ISOSTASY AS A WORKING HYPOTHESIS 271 



Mobility may be, furthermore, a condition of a mass or of a molecule. 

 Evidently, if mobility be a condition temporarily induced in rock by a 

 rise of temperature which first produces effects of recrystallization, it will 

 be at first a molecular condition. But it may pervade an entire mass, if 

 the rise of temperature affects a sufficient number of molecules to such a 

 degree as to cause general recrystallization, followed possibly by fusion. 

 Barrell expresses the concept of magma diffused through the solid rock- 

 mass. Mechanical conditions would, it seems to me, result in a thinly 

 sheeted, lenticular distribution of newly formed crystals and of the ulti- 

 mate melt. 



Xucleal heterogeneity is a concept introduced by Gilbert into the theory 

 of isostasy in his last paper.^^ Whereas Hayford and Bowie implicitly 

 assume perfect isostatic adjustment to be permanently established in the 

 nucleus, Gilbert recognizes reasons for the existence of masses differing as 

 to density and not necessarily in balance among themselves. His view is 

 based on the independence observed between crustal activity and latitude, 

 on the lack of relation between crustal activity and ^^things visible." For 

 explanation he appeals to ^^primordial heterogeneity of earth material, 

 a heterogeneity which gives diversity to the flow of heat energy and to 

 physical and chemic changes of crustal regions." 



While agreeing with Gilbert in his argument for "primordial hetero- 



slow spreading toward the periphery and consequent accumulation of stress. The resist- 

 ing mass is, furthermore, unloaded by erosion, and herein lies a third important dis- 

 turbing effect, though not a primary cause of diastrophism. The fourth condition of 

 periodic movement inheres in the storage of heat in an outer shell, as deduced by Cham- 

 berlin, the effect being generally to lower the internal resistance of the masses and pro- 

 mote the tendency to plastic flow and locally to induce special conditions of melting and 

 extrusion. More or less effective combinations of these effects appear to be inherent in 

 the heterogeneous structures of the lithosphere, and to be adequate to account for the 

 diversities of local diastrophism, as well as the universality of deformation during the 

 great epochs. . . . 



"If the molecular rearrangements occur in the zone of contact of two adjacent masses, 

 one of which is denser than the other, the net result of all the movements will be from 

 the denser toward the lighter under the directive influence of the heavier. There will 

 thus be established a stress or tendency to flow which in course of time may become 

 suflicient to overcome the resistance of the solid rock and occasion actual flow from the 

 suboceanic toward the subcontinental mass. It is conceivable that this molecular process 

 is only one of several which may give rise to a similar movement, but in view of the 

 rearrangement of minerals in the development of flow cleavage in deep-seated rocks, 

 recrystallization Is a process of notable potency and appropriate orientation." 



Even though Barrell misunderstood these statements, he later came to recognize the 

 potency of recrystallization as a mode cJf flow and named it, along with rising tem- 

 perature, as a principal cause of movement in the asthenosphere : 



"A fourth mode of rock flowage is by recrystallization. It is the chief factor, as Van 

 Rise has shown, in the deformation of crystalline foliates. It is thought that this is 

 the method by which the asthenosphere yields, and that a readiness of recrystallization 

 under unbalanced stresses of a permanent nature is the cause of the weakness of the 

 asthenosphere." (Strength of the earth's crust, page 428.) 



28 Op. cit., 1913, p. 35. 



