288 B. WILLIS DISCOIDAL STRUCTURE OF THE LITHOSPHERE 



let it be pointed out that the non-uniform stress is not supposed to cause 

 recrystallization. That is the work of rising heat energy. With regard 

 to the elastic stress it is postulated only that in a state where the tem- 

 perature of the minerals is rising to the point of solution or of melting, 

 as in Wright's experiment, it is sufficient to cause a mineral to pass into 

 solution or to melt in the locus of maximum pressure before it does so in 

 that of least pressure. Eecrystallization in a form better adapted to 

 the environment is assumed to occur. The system is, therefore, close to 

 equilibrium, and a very small difference of stress would probably be 

 effective.*^ 



(d) It may with reason be regarded as probable that the elastic stress, 

 which accumulates very slowly, is steadily converted into a permanent 

 strain, which, being within the elastic limit, is equivalent to a separation 

 of the molecules in the direction of elastic stress. The energy thus taken 

 up is potentialized in the mass and, as argued by Van Hise, is available 

 for service, as, for instance, to promote chemical reaction.** The direc- 

 tion of this force would be the same as that of the original stress, and it 

 would therefore have the same effect in orienting recrystallization. 



Thus it appears not unreasonable to assume that the elastic stress cir- 

 cuit, which constitutes a condition which has pervaded the heterogeneous 

 lithosphere since its earliest development, has directly or indirectly acted 

 to orient foliation in rocks in process of recrystallization. This is par- 

 ticularly true of deep-seated rocks in the zone where the weight of the 

 load exceeds the strength of rock and high temperatures are liable to 

 variation. 



DISTRIBUTION OF FOLIATION 



In the preceding discussion a certain role is assigned to the elastic 

 stress set up in the lithosphere by loading and unloading due to erosion 

 and deposition. That role consists in orienting foliation in a vertical 

 plane, in a curve which rises from under the loaded mass into an adjacent 

 unloaded mass. Clearly, if this be the case, the outcrop of the foliation 

 in a horizontal plane — that is, its strike — should conform to the outlines 

 of the loaded and unloaded masses. The outlines may be straight or 

 curvilinear, but, in the nature of the case, since we are dealing with 

 more or less deformed rounded bodies, they will commonly be curved. 



Suess traced the winding leitlinien, or directrices of the continents, in 

 his great survey of the face of the earth. He linked in one chain, which 



*3 John Johnston and Paul Niggli : The general principles underlying metamorphic 

 processes. Chicago Jour, of Geol., vol. 21, 1913, p. 206. 



** C. R. Van Hise : Treatise on metamorphism. U. S. Geol. Survey, Monograph 47, 

 1904, pp. 46-47 and 690-692. 



