ON THE CIRCULATION OF UNDERGROUND WATERS. 231 



are brought about by tbe percolation of water through them. But while 

 engaged in work of this kind, one is struck with the great volume of 

 water which the rocks of the earth are capable of absorbing, and a know- 

 ledge of this is important both as regards water-supply and the suit- 

 ability of stone for building purposes. Much information on the subject 

 has already been obtained by the investigations of the Rivers Pollution 

 Commission, the Commission on "Water Supply, and on the Selection and 

 Decay of Stone for the Houses of Pai'liament. Also by the Committee 

 appointed by the British Association for the Investigation of the Circula- 

 tion of Underground Waters, and by Mr. De Ranee, C.E., F.Gr.S., in his 

 book on the Water Supply of England and Wales. 



The method which I have adopted for arriving at the results con- 

 tained in this paper is that recommended by Dr. Sterry Hunt in the 

 Geological and Chemical Essays. ^ The portions of rock selected for the 

 work were struck off by a blow with a hammer. By this means I was 

 enabled to get clear and natural surfaces exposed. The whole of the 

 results obtained are given in the annexed table. 



The Arenaceous JRochs. — The oldest rocks which I have examined are 

 those of the Old Red, and all the specimens were selected personally. 

 Taking first the Old Red Sandstone we get an average specific gravity 

 of 2"61, but the specimens from near Bristol have a decidedly lower 

 specific gravity compared with others which are given. Excluding the flags 

 from Caithness, the volume of water absorbed by a cubic foot of the rock 

 is 0-707 of a gallon, or 59,000,000 gallons to the square mile 3 feet thick. 

 The specimen of flags from Caithness absorbed much less Avater, and I 

 found the same thing to apply to Old Red flagstones from other districts. 

 The conglomerate beds are more absorbent than the sandstones : the 

 average specific gravity is 2-58 and the volume of water absorbed by a 

 cubic foot 0-805 of a gallon, or 67,000,000 gallons to a square mile .3 feet 

 thick. 



I next take the Millstone Grit. The specimens from Bristol, South 

 Wales, and the Forest of Dean were selected personally, and for those 

 from Sheffield I am indebted to the kindness of Dr. Sorby, P.R.S. The 

 Millstone Grit which underlies the Bristol coalfield is something like 

 1,000 feet thick ^; the chemical composition, the mean of five analyses, 

 is as follows : — 



Silica 97-80 



Alumina ......... '47 



Oxide of Iron -80 



Lime ......... -44 



Carbon . -17 



Carbonic acid -39 



Moisture -22 



100-29 



In some of the specimens of this grit, microscopically examined, the 

 grains of silica appear to cement themselves together, and so closely, 

 that it is difficult to distinguish their outline. The gi-ains are, for the 

 most part, sub-angular, and are either colourless or have a slight pink 

 tinge imparted by oxide of iron. The specific gravity averages 2-60, 

 and the amount of water absorbed by a cubic foot of the rock is O'OSO of 

 a gallon, equal to 6,000,000 gallons to a square mile 3 feet thick. 



' Pages 165-7. 



^ Proc. Bristol NaUt/ralists' Society, 1875-6, page 336. 



