616 PHYSIOGRAPHY 



nothing to do with folding, and one speaks involuntarily of a broad 

 elevation, since here extensive masses have been brought above sea- 

 level. From a physiogeographic standpoint we cannot determine 

 whether this elevation is identical with a centrifugal movement of 

 the mass with reference to the earth as a whole; the answer to this 

 question lies in the hands of geodesy and geophysics, which alone 

 may with certainty determine the degree of mobility possessed 

 by the different levels of the earth's crust. 



The latter are certainly not rigid, but so long as we do not know 

 the degree of contraction suffered by the terrestrial sphere during 

 the course of geologic time, we never go further in geomorpho- 

 logical considerations of elevations and depressions than to refer 

 them to sea-level, since the position of this surface determines all 

 physiographic work upon the earth's surface. From the geomor- 

 phological standpoint one cannot say more than that the physical 

 surface of the earth, however mobile it may be with reference to 

 sea-level, possesses certain peculiarities which we cannot assume 

 it to have acquired during present times alone. Among these 

 peculiarities the one of most significance is its geometric charac- 

 teristic as a complex of slopes. This has a physical basis in the 

 small strength of the rocks as compared to the attraction of gravity. 

 Even where we see steep projections, these show themselves to be 

 transitory, they fall down and form slopes entirely without the 

 cooperation of running water. We may, therefore, consider that it 

 is impossible that there ever have been such overhanging rock- 

 masses at the earth's surface as might correspond to the recumbent 

 folds exhibited in mountain-masses. Such bold stratigraphic folds 

 can only have originated at considerable depths. The processes 

 of folding, whose products we meet with in folded mountain ranges, 

 appear to us to be deep-seated, a conclusion already reached twenty 

 years ago by G. K. Gilbert also. No doubt these folds were repre- 

 sented on the surface by other results which must have been super- 

 ficial and could not have extended down to an unlimited depth, for 

 a stratigraphic folding of the rocks indicates simply a decrease in 

 area which is not connected with a decrease in volume, since rocks are 

 but slightly compressible. The rocks must spread themselves out 

 upward or downward to correspond with the compression that they 

 suffer by folding, and since this redistribution of mass has taken 

 place only in moderate amount, the processes of folding must have 

 been limited to comparatively thin layers. 



A second important peculiarity of the land surface is that its greater 

 features present a surface of isostatic equilibrium. Measurements of 

 the length of a geographical degree and pendulum observation have 

 long since shown that the elevated masses upon the earth's surface are 

 compensated by decrease of mass below them. The whole surface of 



