222 T. C. CHAMBERLIN THE GREATER EARTH 



This zone of differential resistance is closely related to the zones of frac- 

 ture and flowage made familiar by the work of Van Hise and Leith. 

 These, however, are primarily matters of texture and structure and sec- 

 ondarily of pressure. Like the preceding, though in a different sense, 

 they are not strictly surficial, as pointed out by Leith in his recent vice- 

 presidential address, though, like the preceding, they are most pro- 

 nounced in the surficial zone. 



Because of the special influences of the surficial zone, it has become a 

 question how far the special forms of yielding seen at the surface are 

 simply expressions of surficial dynamics and how far they are reliable 

 signs of the agencies which primarily actuate the diastrophism. To throw 

 light on this and related questions, a special mode of study has come into 

 use in which the reliefs are made the key to the inquiry, the effort being 

 to deduce from these the amount of surface shortening, the depth, and 

 the under-confLguration of the shell actually deformed, and so reach an 

 approximation to the totality of the deformation in so far as it takes a 

 distinctive surficial expression. This should aid in distinguishing such 

 deformation as is merely an expression of surface conditions from such 

 as constitutes a true index of the primary source of the diastrophism. 

 The results of first trials of this special type of inquiry may be found in 

 the papers of R. T. Chamberlin on the Appalachians of Pennsylvania and 

 on the Rocky Mountains of Colorado.' 



The Gravity-pull within the Earth 



The interior of the earth is pervaded by more intense and more com- 

 plicated dynamical qualities. The factors most needed to fill out our 

 series are those that spring from a continuation of the gravity effects 

 which, in the form of the sphere of control, was our starting point. 

 These now assume two phases, namely, the continued gravity-pull, or 

 attraction of the diminishing sphere, and the rising gravity pressure, or 

 weight of the increasing superincumbent mass. In a sense, these are 

 reciprocals, for as the one declines the other increases. The accelerating 

 power of the earth is greatest at or near the surface. In some cosmo- 

 logical applications it is of acute importance to recognize that this maxi- 

 mum gravity is a variable in the sense that it increases with every stage 

 of contraction of the earth, and this is not to be overlooked by students 

 of relatively recent diastrophism ; for, though the value of shrinkage may 



■^ R. T. Chamberlin : The Appalachian folds of central Pennsylvania. Jour, of Geol.. 

 vol. xviii, 1910, pp. 228-51 ; The building of the Colorado Rockies. .Jour, of Geol., vol. 

 xxvil, 1919, pp. 248-51. 



