33 PHYSIOGRAPHY. [chap. 



a moment be supposed that they lie in a deep hollow 

 anything like that of an ordinary domestic basin, or even 

 like that represented by the curves in the figure. It is 

 true the rocks to the north and south of London slope 

 gently inwards, and thus produce a depression, but it is 

 a depression of the very shallowest kind. " The inclination 

 is indeed so gentle that it can scarcely be shown in a 

 diagram brought within the compass of a page of this 

 book. Hence most diagrammatic sections, as already ex- 

 plained (p. 17), in the description of the basin of Thames, 

 are necessarily exaggerated, and false notions have thus got 

 abroad as to the nature of the London basin. It is only 

 when true sections are drawn, having the vertical heights 

 represented on the same scale as that of the horizontal dis- 

 tances, that its extremely gentle concavity becomes apparent. 

 With this caution, we may proceed to examine more 

 closely the section in Fig. 11. This may be taken to 

 represent an exaggerated section across London, from 

 north to south ; A representing the London clay, and B 

 indicating the position of certain underlying rocks which 

 geologists are in the habit of calling the Lower London 

 Tertiaries. These lower strata are extremely variable, but 

 in the neighbourhood of London they consist for the most 

 part of sandy deposits, and are therefore highly permeable. 

 Thin beds of clay, spread out at different levels in the 

 sands, serve to retain the water ; and the supply, thus stored 

 up, has been utilized by borings carried down through the 

 overlying London clay. Such borings were frequently 

 made in the early part of this century, and an abundant 

 supply of water was obtained from depths varying from 80 

 to 140 feet. So numerous, however, were these wells, that 

 a great drain was thrown upon the water-yielding power of 

 the strata, and the supply ultimately became unequal to the 



