BENEATH THE LONDON CLAY. loo 



wards or N.W. from tliis, we come to lines in succession in which 

 it would rise lo, 20, 30 feet above the O.D., while to the S.E. 

 we should come to lines along which it would rise to levels 10, 20, 

 etc., feet below the O. D. There is much to be learnt from these 

 maps, which the limitations of time and my proper subject 

 prevent me from now entering on. I would merely remark 

 further, that these lines, if correctly laid down, enable us to 

 determine the general direction of the flow of the water in the 

 chalk at any place, which, as the water must move from the higher 

 to the lower level — that is, from the place of higher pressure to 

 that of lower — cannot deviate much from the direction at right 

 angles to the line of equal level through that place. 



As to the quality of the water in the chalk, considering the 

 extent and position of the gathering ground on the chalk-hills 

 which surround the Thames basin, there is obviously little chance 

 of any organic impurity polluting it; and if there were such pollu- 

 tion in certain localities, it would in its long course through the 

 chalk beds clear itself, both by dilution with water from uncon- 

 taminated sources and by becoming oxidised, and so rendered 

 innocuous. Its organic purity is, therefore, unquestionable. We 

 should expect, however, to find a considerable amount of inorganic 

 and mineral matter dissolved in the water. For, though chalk is 

 almost insoluble in pure water, water which contains (as rain water 

 always does) a certain quantity of carbonic acid readily dissolves 

 it. Chalk (chemists tell us) is carbonate of lime (calcium carbon- 

 ate), with about 2 per cent, of clay and a little silica ; sometimes 

 also containing small quantities of magnesia and calcium chloride. 

 The carbonic acid of the water combines with the carbonate of 

 lime to form bicarbonate of lime, which is soluble in water. Thus, 

 in the chalk water a considerable amount of calcium may be 

 expected to be held in solution, and very little else ; and we 

 should probably expect a priori that, as the water remains in the 

 chalk, very little variation would be found in its constitution in 

 different localities. Such, however, as I shall proceed to show, is very 

 far from being the case, even in wells in which the entry of all 

 water from the surface or from strata above the chalk has been 

 carefully and effectually guarded against. 



Let us first examine water from wells in the chalk, where it is 



