512 JOSEPH BARRELL 



MODES OF LITHOSPHERIC YIELDING AND THEIR RELATION 

 TO STRENGTH 



The relationship of strength to depth which has been derived 

 in this study and which was expressed in the curve of strength at 

 the end of Part VII is to be connected with the physical qualities 

 discussed in this part. Here it is seen that it is a curve of elastic 

 limit. When that limit is exceeded, permanent deformation must 

 take place; by one means at the surface, by another within the 

 body of the lithosphere, by still another at its base. 



At the surface the typical mode of yielding is by jointing and 

 faulting, in stratified beds by folding also. The movements in this 

 zone of fracture and in the transitional zone of combined fracture 

 and flow may be regarded as merely the responses in a thin, brittle, 

 and relatively weak outer layer to deformative movements pro- 

 gressing in the great thickness of the lithosphere below. But the 

 rocks of deeper origin which have been exposed at the surface by 

 profound erosion show that they have yielded in another fashion. 

 Their foliated structures and crystalline textures testify to yielding 

 by massive flowage. Fracturing appears to have been absent, 

 except in so far as it was produced by intrusions from below, giving 

 rise to complexes of dikes and sheets. These visible exposures 

 suggest that at still greater depths, notwithstanding the great 

 strength of that zone, open fracture planes disappear and rock 

 flowage both by granulation and by recrystallization is still more 

 distinctive. This appears then to be the mode of yielding of the 

 great body of the lithosphere. 



Recently Becker has suggested that fracturing may enter into 

 the problem of isostasy in the following way: The demonstrated 

 capacity of small cavities to remain open under great pressures 

 may permit Assuring and jointing to extend deeper into the crust 

 than had been previously thought possible. To the degree to 

 which fractures and porosities do exist they must decrease the 

 specific gravity of rocks. If shattering pervaded the rocks of one 

 region and not another, even though the rocks were exactly alike 

 in composition, the densities would become different. To give 

 isostatic equilibrium the region of shattered rocks would have to 

 stand higher than the other. This would be the initial effect as 



