252 B. WILLIS DISCOIDAL STRUCTURE OF THE LITHOSPHERE 



beyond the fact that it is heavy and behaves like an elastic solid. This 

 discussion deals only with a relatively superficial shell. 



Mechanical State of the isostatic Shell 

 effect of pressure 



During the past dozen years the experimental researches of Dr. F. D. 

 Adams, of Montreal, and also of the Carnegie Geophysical Laboratory 

 have greatly increased the available data which are helpful in seeking to 

 comprehend the state of rock under high pressure and at high tempera- 

 tures, and it is now possible to speculate somewhat more surely than we 

 previously could regarding the general conditions. The limitations of 

 experiment are such, however, that it is necessary to extrapolate beyond 

 them to a degree which renders inferences suggestive rather than con- 

 clusive. It is with a full appreciation of this qualification that the 

 following discussion is offered. 



Since the publication of Heim's studies of alpine structure^ geologists 

 have recognized that solid rocks "flow" under the unequal pressures which 

 develop in the earth. Since rocks do not flow at the surface, and since 

 the pressure which may cause them to flow is an effect of their own weight, 

 it has been assumed that a condition of plasticity corresponding to flowage 

 develops at some depth in the earth where the weight of the superincum- 

 bent column is sufficient to overcome the rigidity of the rock. 



The terms plasticity and flow involve the idea of mobility, and even 

 when used with the distinct statement that the flowing rock is a rigid 

 solid, they persist in suggesting ease of movement. Gilbert was, so far 

 as I know, the first to postulate that rock may grow stronger, may become 

 more rigid under pressure than it is under surface conditions, and that 

 even where it moves it may present greater resistance to distortion than 

 does rock as we know it. He expressed this view in discussing the 

 conditions of experiments on folding strata in 1887, and the concept 

 which I then gained from him was subsequently phrased as follows :^ 



"The crushing strengths of stones . . . probably increase with depth in 

 the earth's crust and in an unknown ratio ; but it is not likely that the incre- 

 ment of strength is as great as the increment of pressure. ... At five miles 

 below the surface the pressure exceeds the maximum resistance of the rocks 

 at the surface, and at ten miles the pressure is more than double the resist- 

 ance. This means that somewhere between five and ten miles beneath the 

 surface the weight of the superficial crust is suflacient to crush its support. 



5 Albrecht Heim : Mechanismus der GebirgsbUdung, 1878. 



« Bailey WiUis : Mechanics of Appalachian structure. U. S. Geol. Survey, 13th Ann. 

 Report, 1891-92, p. 237. 



