Ixxiv rROCEEDlNGS OF THE GEOLOGICAL SOCIETY, [May 1 899, 



reproduces that old, old section of the London Basin, with the Lower 

 Greensand forming a thick and continuous base. I must forbear, 

 however, from entering on the controversial question of London 

 water-supply, and may leave it to our Foreign Secretary (Sir John 

 Evans) to say what he thinks of a scheme that proposes taking some 

 hundred or more million gallons a day from wells in the Chalk, in 

 addition to the amount already obtained in that way, within the 

 metropolitan district. 



Not only do we find that beds pierce 1 at great depths often have 

 a character different from that which they put on at their outcrop, 

 but also that waters found at great depths often vary much in their 

 mineral contents from those in the same beds much nearer the 

 surface. A well-known case of this sort is that of the waters 

 in the Chalk under London, where the Chalk is thickly covered by 

 Tertiary beds, those waters differing greatly from the waters 

 in the bare Chalk northward and southward, in the increase of 

 alkaline salts and the decrease of lime-salts. Having, however, 

 alluded to this subject some years ago in a Geological Survey 

 Memoir, and again lately, in an address to another Society,^ I need 

 not enter into it now. 



Other like cases have been described in waters from Jurassic beds, 

 as at Swindon and at Woodhall Spa, in both of which a large amount 

 of common salt occurs, while in the latter case there is a regular 

 mineral water. It is found, too, that waters from wells in the 

 sandy beds of the Wealden Series often contain a goodly proportion 

 of carbonate of soda. 



Such matters, and the occurrence of mineral waters generally, 

 point to the need of alliance with chemists, and the advantage of 

 getting full analyses of well-waters, which show the mineral 

 contents and do not merely refer to organic purity or impurity. 

 With this help we may be able not only to trace the origin and 

 history of a water, but may also some day learn something of those 

 slow, quiet, unseen changes that go on underground, through the 

 agency of water in the rocks : a subject of which, I think, we know 

 little as yet, at all events in this country. 



Although water-supply seems to be the chief sanitary subject in 

 which the knowledge of the geologist is often required, there are 



^ Mem. Geol. Surv, * The Geology of London & of Parts of the Thames Valley/ 

 vol. i (1889) pp. 514-516, 533, with folding table ; and Trans. Herts. Nat. Hist. 

 Soc. Yol. X, pt. i (1898) pp. 11-13. 



