WATER-SUPPLY 217 



sources, it is controlled by the geological structure 

 of the district. Although any supply of water 

 depends primarily on the rainfall in the catchment- 

 area, it is the relative porosity of the strata, their 

 retentive capacity, and the position of their out- 

 crops that together determine the position of springs, 

 and whether such springs shall be permanent or 

 intermittent. In cases of supplies obtained from 

 some underground porous strata, it is the geologist 

 who informs the well-sinker as to the thickness of 

 strata through which he will have to bore before 

 water is obtained, and who by a careful delineation 

 of geological boundaries knows whether the supply 

 is likely to prove lasting. From his knowledge of 

 the strata he would be able to state whether the 

 water was likely to be fresh or salt, hard or soft, 

 and in addition he would be in a position to advise 

 what steps should be taken to preserve it from 

 contamination. 



It is obvious, therefore, that detailed mapping 

 of the geological formations at the surface, and 

 calculations as to their underground position and 

 extent, are 'necessary for a proper understanding 

 and conservation of our supply of pure water. 



Fortunately, many of our large towns are situated 

 on impervious strata that rest upon porous rocks of 

 which the outcrop is situated beyond the region of 

 possible contamination, such for instance as London 

 and many towns in central and eastern England. 

 The London Clay and other deposits overlying the 



