154 A TREATISE ON METAMORPHISM. 



The greater the speed of the moving water the greater the internal 

 friction, because of the differential movements both in the moving water 

 and between the moving water and the films fixed to the walls. Where 

 the rate of movement is sufficiently slow the internal friction due to viscosity 

 drops to an almost inappreciable factor. Therefore where the movement 

 is very slow, even if the passages be long and small, the pressure due to 

 head may diminish very slowly. Indeed, nearly the full pressure may be 

 maintained for long distances — many or even hundreds of kilometers. 

 This principle is of the utmost importance in the flowage of ground water, 

 and its applications are later developed. (See pp. 585-588.) 



GENERAL STATEMENTS. 



In general it may be said that in proportion as the driving forces, 

 gravity, mechanical action, etc., are great, circulation is likely to be rapid. 

 In proportion as the opposing force, internal friction, is great, circulation is 

 likely to be slow. In proportion as the openings approach the circular 

 form, circulation is likely to be rapid. In proportion as the openings are 

 continuous, the circulation is likely to be rapid. In proportion as the pore 

 space is great, circulation is likely to be rapid. 



However, of all these various factors dependent upon the character of 

 the openings, that of size is probably the most important; for rocks which 

 do or do not readily transmit water may have the same proportion of pore 

 space. For instance, if the grains be supposed to be spherical, of the same 

 size, and arranged in the most compact fashion possible, the unoccupied 

 space is 0.26 of the entire space, without reference to the size of the grains. 

 Thus the relative proportion of the openings in a great bowlder conglom- 

 erate and a fine-grained clay may be the same. But the capacity for the 

 transmission of water by the former will be indefinitely greater than by the 

 latter. As illustrating this, an experiment showed that a quartz sand, the 

 water of saturation of which was the same as that of a certain chalk, trans- 

 mitted water under a certain pressure six hundred times as fast as the chalk." 

 In the compact soils, the particles of which are exceedingly small (see 

 pp. 138-146), the openings between the particles are of capillary or sub- 



"Prestwich, Joseph, Geology, chemical, physical, and stratigraphical, Clarendon Press, Oxford, 

 vol. 1, 1S86, p. 159. 



