140 



PRINCIPLES OF STRATIGRAPHY 



Porosity of Rocks. 



The porosity of a rock or soil mass is determined by the frac- 

 tional part of the mass occupied by open spaces or voids. (Slichter- 

 37:/(5.) If a cubic foot of sandstone holds, on saturation, one- 

 quarter of a cubic foot of water, the porosity of the sandstone is 25 

 per cent. The following table of tests made by Dr. E. R. Buckley, 

 State Geologist of Wisconsin, shows the variation in the porosity of 

 various building stones of that state : 



The porosity of quartz sand usually varies between 30 and 40 

 per cent., and that of clay loams between 40 and 50 per cent., de- 

 pending on the variety of size of grains in the mixture, and on the 

 manner of packing the particles. ( Slichter, 34, p. 17.) The pore 

 space of fresh, strong granite varies from 0.2 to 0.5 per cent., the 

 absorption of water being 0.08 to 0.20 per cent, by weight. Ordi- 

 nary compact limestone varies from 2.5 to 12.5 per cent, (absorp- 

 tion I to 5 per cent, by weight of water), although the more porous 

 limestone can absorb 10 per cent, by weight of water, corresponding 

 to a pore space of 25.0 per cent. The more compact types of lime- 

 stones, however, fall as low as 0.55 per cent, of pore space. Sand- 

 stones generally have a pore space ranging from 5 to 28 per cent. 

 Chalk has been credited with the ability to absorb 20 per cent, by 

 weight of water corresponding to a pore space of about 41 per cent. 

 (\'an Hise-43 : 12 f,.) The pore space of an organic limestone from 

 the Gulf of Naples was over 35 per cent. (Walther), while recent 

 sediments of the Mississippi delta contained, according to Hilgard, 

 a pore space ranging from 23 to 61 per cent. The actual size of the 

 grains is of less importance in influencing the porosity of the mass 

 than the variation in size within the same mass, or the arrangement 



