112 J. V. ELSDEN — POST-TEETIAEY DEPOSITS. 



drift gravels seem insignificant in comparison with, the great Chalk 

 water-system, yet it would appear that even the water in the chalk 

 is to some extent influenced by overlying deposits. Not only 

 does the quality seem to be affected in a remarkable way, but the 

 amount of percolation into the chalk is also largely influenced by 

 extensive coverings of impervious boulder-clay, and permeable sands 

 and gravels. Concerning the influence on quality, analyses show 

 that water from chalk covered by clay contains less calcic carbonate 

 and more soda-salts than from chalk not so covered.* As to its 

 influence on quantity, while beds of sand and gravel absorb th.e 

 rainfall and remove it from the influence of evaporation, the im- 

 pervious clays throw off" the water into the natural drainage- 

 channels. Some idea of the amount thus thrown off', including what 

 issues from springs, may be got from the fact that, with an annual 

 rainfall of about 26 inches, the total discharge of waters from the 

 Lea, Beane, Rib, and Mimram at Hertford is estimated at 6159 

 cubic feet per minute, f Turning now to the middle sands and 

 gravels as water-bearing strata in themselves, we cannot but be 

 struck with their influence on the distribution of population. Mr. 

 Prestwich has pointed out that the site of ancient London coincided 

 with the beds of gravel resting on London Clay, and many of the 

 suburban villages were grouped around a spring of water or a bed 

 of gravel whence water could be got by shallow^ wells. | 



In conclusion, I have only to express a hope that, although 

 much must necessarily have been omitted, yet enough has been 

 said to give some idea of the interest which attaches itself to those 

 deposits which, under the name of drift, are so often dismissed 

 from further consideration. 



* ' Proc. Inst. Civ. Eng.,' vol. xlvii, pt. 1. 

 t lb., vol. xiv, p. 42. 



X " Anniversary Address, Geol. Soc, Lond. 1872," ' Quart. Journ. Geol. Soc.,' 

 vol. xxviii, p. liii. 



