arrangement of the 

 strata which constitute its floor. 



1 



