PORE SPACE AND UNDERGROUND CIRCULATION. 155 



capillary size. In the case of the fine soils and clays the pores may be almost 

 wholly snbcapillary, or the water is that of imbibition. In this fact we 

 have the explanation of the retention of soil moisture in fine clays. The 

 moisture is glued to the grains. There is practically no circulation, and 

 the water is removed only by high temperature or high pressure, or the 

 two combined. It follows from the foregoing that, under given conditions 

 with a given pore space, the coarse conglomerates furnish a much larger 

 flow than fine conglomerates, the fine conglomerates a larger flow than the 

 sandstoues, and these a vastly greater flow than the soils, clays, and shales. 



Bedding, fault, joint, and fissility openings may be so close together 

 that the pore space is very large. Ordinarily fault openings are wider 

 spaced but larger than the joint openings, and joint openings are wider 

 spaced and larger than the openings of fissility. It can not be said which 

 kind of opening gives, on the average, the larger pore space. Since, 

 however, large openings are favorable to rapid flow, for a given pore space 

 the fault openings are likely to give a greater flow than joint openings, and 

 joint openings a greater flow than those of fissility. This follows from the 

 greater size of the fewer openings. To this is to be added the element of 

 greater continuity of the larger openings, as explained on pages 130- 

 131. Therefore, with a given pore space the flow may be vastly greater 

 in the case of faults than in the case of joints, and much greater in the 

 case of joints than in the case of fissility. 



In this connection it may be said that the capacity of a rock for 

 imbibition gives a xerj good idea as to its power of transmission. The 

 water of imbibition, it may be recalled (see p. 124), is the amount which 

 adheres to the walls of the openings. It is evident that in rocks containing 

 the same percentage of water when saturated the power . of transmission 

 varies inversely as their capacity for imbibition. If the openings of a rock 

 be very small, but numerous, there is in a cubic centimeter a large surface 

 to which the water can adhere. If the openings be subcapillary, the water 

 of imbibition and saturation are the same and the powers of transmission 

 practically nil. If the spaces be capillary, the water of imbibition is much 

 less and the power of transmission greatly increased. If the spaces be 

 supercapillary, the water of imbibition is slight in amount and the power 

 of transmission very great. 



