MECHANICAL STATE OF THE ISOSTATIC SHELL 265 



shear. The solid rock is capable of stress orientation. The molten rock 

 is not. 



The deformation of molten rock obeys the laws of flow under hydrostatic 

 pressure. The deformation of solid rock under non-uniform stress beyond 

 the elastic limit proceeds by shearing. Although an excessive stress will 

 be transmitted throughout the mass in all directions, the law of shear 

 will control deformation, and any displacement will take place on planes 

 at 45 degrees to the instantaneous stress^^ (plate 10). 



SUMMARY AS TO THE MECHANICAL STATE 



In conclusion, we may draw from the preceding qualitative survey of 

 the data on the mechanical state of the lithosphere, the following in- 

 ferences : 



1. The internal friction,i or the strength which rocks oppose to stresses 

 tending to deform their masses, increases with pressure. The increase is 

 very rapid in the outer 10 miles of the isostatic shell ; is less rapid below 

 that; but, in spite of the normal rise of temperature, it continues at a 

 rate which may reasonably be assumed to be related to the increase of 

 density. 



2. With reference to the support of the load resting upon anv unit 

 mass of rock within the lithosphere, the strength of rock does not increase 

 as fast as the superincumbent weight. The quotient of the strength 

 divided by the load diminishes in such manner that it reduces to 1 at a 

 depth of about 40 miles, or less making allowance for rise of tempera- 

 ture. At that depth a non-uniform stress, equivalent in pressure to the 

 weight of a column of rock, say, 40 miles high or somewhat less, will 

 cause mass movement. No less stress will. Below that depth an even 

 greater non-uniform stress is required, but the ratio to the superincum- 

 bent load becomes less than one. 



3. The strength of rock in the isostatic shell under normal conditions 

 of temperature is so great that the stresses set up by erosion and sedimen- 

 tation, whose maxima do not exceed an unloading or loading by an 

 amount of 6 miles of rock, are far within the elastic limit and are, there- 

 fore, incompetent to produce mass deformation. 



4. Since the elastic volume of the lithosphere is determined by the 

 balance between compression and thermal expansion, and since the op- 

 posed forces are instantly and constantly active, the lithosphere is very 

 sensitive and responsive to stresses within the elastic limit. 



18 B, Willis: Mechanics of Appalachian structure. U. S. Geol. Survey, 1,3th Ann. 

 Rept.. 1891-1892. See plates xciii and xciv and the explanation thereof. 



