134 A TREATISE ON METAMORPHISM. 



irregular fractures of rocks. In rocks where the openings are exceedingly 

 irregular in form the flowage of water is limited by the continuous openings, 

 however small they may be. 



Irregular openings may be of any form. In the lavas they are fre- 

 quently spherical or ovoid. In the compact rocks they are confined to the 

 very minute, exceedingly irregular interspaces between the mineral par- 

 ticles, which apparently are in perfect contact. As already seen, in the very 

 vesicular lavas the pore space may vary from a small per cent to a very 

 large* amount, even to 75 per cent or more. The openings are 'more likely 

 to be continuous where the pore space is large than where it is small. But 

 even where the pore space is very large the openings of lavas are not nearly 

 so continuous nor the minima of the tubes so large as in sands. In the 

 igneous rocks and in the rocks metamorphosed under deep-seated conditions 

 the openings are minute; they are controlled by the form of the grains. 

 They are, therefore, very irregular and discontinuous. 



SIZE OP OPENINGS. 



The size of the openings is very important in the circulation of ground 

 water. The size of openings must be discriminated from the amount of 

 pore space. The amount of pore space may be the same in two cases, but 

 in one the openings may be very few and large, and in the other very 

 numerous and small. The flowage in the two cases, other conditions being- 

 equal, is very different. For a given mass of water the internal friction, 

 both within the moving water and between the moving and fixed water 

 increases very greatly as the openings decrease hi size. It is, therefore, 

 necessary to consider the various classes of openings in reference to size. 



Upon the basis of size openings in rocks may be divided into (a) open- 

 ings larger than those of capillary size, or supercapillary openings: (b) 

 capillary openings, and (c) openings smaller than those of capillary size, or 

 subcapillary openings. 



For water, openings larger than capillary opening's, according- to 

 Daniell," may be circular tubes which exceed 0.508 mm. in diameter, or may 

 be sheet openings, such as bedding partings, faults, joints, etc., the widths of 

 which exceed one-half of this, or 0.254 mm. To movement of water in such 



"Darnell, Alfred, A text-book of the principles of physics, 3d ed., Macmillan Co., New York, 

 1895, pp. 315-317. 



