1 84 ALEX. L. DU TOIT. 



A microscopical examination of the material indicated: — 



1. That the water is found equally in very fine and in 



moderately coarse grained sandstones. 



2. That the fine sandstones are rather even in texture, 



the mean diameter of the particles ranging from 

 0*15 to 0*2 millimetres. 



3. That in the coarser sandstones, though there are 



grains up to 1 or 2 mm. in diameter, or even more, 

 their relative proportion is low, the bulk of the 

 rocks being composed of grains ranging from 0*2 

 to 0*4 mm. in size, while there are many smaller 

 particles that help to fill up the larger pores, and 

 hence bring down the average diameter appreci- 

 ably. 



Altogether, considering both fine and coarse varieties, 

 the mean effective size of particles may be taken to possess 

 a diameter of from 0*2 to 0*3 mm., especially if one takes 

 into account that the grains are never well rounded, that 

 there are always smaller angular fragments present, and 

 that the particles are bound together by a certain, though 

 perhaps sometimes negligible, amount of rather incoherent 

 kaolinic cement. 



The following formula for the transmission of water 

 through a bed of sandstone is borrowed from Slichter: — l 



a- P^s 



y -96MB(l-m) 

 where q equals the volume of water in cubic feet per 

 minute, flowing through s square feet of sandstone of 

 porosity m, and effective diameter of grain d (in milli metres), 

 under a head of p feet of water through a horizontal dis- 

 tance of h feet ; B represents a variable dependent upon the 

 closeness of packing of the grains. 



1 C. S. Slichter, U.S. Geol. Surv., 19th Ann. Kept, pt. 11, p. 322, 1899. 



