500 MESSES. WHITAKER AND JUKES-BROWNE ON [Allg. I 894, 



rises is owing to the height of the outcrops of that formation, and 

 probably also to the somewhat free passage of water through it, as 

 well as to the fact that the source has now been tapped for the first 

 time in the district. In the case of the Chalk, on the other hand, the 

 nearest outcrop is at the level of the Thames, and a large quantity 

 of water is taken by various wells. One can hardly expect, however, 

 that the high level of the water from the Lower Greensand will be 

 maintained, but rather that it will fall slightly after some time, 

 especially if other successful borings be made. 



[In a communication to this Society, made a short time ago and 

 not published until after this paper had been sent in, 1 Prof. Hull 

 gave it as his opinion that the water came from the southern outcrop 

 of the Lower Greensand rather than from the northern one. Iu 

 this we agree, as the former is broad and unbroken, whereas the 

 latter is narrower and broken. We do not, however, see with him 

 (as one of us said at the time) that there is any evidence that 

 the southern outcrop is divided from the northern one, near 

 Windsor, by the uprise of older rocks (as shown in the section 

 which he exhibited). It may be so ; but, on the other hand, it may 

 not. There may be continuity here, though there is discontinuity 

 under London ; and at present we are without evidence, knowing 

 only that water-bearing Lower Greensand is present. 



The woodcut from this section has been altered to show the 

 continuity of the Gault over the ridge of old rocks, in deference 

 to remarks made in the discussion on the paper. 2 While thanking 

 the author for this courteous acknowledgment, we must be allowed 

 to differ from him in the opinion that " the question is one which 

 is entirely conjectural"; for of the many borings in the London 

 Basin that have been carried deep enough, every one has proved the 

 presence of Gault, and in one only, at Harwich, has it been found 

 to be thin.] 



The following chemical analysis of the water from this boring is 

 not without geological interest, for in containing a comparatively 

 large amount of common salt it follows the rule of many other 

 deep well-waters, both from Cretaceous and from Jurassic beds. 

 The difference between the analyses of \v T ater, from wells sunk through 

 a good thickness of Tertiary beds into the Chalk and from wells in 

 which the Chalk is at or near the surface, has been commented on 

 by one of us, 3 who, however, had not at the time read a paper 

 by Mr. II. Warington, in which much light is thrown on the 

 subject. 4 It seems as if the salts once held in the higher outcropping 

 parts of a permeable formation had been gradually dissolved out by 

 the long-continued passage of water down and through them, such 

 silts being then carried in the water to lower parts, whereas in 

 those parts where the beds are satuiated and from which the water 

 cannot escape the contained salts are retained. 



1 Quart Journ. Geol. Soc. vol. 1. pp. 152-155 (May 1894). 



2 Ibid. p. 154. 



3 Geol. Surv. Mem. 1889, ' Tbe Geology of London and of Part of the 

 Thames Valley,' vol. i. pp. 514-510, 533 and table opp. 



* Journ. Chem. Soc. vol. li. (1887) pp. 544, 545. 



