PORE SPACE IN ROCKS. 127 



of soil and sand are not spherical in shape, or of uniform size. In so far as 

 the grains vary from regular forms and uniform magnitude the pore space 

 would be less than calculated; but in so far as the method of packing is 

 not the most compact possible the pore space would be greater than calcu- 

 lated. Thus these two factors neutralize each other to a considerable 

 degree, and we are obliged to turn to experiment to ascertain approxi- 

 mately the facts. It is probable that King's experimental results" on sands 

 composed of well-rounded grains of nearly uniform diameters, where the 

 pore space was between 32 and 40 per cent, represent approximately the 

 original pore space in the coarser assorted mechanical sediments. The 

 more porous sandstones, where the pore space, as ascertained by Buckley, b 

 varies from 18 to 28 per cent, have a crushing strength varying from 172 

 to 413 kilograms per square centimeter; indeed, are strong enough to serve 

 for building stones. It is clear that a considerable amount of cementing 

 material has been added, and that the pore space measured is much less 

 than the original space in the sands before cementation. Hence it appears, 

 both from experimental work by King and by deductions from actual 

 measurements of the space in partially cemented sandstones, that the 

 original pore space in clean, well-assorted sands probably varies from one- 

 fifth to as much as two-fifths, with a probable average of about one-third. 



It is much more difficult to give a statement as to the average pore 

 space of the lavas. Some of these rocks are rather dense and had orig- 

 inally a very small amount of pore space; Others are exceedinglv vesicular 

 and originally had pore spaces amounting to 50 to 75 per cent, or even 

 more. It is rather probable that where a succession of thin-bedded basic 

 lavas are piled up one on the other, as in the Keweenawan of the Lake 

 Superior region, the pore space averages as much as in ordinary sandstones ; 

 but from this maximum the average runs clown as the lava flows become 

 thicker and as they become more acid. Therefore the average pore space 

 of the vesicular lavas is probably not more than one-third to one-half as 

 great as in the mechanical sediments. 



It is even more difficult to make an estimate of the amount of pore 

 space due to fractures in the rocks, such as faults, joints, fissility, the op'en- 



°King, cit. , pp. 147-157. 



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

 Hist. Surv. No. 4, 1S98, pp. 393-395, 402-103. 



