and Subterranean Water Storage. 71 



The facility with which sandstones absorb water is illustrated 

 by the quantity of water they contain both in their ordinary 

 state and when saturated. Even granite always contains a certain 

 percentage of water, and in the dry state is rarely without one and 

 a half pint in every cubic foot. Sandstones, however — even 

 those fit for building purposes — may contain half a gallon per 

 cubic foot, and loose sands at least two gallons. When the water 

 is present in any part of a rock, it readily diffuses itself owing 

 to the force of capillary attraction ; but although the diffusion is 

 rapid from a moist to a dry part of any rock, however close 

 its texture, the rate at which a fluid collects in cavities from 

 a stone not absolutely surcharged is extremely slow. This is 

 especially the case with sandstones. 



Limestones contain very large quantities of water, not only in 

 cavities underground, but in crevices of the rock, in spaces 

 between strata, and in faults. Dry compact limestones contain 

 half a gallon of water in every cubic foot. Bath stone contains 

 at least a gallon, and some magnesian limestones one and a half 

 gallon. Chalk is as absorbent as loose sand, and contains at least 

 two gallons per cube foot when saturated. 



It is not easy to realise the magnitude of these quantities, 

 although the results have been determined very accurately 

 by calculation and experiment. If we limit our estimate to an 

 area of the chalk downs 50 miles in length, 10 miles wide, and 

 300 feet thick, we shall find that the total annual rainfall on the 

 surface (taken at 30 inches per annum) will amount to 225,750,000 

 gallons ; while the water contents of the rock, if only half satu- 

 rated, would be more than 660,000,000 gallons, or nearly three 

 years' total rainfall, and fully twelve years' average supply even 

 if there were no loss by evaporation, and no circulation under- 

 ground. It must be evident then that there is an unlimited 

 power of absorption in such rocks ; and as water is distributed 

 through them rapidly and thoroughly, they may be regarded as 

 large receptacles partly filled, but in which the water is con- 

 stantly in circulation, rising and falling according to the influence 

 of past and present Aveather. The longest succession of the 

 driest seasons can never exhaust them : the heaviest rains re- 

 peated for years can never fill them. Other absorbent rocks 

 exhibit the same general features in a different degree, and all 

 assist in the general circulation, the water-level rising after rain, 

 and sinking by evaporation during drought, so as never to leave 

 the surface either absolutely wet or perfectly dry. 



And this is the case not only with chalk, which is itself 

 absorbent in the strict sense of the word, but with hard, brittle 

 limestones, that seem to admit water only by the cracks and 

 fissures at their surface. I have noticed in the Ionian Islands 



