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GEOLOGY: B. WILLIS 
c. Both lighter and heavier masses are distributed throughout con- 
tinental areas and oceanic basins, but the lighter masses prevail in the 
continents and the heavier masses beneath the oceans; 
d. The prevaiKng structure of deep-seated rocks is a foKated one 
and is the result of crystallization in response to appropriate conditions 
of stress and temperature; 
e. Stresses competent to orient foliation in the elastic earth are set 
up by the effort of hghter and heavier masses to assume positions of 
isostatic equihbrium. These stresses are general, but they are locahzed 
and intensified by unloading and loading of adjacent areas through 
the process of erosion. 
/. Periodic rises of temperature, local recrystallization, and local 
fusion of moderately deep-seated rocks result from the outward flow of 
the internal heat of the earth. 
g. The strength of rocks increases with increasing pressure, as shown 
by Adams, but not as rapidly as the load increases from the surface 
of the earth downward. There is, therefore, a level at which the load 
equals the strength of any rock. Below that level for the strongest 
rock the earth is a potentially f aihng structure, which will fail by recrys- 
tallizing or by shearing under a differential stress which equals the 
critical load. 
h. High temperature and fusion are potent factors in producing failure 
of the rigid lithosphere under its own weight. 
It is obvious that these postulates rest upon the work of many in- 
vestigators, among whom are Hayford and Bowie on gravitation, 
Button, Gilbert, and Barrell on the strength of the lithosphere. Van 
Hise, Leith, Johnson, and Bowen on schistosity, and Chamberlin on 
the origin of the earth and its internal heat. The contribution which 
this paper seeks to make is one toward an explanation of the mechanics 
of the Hthosphere. Its conclusions, if vaHd, bear upon the growth of 
mountains, the deformation of superficial rocks, the distribution of 
volcanoes, the connections between continents, and the permanence of 
ocean basins. 
The argument which lead* to the recognition of the discoidal structure 
of the lithosphere proceeds as follows : 
From an early stage of the growth of the earth onward the lighter 
and heavier masses of the elastic lithosphere have tended toward an 
equilibrium which may be described as an equilibrium in isostatic 
adjustment. It is conceivable that the departure from equilibrium may 
at some time have been such that the stresses set up in the lithosphere 
were adequate to cause flow and mass movement, until a close approach 
