650 LOUIS VESSOT KING 



down from an initially unstrained state 1 at each stage of the process. 

 The constants of plasticity and internal friction, as determined by 

 the present investigation, refer to rock which is being made to flow 

 continuously. This state of affairs, however, approaches more 

 nearly to that occurring in nature during slow geological deforma- 

 tions than to conditions existing when the rock is broken down from 

 an initially unstrained state. 



Under ideal conditions the curves for the observations taken 

 with the nickel-steel jackets of the two wall thicknesses should be 

 identical. Actually, however, they differ to some extent, indicating 

 that the effect of stresses set up by the deformation of the nickel- 

 steel has not been entirely eliminated. The two sets of observa- 

 tions are, however, sufficiently close to give approximate estimates 

 of the relation between the principal stresses which must exist 

 before the rock can be made to flow under conditions existing in 

 the earth's crust. It will be noticed from the curves of Plate I that 

 for the harder rocks, such as diabase and granite, the curves along 

 which breakdown takes place show the existence of a very large 

 coefficient of internal friction. Since the hydrostatic pressure is 

 given by ^(2xx-\-zz), this is equivalent to the statement that the 

 stiffness or limiting shearing stress required to break down the 

 rock increases with the hydrostatic pressure to which the rock is 

 submitted. In other words, we come to the important conclusion 

 that the stress-difference required to break down rock material under 

 conditions of pressure existing in the earth's crust increases with the 

 depth. In the application of this result to geophysical problems, 

 the foregoing conclusion may have to be somewhat modified to 

 take into account the rise of temperature with depth. It is highly 

 desirable that further experiments be carried out with a view to 

 ascertaining the influence of this factor. 



NOTE ON APPLICATIONS TO GEOPHYSICAL PROBLEMS 



Up to the present the only quantitative data available for use 

 in geodynamical problems have been obtained by crushing cubes 

 of various rocks in a testing machine according to the ordinary rules 



1 Compare Karman's observations on marble and sandstone, Zeit. des Vereins 

 deutscher Ingenieure, October 21, 191 1. 



