PORE SPACE IN ROCKS. 125 



POKE SPACE OF ROCKS. 



The pore space of rocks varies from a small fraction of 1 per cent to 50 

 per cent or more. The pore space in compact, strong, igneous rocks is 

 exceedingly small. For instance, in fresh, strong granites the percentage 

 of water absorbed by the dry rock varies from 0.08 to 0.20 per cent, which 

 corresponds to a pore space of 0.20 to 0.50 per cent. The more compact 

 limestones also contain very little pore space. Some of them absorb as 

 small an amount as 0.20 per cent by weight of water, which corresponds to 

 a pore space of about 0.55 per cent. 



Ordinary compact limestones used for building material, when satu- 

 rated, contain from 1 to 5 per cent of water by weight, and this corre- 

 sponds to a pore space of about 2.5 to 12.5 per cent. The more porous 

 limestones are capable of absorbing 10 per cent or more of water by weight. 

 Sandstones are ordinarily very porous, holding from about 2 or 3 to 15 

 per cent of water by weight. This corresponds to a pore space of from 

 about 5 to 28 per cent Capacity to hold about 10 per cent by weight, and 

 therefore a pore space of about 20 per cent, is very common in sandstones. 

 The extreme of porosity for sandstones yet reported is the Dunnville sand- 

 stone of Wisconsin, which, according to Buckley, contains a fraction more 

 than 28 per cent of air space when dry," and therefore when saturated is 

 capable of having 28 per cent of its volume occupied by water. According 

 to Merrill, 6 chalk may contain as much as 20 per cent by weight of water. 

 Supposing the specific gravity of the chalk to be 2.8, this corresponds to a 

 pore space of about 41 per cent. However, in coherent rocks, pore spaces 

 of more than 25 per cent are rather uncommon. 



In unconsolidated rocks where cementation has not taken place at all, 

 and in products of the belt of weathering, the pore space may be even 

 greater than the above amounts. If grains of sand are spherical, of uniform 

 size, and " are arranged in the most compact manner possible, each grain 

 will touch the surrounding grains at twelve points." c In this case the pore 

 space will be 25.95 per cent.'' If the particles be spherical, of uniform 



o Buckley, E. R., Building and ornamental stones of Wisconsin: Bull. Wisconsin Geol. and Nat. 

 Hist. Survey, No. 4, 1898, p. 225. 



ft Merrill, G. P., Rocks, rock-weathering, and soils, Macmillan Co., New York, 1897, p. 198. 



cSlichter, C. S., Theoretical investigation of the motion of ground waters: Nineteenth Ann. Rept. 

 U. S. Geol. Survey, pt. 2, 1899, p. 306. 



^Slichter, cit., p. 310. Becker, G. F., Geology of the quicksilver deposits of the Pacific coast: 

 Mon. U. S. Geol. Survey, vol. 13, 1888, p. 399. 



