NOTES OX ISOSTASY 305 



Eeclucing the extension in the colnmn of rock, 60 miles in height, to a 

 daily basis, we have for the ratio of the two extensions, 



e' ^198 n2X10-^^ ^^gQ. 

 e 43 241 X 10-^^ 



in other words, the tape increased in length b}'' a process of flow (elastic 

 after-effect) during the interval, 110-126 days, at a rate which was -160 

 times the elongation in a colnmn of rock 60 miles in height, necessary to 

 produce a momitain range 25,000 feet in height in 5,000,000 years by the 

 flow of material into the base of the colnmn. 



A load of 10 kilograms on the tape produced a stress of about 13,800 

 pounds per square inch, which is the same as the pressure at the bottom 

 of a column of water 31,800 feet (6.0 miles), or a column of rock 12, TOO 

 feet (2.1: miles) in height. Considered as a mechanical property of 

 matter, therefore, there would seem to be nothing exceptional in the de- 

 velopment or erosion of a mountain range by the process of isostatic 

 adjustment. The mechanism is due to the conduction of heat and the 

 redistribution of weight on the crust of the earth brought about by evap- 

 oration, changes in mean annual temperature, deposition of sediments, 

 redistribution of large bodies of water, etcetera. The rocks within the 

 isostatic shell are always under the influence of a system of forces which 

 is never in equilibrium, and the resultant of this system of forces tends 

 always to cause the rocks to flow very slowly, in the direction of least 

 resistance. 



Elastic Yieldixg of the Earth's Crust uxder a Load of 

 Sedimextary Deposits; by Walter D. Lambert 



The strata principally concerned in the elastic yielding of the earth's 

 crust are doubtless chiefly composed of material similar to the basic in- 

 tru^ve rocks. The ranges of the elastic constants of the four specimens 

 of such rock tested by Adams and Coker^ were 



^c = modulus of rigidity from 2.670 X 10^^ to 4.380 X 10"' 



C. G. S. units 

 K = modulus of cubic compressi- from 4.650 X 10^^ to 7.329 X 10^^ 



bility C. G. S. units 



^ r=r Young's modulus from 6.71 X 10" to 10.80 X lO'"^ 



C. G. S. units 



^ F. D. Adams and E. G. Coker : An investigation into the elastic constants of rocks, 

 more especially with reference to cubic compressibility. Washington (Carnegie Institu- 

 tion), 1906, p. 69. 



