Notices of Memoirs — The Water- Supply of London. 419 



The Oxford Clay is impermeable, and attains in Oxfordshire a 

 thickness of 400 to 500 feet. 



The Great Oolite and subordinate beds may for our purposes be 

 taken together. They form in Oxfordshire an important group of 

 permeable strata 250 to 300 feet thick. They have a collecting area 

 of about 300 square miles, and give rise to a number of fine springs 

 amongst which those of Ampney, Bibury, Box well, and Thames 

 Head have been described by Mr. Bravender and Mr. Brown, and 

 are stated by Mr. Pole to have been yielding at the time of his visit 

 probably not less than 10 million gallons of water daily. 



Most of the springs of this series are thrown out by the Fullers 

 Earth, an impermeable bed of no great thickness in this district — 40 

 to 60 feet — and persistent only over a limited part of the area. 



The Inferior Oolite and underlying sands reposing on the Lias form 

 another important water-bearing formation. They are from 300 to 

 320 feet thick, and extend in the Thames basin over about 180 square 

 miles. As the hills of this formation rise 230 to 300 feet above the 

 valleys, and have a considerable range, the head of underground 

 water is large, and furnishes several important and perennial springs, 

 such as those of Syreford and the Seven Springs, of which the yield 

 is stated by Mr. Pole to have been at the time of his visit from three 

 to four million gallons of water daily. 



These various Oolitic strata consist of beds of rubbly limestones, 

 soft freestones, sands and fissile sandstones, through which the water 

 passes chiefly by fissures ; and although often traversing a great 

 thickness of strata, it is not filtered to the same extent as it is in the 

 Chalk and Lower Greensands. 



Mr. Hull has shown ^ that the Inferior Oolite and underlying sands 

 which in the neighbourhood of Cheltenham are about 320 feet thick, 

 thin off as they range eastward, and probably die out about the centre 

 of Oxfordshire. In the same way he shows that the Great Oolite 

 and accompanying beds, there about 300 feet thick, also thin to the 

 eastward, and they apparently do not extend more than a few miles 

 further east than the Inferior Oolite. 



It follows that the underground passage of water through these 

 Oolites, which might, had these formations ranged in full force east- 

 ward, have been carried as far as London, the dip being in that direc- 

 tion, is stopped by the thinning out of those beds, and by the closing 

 in, as it were, of the Lias, Oxford Clay, and Kimmeridge Clay. 

 Although, therefore, the surface drainage of the Cirencester and 

 Bampton districts runs into the Thames valley, the subterranean 

 water channels are intercepted and do not reach London, and the 

 Oolitic series must be excluded as a possible source of supply by 

 deep wells in the London district. 



The exact proportion of the rainfall absorbed by the different per- 

 meable and porous strata, and which is given out again in the form 

 of springs, has yet to be determined. It varies according to the 

 lithological character of the water-bearing strata. The general 

 results are, however, known in many cases. Thus the annual flow 

 ^ Quart. Journ. Geol. Sue. Vol. xyi. p. 63. 



