428 JOSEPH BARRELL 



regarded here as the mode of deep rock flowage involved in the 

 isostatic readjustment of unfolded tracts. 



Thirdly, flowage may take place in some minerals, as calcite 

 and ice, by gliding upon the cleavage planes. But such gliding is 

 not regarded as the mode by which the foHated rocks are developed. 

 It requires furthermore far greater force than that which is given 

 by the departures from isostatic equiHbrium. 



A fourth mode of rock flowage is by recrystallization. It is the 

 chief factor, as Van Hise has shown, in the deformation of the 

 crystalline foHates. It is thought that this is also the method by 

 which the asthenosphere yields and that a readiness of recrystalli- 

 zation under unbalanced stresses of a permanent nature is the cause 

 of the weakness of the asthenosphere. 



The vibratory forces transmitted as earthquake shocks and those 

 due to tidal strain, from this standpoint, are both rapid. Under 

 such conditions the asthenosphere could show high order of strength. 

 It is argued that the ease of recrystallization under constant strain 

 becomes more marked the nearer the temperature approaches to 

 that of fusion, or to express it better from the physico-chemical 

 standpoint, the nearer the temperature approaches to the mutual 

 solution point of the constitutents involved. The result is that at 

 such temperatures the rigidity may be high and not greatly different 

 from that at low temperatures, but for permanent stresses the 

 elastic limit becomes low. The movement of continental glaciers 

 with a low surface gradient, accompHshed by recry^stallization, 

 illustrates the condition which it would appear exists to even a 

 higher degree within the asthenosphere. 



This conclusion carries with it the idea that within the litho- 

 sphere the temperature is in general considerably below that of 

 fusion; whereas below, in the thick zone of weakness, the tempera- 

 ture must He close to, or at, that of molten rocks. For fusion there 

 is needed, however, the energy necessary to supply the latent heat 

 and volume expansion. Unless this energy is supplied, the astheno- 

 sphere remains soKd rock, but the least accession of internal heat, 

 or rehef from external pressure, wdll generate a proportionate 

 amount of magma, diffused as Hquid throughout the soKd. To 

 gather into reservoirs temporarily molten, the magma must con- 



