SEGREGATION OF MATERIALS OF MOUNTAINS. « 927 



stratum, and thus the necessary crustal shortening demanded for a large 

 segment of the earth may be concentrated in a single mountain mass. 

 This theory of the deformation of the zone of flowage by recrystallization 

 meets the needs of the geologist who demands a substratum sufficiently 

 plastic to be deformed under long-continued moderate stresses, and at the 

 same time meets the demands of the physicist for a material so strong as to 

 resist sudden stresses. 



But it may be said that this hypothesis is" a mere speculation, 

 unsupported by any observation and with no possibility of support by 

 observation. This is true if we consider the existing mountains, the last 

 uplift of many and perhaps the majority of which occurred in Tertiary 

 time. But before these mountains existed more ancient mountains existed, 

 and denudation since early geological time has exposed man}^ parts of the 

 lithosphere over vast regions which were once thousands of meters below 

 the surface. For instance, the Appalachian system of the United States, 

 from Maine to Georgia, including the New England and Piedmont 

 plateaus, has undergone deep erosion since Paleozoic time. Vast regions 

 in Canada and in northern Europe have been subject to enormous, although 

 perhaps interrupted, erosion since Cambrian time. In such regions obser- 

 vations show that there now exist at the surface extensive areas of rock 

 which were deformed in the zone of rock flow. 



Is there any evidence in such regions, from the character of the 

 deformation and the structures which have been produced, that shearing 

 has taken place in the same direction for considerable areas? Such 

 evidence would support the view that equivalent shearing may have taken 

 place in the present zone of rock flow, and thus have segregated the 

 existing mountains. In another place I have shown that monoclinal 

 cleavage may result from shearing motion in rocks parallel to the surface 

 of the earth, there being no elongation or shortening in consequence of it. a 

 This is illustrated by fig. 25, where a mass represented by abed in the 

 diagram at the left is sheared into the form shown in the diagram at the 

 right, the lines a-b, c—d in both diagrams being supposed to be parallel to 

 the surface of the earth. The position of the cleavage is indicated by the 

 longer diameters of the flattened ellipsoids. If this sort of deformation 

 should take place over an extensive area in the zone of flow, it might 



a Van Hise, C. R., Estimates and causes of crustal shortening: Jour. G'eol., vol. 6, 1898, pp. 29-31. 



