138 A TREATISE ON METAMORPHISM. 



enough to make the pores capillary at the smallest section have super- 

 capillary pores in other parts of the section. Hence, it may be said that 

 sandstones and conglomerates the grains of which exceed 3 mm. in diameter 

 have tubes which are greater than those of capillary size. But the grains 

 in the great majority of sandstones average less than 3 mm. in diameter, 

 and hence the pore openings in sandstones are for the most part capillary, 

 and are considered under the next heading. 



It is through openings exceeding those of capillary size — that is, cir- 

 cular tubes larger than 0.508 mm. in diameter and sheet openings greater 

 than 0.254 mm. in diameter — that the rapid circulation of underground 

 water is accomplished. For instance, the openings through which springs 

 of large size issue mainly exceed those of capillary dimensions. 



capillary openings. — Capillary openings include the great majority of the 

 openings of sands and sandstones, many of the openings of fine conglom- 

 erates, many of the openings of porous lavas, and many of the openings 

 produced by fracture. As already noted, the superior limit of size of 

 grains of sands and sandstones composed of grains of uniform size, the 

 smallest opening's of which are capillary, is 3 mm. in diameter. The inferior 

 limit of size are grains, the diameters of which are six times the maximum 

 diameter of subcapillary tubes, or 0.0012 mm. The majority of the par- 

 ticles of most clays, shales, and slates are much smaller than this, and 

 therefore the openings of these rocks are subcapillary. Hence capillary 

 openings in mechanical sediments range from very fine sands to very 

 coarse sands. Many of the openings of fissility are capillary; but the 

 majority of bedding partings, fault openings, and joint openings are partly 

 supercapillary, although often the walls of such fractures are so close 

 together as to make even these openings capillary in part. 



In capillary openings the resistance to flow increases very rapidly as a 

 tube diminishes in size. This is due to the fact, already explained, that 

 the area of contact between the moving liquid and that fixed to the wall 

 increases inversely as the size of the openings. Indeed, the friction between 

 the moving and the fixed liquid becomes the dominant factor in the resist- 

 ance to flowage in capillary tubes. As openings decrease in size, at the 

 diameter at which this factor controls for a given liquid the openings 

 become of capillary size for that liquid. 



