28 PHYSIOGRAfuv. [chap. 



Holywell, Bagnigge Wells, and other localities. Wells 

 supplied from this gravel constituted for centuri&s the sole 

 water-supply of the metropolis, and Professor Prestwich has 

 well pointed out how " the early growth of London followed 

 unerringly the direction of this bed of gravel." ^ As long 

 as this was the case, settlement was quite impossible where 

 the gravel was absent and the clay exposed. Indeed, it 

 was not until an independent source of water was supplied 

 by the great water-companies, that a population was 

 established on the clay-districts of Camden Town, St. John's 

 Wood, Netting Hill, &c. 



Along the banks of the Thames and its tributary streams 

 there is a bed of valley- gravel, at a lower level than that to 

 which reference has just been made. This low-lying gravel 

 also forms a source of water-supply to shallow wells, and has 

 determined the site of many centres of population. Thus, 

 Westminster, Battersea, Hammersmith, Brentford, Eton, 

 Maidenhead, and many other towns up the Thames, were 

 originally dependent upon this source for water. 



From the cases hitherto considered, namely, those in which 

 a porous rock rests upon one that is not sensibly porous, it 

 is desirable to advance to the case in which the porous 

 material is not only supported, but is also covered, by an 

 impermeable stratum ; the pervious substance being thus 

 inclosed between two impervious beds, one forming its 

 floor and the other its roof Thus, the sandy stratum B, 

 in Fig. 8, is supported by one bed of clay, C, and covered 

 by another, A. As long as the strata remain in the 

 horizontal position here represented, the rain which falls 

 upon the surface of A is effectually prevented from reaching 

 the porous material B, save only through any cracks which 



1 Anniversary Address to the Geological Society 1872, Quart. Joiirn, 

 Cc'ol. Soc. vol. xxviii. No. no. p. liii. 



