272 VAN HISE—METAMORPHISM. OF ROCKS AND ROCK FLOWAGRE. 
cending columns resulting from heat abstracted from the rocks due to 
depth alone.* 
(0) Openings in rocks may be divided into (1) openings which are larger 
than those of capillary size, (2) capillary openings, and (8) subeapillary 
openings. For water, openings larger than capillary openings may be 
tubes which exceed .0508 centimeter in diameter, or may be sheet open- 
ings, such as those furnished by faults, joints, etcetera, the width of which 
exceeds .0254 centimeter. To movement of water in such openings the 
ordinary laws of hydrostatics apply. Capillary openings for water solu- 
tions include those which, if tubes, are smaller than .0508 centimeter in 
diameter, or, if sheet spaces, are narrower than .0254 centimeter, and | 
-which in either case are larger than the openings in which the molecular 
attractions of the solid material extend across the space. To the move- 
ment of water in openings such as these the laws of capillary flow apply. 
By subcapillary openings are meant those in which the attraction of the 
solid molecules extends from wall to wall. Such openings in the case 
of tubes are those smaller than .00002 centimeter in diameter, or, if 
sheet passages, are below .00001 centimeter in width. 
Large openings are favorable to a somewhat direct course. This may 
be illustrated by the limestone regions, where there are numerous large 
joints and caves within which the water is quickly concentrated, and 
thus the underground course of the water is very largely in the upper 
part of the zone of fracture (see page 294). Small openings are favorable 
to a circuitous route. In order, for instance, to pass from one point to 
another, a wide range of openings must be used in order that consider- 
able flowage shall occur. Large openings are favorable to rapid flowage ; 
small openings are unfavorable to rapid flowage. ‘This results from the 
rapid increase in resistance per unit volume with decrease in the size of 
the openings. In capillary tubes this resistance increases greatly with 
decrease of size, and in subcapillary openings the resistance is so great 
that the flowage practically ceases. 
(c) The elements entering into viscosity are the concentration of the 
solutions and the temperature. The more concentrated the solutions, 
the greater the viscosity ; but as the underground solutions of water are 
not strong, this is probably not an important element. The viscosity of 
water is inversely as the temperature, being only about one-fifth as much 
at 100° C. as at 0° C. Since the resistance to flowage of underground 
water is largely due to the internal friction of viscosity, it is seen that 
temperature is a factor of the greatest importance in its circulation. As 
the underground temperatures are 100° C. ormore at comparatively mod- 
*The sea mills of Cephalonia, by F. W. Crosby and W. O. Crosby: Technical Quarterly, vol. 9, 
1896, pp. 6-23. 
