132 



A TREATISE ON METAMORPHISM. 



in contact with any grain. (Fig. 4.) In any other than the one direction, 

 where the grains are naturally arranged, the tubes are ordinarily interrupted 

 In any case the continuity of the tubes in straight lines persists so far as the 

 arrangement of grains is by one system of piling. Slichter has shown that 

 the openings in the directions in which the tubes are not straight may be 

 neglected so far as the no wage of water is concerned; he therefore con- 

 cludes the quantity of flowage to be dependent upon the continuous straight 

 or nearly straight tubes. These of course vary in size, but the water 

 may be reckoned as passing through continuous tubes of the minimum size, 

 made by the cross section between three grains arranged in a plane at right 

 angles to the direction of the tubes. Of course it is understood that any 



Fig. 3.— Triangular cross sections of pore space. After Slichter, 



one system of arrangement does not extend indefinitely, and that where 

 one system of packing changes into another there are, ordinarily, bends in 

 the tubes. 



Slichter further shows that the amount of space in mechanical sediments 

 before cementation takes place is largely dependent upon the system of 

 packing. It is also dependent upon the regularity of the grains and their 

 variation in size. The more nearly spherical the grains and the more 

 nearly uniform the size, the greater is the pore space. 



Ordinarily the continuous tubes of mechanical sediments are limited 

 by the boundaries of a stratum or formation. However, a porous formation 

 may extend for hundreds of kilometers and have a thickness of hundreds 



