MAPPING OF THE CRYSTALLINE SCHISTS 86 1 



rocks of well-developed flow structures and well-marked fracture 

 structures independent as regards their direction of the flow 

 structures, is strong evidence that the deformation by fracture 

 was subsequent to that by flow. Moreover, in the very nature 

 of the problem it is to be assumed that a fracture structure 

 will in most cases be superimposed upon a flow structure, owing 

 to the fact that degradational forces, as they continuously 

 remove the upper layers of the lithosphere, gradually relieve the 

 load upon the deeply buried rocks, until these are at last 

 exposed to our view. This, however, is not the only reason 

 why we should assume a fracture structure to be the later when 

 found in conjunction with a flow structure, for fracture struct- 

 ures are quickly effaced by flow, whereas flow structures, though 

 disturbed and distorted, are not obliterated by fracture 

 structures. 



THE MECHANICS OF DEFORMATION BY FLOW. 



The mechanics of this problem is a large subject fully treated 

 elsewhere 1 and may well be largely omitted from this discussion. 

 Deformation by flow produces flexuring or folding of the initially 

 horizontally bedded rocks. The position and the shape of the 

 individual flexures will be determined by many factors : as, the 

 position of old shore lines with their planes of weakness, and the 

 initial dip of the beds resting upon them ; the position of masses 

 of igneous material (which by refusing to yield as readily as the 

 bedded rock may elevate a portion of the latter above the zone 

 of flow and cause rupture by thrust at the planes of maximum 

 mashing); the position of earlier planes of rupture or foliation, 

 etc. Flexuring is thus the method by which the heavily loaded 

 rocks accommodate themselves to lateral compression, and results 

 in a crustal shortening and thickening. In every fold there is a 

 local compression normal to the beds in the limbs of the fold, 

 and a local tension in the same relative direction in the arches, 

 the tendency of which is to cause a local transfer of rock mate- 



1 Willis, " Mechanics of Appalachian Structure," Thirteenth Annual Report U. S. 

 Geo/. Survey, Part II, 1891-2, pp. 211-282. Van Hise, loc. cit., pp. 589-668; see 

 also Suess, Antlitz der Erde, Vol. I, pp. 142-189; MargERIE ET Heim, loc, cit., 

 49-87. 



