CHARACTER OF OPENINGS IN ROCKS. 129 



meter. Supposing* the pore space for the upper part of the zone of fracture 

 to be one-fifth of that suggested by Dana and to diminish to zero at the 

 lower part of that zone, this would give an average pore space for that 

 zone of 0.69 per cent. Supposing that the zone of fracture extends to a 

 depth of 10,000 meters and that the pore space is saturated, the amount of 

 contained water, if concentrated to the exclusion of rock, would make a 

 sheet 69 meters thick, extending throughout the continental areas. This 

 calculation is of course made upon an hypothetical basis (see pp. 569-571), 

 but it shows that the underground water is truly a great subterranean sheet. 

 This subterranean sheet may be compared to the blood of an organism, 

 and the comparison has force to the degree that it is the chief medium 

 through which the transformations of the rocks are accomplished. 



CIRCULATION OF GROUND WATER. 



Subterranean water must be considered from two points of view — 

 its circulation and its work. 



The actual ground-water circulation depends upon the openings in the 

 rocks, the forces producing water circulation, and the forces opposed to 

 circulation. 



OPENINGS IN ROCKS. 



The rate and amount of flowage of water is largely dependent upon 

 the openings in rocks. The openings in rocks in reference to flowage 

 need to be considered from the following points of view: The form and 

 continuity of the openings, the size of the openings, and the percentage of 

 openings, or pore space. 



FOKM AND CONTINUITY OF OPENINGS. 



For a given cross section, in proportion as an opening approaches a 

 circular form — that is, as it approaches a minimum of wall area per unit of 

 volume — the flow increases, because the friction between the moving water 

 and the film of fixed water upon the walls is less per unit volume. In 

 proportion as the openings are continuous in rocks the flow increases. 



The openings in rocks include (1) those which are of great length and 

 breadth as compared with their width, and thus are essentially flat parallel- 

 opipeds; (2) those in which the dimensions of the cross sections of the 

 openings are approximately the same, and therefore resemble tubes of 

 various kinds; and (3) irregular openings. 

 mon xlvii — 04 9 



