584 A TREATISE ON METAMORPHISM. 



shown by springs. These facts are important in connection with the varia- 

 tion in the size of the openings. The earlier movements are in the capillary 

 openings, where the resistance to rapid flow would be great ; but here the 

 movement is slow, and consequently the friction is very small. (See p. 

 581.) As the water passes into the supercapillary branch and trunk 

 channels, the important factor in friction is not that between the moving 

 water and the adherent films, but the internal friction. T he total friction 

 in these larger channels is almost indefinitely small as compared with the 

 friction of the same amount of water moving at the same rate through the 

 same area subdivided among capillary openings; for the area of contact, 

 and therefore the friction between the moving liquid and that fixed to the 

 walls, is inversely as the size of the openings. (See pp. 136-137.) 



An underground circulating system is ideally illustrated in many arid 

 and semiarid regions. In the United States it is especially well illustrated 

 in the Great Basin and in southern California, In these regions the early 

 parts of the courses of many streams are in mountain gorges, where 

 usually considerable streams are above ground. As the streams leave 

 the gorges and pass out to the plains their courses are over their own 

 deposits, consisting of great alluvial fans of material, coarse and fine, 

 which is called "wash." The streams at these places commonly pass 

 underground, and there, as shown by drilling for water, follow some- 

 what definite channels. Lower in the course of the drainage systems a 

 large part of the underground water frequently issues in a series of springs 

 or by seepage on a large scale, and continues its course above ground. 

 The gravel and coarse sands furnish a close approximation to a" homoge- 

 neous medium and ideally illustrate the laws of flowage given on pages 

 129-152, 572-576. Such an underground circulation as the above differs 

 from an overground system in that the boundaries of the water courses are 

 indefinite, in that the movement as compared with the movement over- 

 ground is exceedingly slow, and in that the underground cross sections 

 are necessarily much larger. The work of Professor Slichter shows that 

 the ground water of the Arkansas River flows in gravels at a rate not 

 greater than 3 to 5 meters a day." Supposing the rate of movement of the 

 overground streams to be 10 kilometers an hour, or 240 kilometers a day, 

 and supposing the rate in the underground streams in the Great Basin or 



"Slichter, C. S., The motions of underground waters: Water-Sup. and Irr. Paper Ivo. 67, IT. S. 

 Geol. Survey, 1902, p. 43. 



