146 A TREATISE ON METAMORPHISM. 



which a portion of the particles are larger than this have subcapillary 

 openings, since the larger openings are occupied by grains as small as or 

 smaller than the above dimensions. The great majority of the clays, 

 shales, and slates are largely composed of particles smaller than 0.0012 

 mm. in diameter and their openings are subcapillary. Minute openings 

 between the grains of the igneous rocks and of the rocks metamorphosed 

 to schists and gneisses are also usually subcapillary. Where practically 

 all of the openings are subcapillary, whether they be the openings of 

 sedimentary, igneous, or metamorphic rocks, such rocks constitute practi- 

 cally impervious strata; for the contained water is in fixed films held by 

 molecular attraction, and the circulation, as already explained, is so slow as 

 to be negligible during short time intervals. 



PERCENTAGE OF OPENINGS, OR PORE SPACE. 



The percentage of openings in the rocks, or the pore space, is a func- 

 tion of the number and the size of the openings. In so far as the openings 

 in rocks are large and numerous, there is a large pore space. It has 

 already been seen (pp. 124-129) that the absolute amount of openings in 

 rocks, as shown by observation, varies from a small fraction of 1 per cent to 

 over 50 per cent. The larger the pore space the more favorable the condi- 

 tions for circulation, but since the variation in pore space is so great it is 

 evident that the flowage of water dependent upon porosity is very variable. 

 Water passes readily through rocks which contain much pore space; water 

 does not flow to an appreciable extent through rocks which have a small 

 fraction of 1 per cent of pore space. Other factors being the same, and the 

 pore space of the same character, the floivage is in direct ratio to the amount of 

 pore space. 



FORCES PRODUCINU WATER (JIRCl'LATIOX. 



The forces producing circulation of ground water are gravity, heat, 



mechanical action, molecular attraction, and vegetation. The dominant 



force, upon which the movement of ground water mainly depends, is gravi- 



tative stress. 



Gravity. — Gravity ever tends to pull the water downward. And this 



never-ceasing force at work throughout the zone of water circulation, on 



the average continuously carries the circulating water to lower levels. This 



condition of affairs is analogous to the work of gravity in earth movements." 



a Van Hise, C. R., Earth movements: Trans. Wisconsin Acad. Sci., Arts, and Letters, vol. 11., 1898, 

 pp. 465-516. 



