926 A TREATISE ON METAMORPHISM. 



unless an equal or a larger mass elsewhere subsides — in short, that the 

 center of gravity of the moved mass is nearer the center of the earth after 

 the movement than before." The expansion theory of mountain making is 

 wholly controverted by the facts of metamorphism. 



But mountain systems have been formed in some way, and during their 

 building the material of the mountains has been uplifted even if material 

 elsewhere has subsided. The uplifted areas are local. Indeed, it is be- 

 cause of localization that uplift can be recognized. As the geoid responds 

 to the stresses within it, at least near the surface, the folding and faulting 

 is largely concentrated in certain belts. It follows that the outer crust of the 

 earth must have moved in some way over the interior. This fact of the 

 localization of mountain masses and the resultant necessity that the crust 

 should shear over the interior is one of the reasons which led the early 

 geologists to adhere strongly to the existence of a liquid substratum. 

 However, the physicists who have studied the earth with reference to tidal 

 deformation insist that its interior is rigid; that if it were a liquid the 

 attraction of the moon and sun would result in tides which would be shown 

 in the crust of the earth. 



It appears to me that the explanation of rock flowage offered on 

 pages 748-759 shows how mountain masses may be segregated, the upper 

 crust of the earth shearing over the deeper seated material for a long 

 way without a liquid substratum. It has been explained that in the zone 

 of anamorphism, under the deforming stresses of the earth, rocks flow 

 by recrystallization. They are solid at the beginning of the process, 

 throughout the process, and at the end of the process; and yet they have 

 accommodated themselves to the form demanded by continuous solution 

 and redeposition. It has further been somewhat fully explained (pp. 

 769-774) that the energy required for such deformation in the zone of 

 flowage is not nearly so great as that required for deformation of an equal 

 mass producing a similar form in the zone of fracture. In other words, the 

 outer shell of the earth, the zone of fracture, is more rigid under slow 

 deforming stresses than is a deeper seated zone which may be deformed 

 by recrystallization. If this be so, we are able to see how the outer part 

 of the lithosphere may shear over the zone below, as if it were a plastic 



a Van Hise, 0. R., Earth movements: Trans. Wisconsin Acad. Sci., Arts, and Letters, vol. 11, 1898, 

 pp. 512-514. 



