BENEATH THE LONDON CLAY, 153 



chalk. It has been estimated by the Rivers' Pollution Commis- 

 sion (6th Report, p. 298), that there are, within 30 miles of 

 London, areas covered by the chalk formations (including in this 

 the upper green sand below the true chalk, which, however, covers 

 a comparatively small area), extending over 635 square miles on 

 the north, and 213 miles on the south of the London clay 

 formation; and within 40 miles of London, 1,298 square miles on 

 the north, and 301 on' the south of the same. Of the rain which 

 falls on this very large extent of surface, such part as is not lost 

 by evaporation, or taken up by vegetation, is absorbed by the 

 chalk. Chalk is capable of absorbing about one-tenth to one- 

 eighth of its weight of water, and when thus saturated, any 

 additional water under pressure would slowly pass through it and 

 fill the joints and fissures in the rock. It is a necessary conse- 

 quence that below a certain level in the Thames basin, at any 

 given time, the chalk is thus saturated, and its fissures filled with 

 water. The height of this level varies with the seasons and the 

 greater or less amount of rainfall, and is determined mainly by the 

 level of the higher springs, which feed the rivers (like the Colne) 

 which run through the chalk district. Below this the chalk must 

 always be saturated, and the only effect of variable rainfall is to 

 slightly increase or diminish the pressure in the water, which is 

 slowly moving through it. Of course if the chalk were everywhere 

 sealed up by clay above it except at its outcrop, the water in it 

 could only escape at the outcrop, and below would remain motion- 

 less in its reservoir ; but the Thames, in its course both west and 

 east of London, crosses uncovered chalk, whence it receives large 

 supplies of water, while there is probably considerable drainage 

 from the chalk into the Thames estuary and the sea itself to the 

 east. It is plain therefore that, from this natural cause, as well as from 

 the artificial drafts upon it from Artesian wells, there must be a flow 

 of water from the higher to the lower parts of the chalk formation. 

 We have, then, I think, satisfactorily accounted for the quantity 

 of water which is found in our county when a boring is made into 

 the chalk from the surface. Whether or not we conclude with 

 the Rivers' Pollution Commissioners, that we have here, within a 

 circuit of 40 miles from the metropolis, a natural reservoir of pure 

 water, amply sufficient for its present wants, as well as for a long 



