FRICTION AND LIMITING STRENGTH OF ROCKS 651 



of engineering practice. The unsatisfactory nature of such data 

 as applied to conditions of stress deep down in the earth's crust has 

 already been pointed out by the writer. 1 The results now available 

 from the observations of Adams and Bancroft supply much needed 

 data for the purposes of geophysics. Quoting from a classical paper 

 by Sir George Darwin, 2 "With regard to the earth we require to 

 know what is the limiting stress-difference under which a material 

 takes permanent set or begins to flow rather than the stress- 

 difference under which it breaks; for if the materials of the earth 

 were to begin to flow, the continents would sink down, and the sea 

 bottoms rise up." In the paper quoted Darwin estimates roughly 

 the stress-difference in the interior of the earth due to a distribution 

 of continental masses corresponding roughly to the actual distribu- 

 tion. For instance, it is estimated that the stress-difference under 

 the continents of Africa and America is at a maximum at more than 

 1,100 miles from the earth's surface and amounts to about 4 tons 

 per square inch. Darwin's conclusion that "marble would break 

 under this stress, but that strong granite would stand" must be 

 modified considerably in the light of the results of Adams and 

 Bancroft, as the limiting strength of the rock material under the 

 enormous pressure at the depth referred to would probably be 

 increased many times. For the purposes of such calculations the 

 curves of Plate I may be employed as they stand. If, for instance, 

 it is desired to investigate the stability of mountain ranges or of 

 continental elevations, the principal stresses at great depths must 

 be derived from the theory of elasticity, making use of elastic con- 

 stants derived from the interpretation of seismological records. If 

 the principal stresses at any point be plotted as zz and xx on such 

 a diagram as that of Plate I, a particular rock material will flow 

 if the point falls between the axis zz and the curve characteristic 

 of the particular rock formation under consideration. The material 

 will be on the point of flowing if the point falls on the curve itself, 

 while the rock will stand the stress if the point falls between the 



1 L. V. King, op. cit., p. 120. 



2 Sir G. Darwin, "On the Stresses Caused in the Interior of the Earth by the 

 Weight of Continents and Mountains," Phil. Trans., CLXXIII (1882), 187-230; 

 Scientific Papers, II (1908), 495. 



