578 A TREATISE ON METAMORPHISM. 



Chamberlin and others in connection with artesian waters;" but the best 

 illustrations of complex systems have been worked out in connection with 

 ore deposits, and in the chapter on that subject a number are discussed. 



Thus far the full effect of gravity upon the character of the circulation 

 has not appeared. The discussion has been carried on as if the movement 

 of the ground water were controlled by simple pressure. But gravity is 

 ever pulling the water downward. This force, in the early part of the 

 jour-ney, accords with the downward movement produced by head, and the 

 result is that the water follows a deeper course than it would if moving 

 through a homogeneous medium, in a manner similar to that of an electric 

 current. 



INCREASE OF TEMPERATURE WITH INCREASE OF DEPTH. 



It has been repeatedly stated that the normal increase of temperature 

 with increase of depth is 1° C. for each 30 meters. It has been pointed out 

 (pp. 140-141) that the viscosity of water decreases as the temperature 

 increases, and that the flowage of water is relatively rapid in proportion as it 

 has low viscosity. Indeed, in capillary openings it has been shown (p. 139) 

 that the flowage is inversely as the viscosity. When it is remembered that 

 the pore spaces of the ordinary mechanical sediments are of capillary size, 

 it is seen that this principle is very important in the flowage of ground 

 water. At a depth of 1,350 meters, supposing the increment of temperature 

 to be normal, the ground waters would have a temperature of 45° C. greater 

 than at the surface. At a depth of 2,700 meters the ground waters would 

 have a temperature of 90° C. greater than at the surface. The viscosity 

 of water at 45° C. is only about one-third as much as at 0° C, and 

 at 90° C. is only one-fifth as much as at 0° C. (See p. 141.) In the lower 

 part of the zone of fracture the temperature of water varies between 

 100° C. and the critical temperature of water, 365° C. (See pp. 566-5'i9.) 

 If we suppose that at temperatures above 90° C. the viscosity decreases at" 



« Chamberlin, T. C, Artesian wells: Geol. of Wisconsin, 1873-79, vol. 1, 1883, pp. 690-691; also 

 vol. 2, 1878, pp. 149-152. Leverett, Frank, The water resources of Illinois: Seventeenth Ann. Kept. 

 U. S. Geol. Survey, pt, 2, 1896, pp. 802-803. Hill, R. T., Geography and geology of the Black and 

 Grand prairies, Texas: Twenty-first Ann. Kept. IT. S. Geol. Survey, pt. 7, 1901, pp. 398-101. 



