Notices of Memoirs — The Water- Supply of London. 415 



while the actual height of the curve is known to vary in accordance 

 with the variation in the rainfall, in many cases as much as 50 to 

 80 feet or more. Where the conditions are favourable to a large 

 underground reservoir, the springs hardly ever run dry. Mr. Beard- 

 more, as the result of many years' observations in the Chalk district 

 of the Lea, sees reason to believe that the storing power of the Chalk 

 hills there holds out at least 16 months. 



Further, some of the water below the lines of permanent level 

 inland has a slow underground movement to still lower levels, unless 

 intercepted or thrown out by faults in the strata or by some other 

 cause. This underground drainage is not, however, coincident with 

 the surface drainage ; and while some of the water-bearing strata of 

 the Thames basin are not available as underground sources of supply 

 by means of wells at or near London, other strata, on the contrary, 

 out of the London basin, are so available from the circumstance of 

 the dip of the beds being towards London. 



Where the permeable strata only cap the hills the springs issue of 

 course on the sides of the valleys at the junction of the impermeable 

 strata. 



In the order of superposition the highest permeable strata near 

 London, excluding the superficial beds of gravel, are — 



The Bagsliot Sands, which are from 100 to 350 feet thick, and 

 extend over an area of 211 square miles. As these strata consist 

 almost entirely of loose quartzose sands, the underground water 

 oozes out commonly at their junction with the London clay, and is 

 rarely conducted into any particular channel of escape so as to form 

 springs, and the loss by evaporation is large. There are, in fact, 

 throughout this area no springs of any imjDortance ; only a few small 

 tributaries of the Thames and the Wey have their sources in this 

 district, and the supply to the wells is not large. The water gene- 

 rally is soft and pure, but in some places it is ferruginous. We 

 cannot look to these sands for any additional water-supply (although 

 they attracted a good deal of attention a few years since), for the 

 whole of the water now delivered by them passes into the Thames 

 or the Wey. None passes elsewhere underground. 



The London Clay underlies the Bagshot Sands and forms a great 

 impermeable bed from 400 to 450 feet thick. 



Loioer Tertiary Sands. — These beds, which are only from 50 to 

 100 feet thick, are of no importance so far as springs are concerned 

 at their outcrop, but they have been useful sources of supply to some 

 of the deep wells under London. Owing, however, to the great 

 increase in the number of these wells, and the fall in the level of 

 the water, the underlying Chalk is now generally resorted to as the 

 better source of supply. In many places round London, where they 

 have not been so drawn upon, they still yield a good supply of water. 



The Chalk, from its large area (1,047 square miles above Kingston, 

 but more than double that in the whole basin), from its great thick- 

 ness — 500 to 1,000 feet — and from its peculiar lithological character, 

 forms a very important source of water-supply, both by springs and 

 by means of wells. Almost all the rain faUiog on its sui-face is 



