DEVELOPMENT OF THE DISCOIDAL HYPOTHESIS 287 



If, owing to a rke of temperature, recrystallization occurs under these 

 stresses, the foliation will he oriented accordingly and will develop a 

 curved structure, hending upward from under the loaded mass toward 

 the surface of the unloaded mass. 



The preceding is a conclusion on which rest all further deductions 

 regarding the structure of border zones subject to erosion and deposition 

 of sediment. It should therefore be examined with care. To reconsider : 



(a) It is assumed that a group of bodies, heterogeneous as to density, 

 has attained a state of isostatic equilibrium and consequently was affected 

 throughout by balanced stresses. It is certainly doubtful whether the 

 hypothetical equilibrium has ever been reached, but a state approaching 

 it doubtless has been, and any residual strains existing in the masses 

 would represent the preceding movements toward adjustment. They 

 would therefore be in the direction of flow, upward in the lighter, down- 

 ward in the heavier element, and from the heavier toward the lighter 

 below. These are the directions of elastic stress due to erosion. It fol- 

 lows, therefore, that any unbalanced residual strain in a system tending 

 toward isostatic equilibrium, but simultaneously subject to transfer of 

 load, is oriented in the direction of the elastic stress concurrently set up 

 by erosion and deposition. 



(h) It is assumed that erosion and deposition do set up elastic stresses 

 in the direction described. It being demonstrated by experiment that 

 rocks under atmospheric pressures are approximately as elastic as glass*^ 

 (E, or Young's modulus of elasticity for plate-glass, 10,500,000; for 

 granites, 5,685,000 to 8,295,000; for basic intrusives, 9,000,000 to 15,- 

 650,000) ; it being determined by seismic observations that elastic vibra- 

 tions are transmitted in the lithosphere rapidly, as through steel; and 

 the isostatic shell being in a state of vibrant sensitiveness to stress, as 

 shown in the preceding discussion, there can be no question but that the 

 underbodies transmit the elastic stresses due to slow unloading and 

 loading. 



(c) The adequacy of the non-uniform elastic stresses to direct the 

 orientation of crystal growth is perhaps open to question. In Wright's 

 experiments a cube of wollastonite glass, for example, was weighted and 

 heated to a state of viscosity at which crystallization began. "It was 

 then in a state of fair rigidity and capable of supporting a certain amount 

 of unequal strain." The amount of strain is not given, but it evidently 

 was within the elastic limit. In the case of foliation in the lithosphere, 



" F. D. Adams and E. G. Coker : An investigation into the elastic constants of rocks, 

 more especially with reference to cubic compressibility. Am. Jour, of Sci., 4th series, 

 vol. xxii, 1906, pp. 121-122. 



