RELATIONS TO OTHER SCIENCES 615 



lacking are old. The sequence in development, when elaborated in 

 greater detail, is called by Davis the geographic cycle. It leads us 

 from the stage of youth, where the elevated surface still predomin- 

 ates, through the stage of maturity, with its surface of adjustment, 

 to the stage of old age with its surface of maximum resistance. 



The dependence of the variety of forms presented by a land 

 surface upon the history of that surface was in the early days re- 

 cognized only with great hesitancy, but to-day, on the basis of 

 the geographical cycle, may be stated with a certain amount of as- 

 surance. In the Rhenish Schiefergebirge, in Western Germany, 

 we have a mountain system of Alpine structure and of plateau-like 

 expression. The region has passed through at least one complete 

 geographical cycle; the inequalities produced by crustal movements 

 were almost completely planed off before it was again furrowed 

 with valleys. The latter process implies a reelevation to have 

 taken place, a change which was not produced by a folding of the 

 strata, but rather in this case by a broad elevation of the whole 

 region. If we imagine this elevation to continue, then the valleys 

 will cut in deeper, and the interstream areas become more and 

 more portions of the valley slopes. An instant in geographic time 

 will come when the plateau surface will disappear entirely and 

 be replaced by mountain ridges whose constancy of elevation over 

 long distances will be the only indication of the former plateau 

 condition. The constancy of elevation of mountain summits is one 

 of the most striking facts which characterize many widely differ- 

 ing mountain ranges of the earth. If, on account of its ambiguity, 

 we do not venture to connect this feature in general with an earlier 

 complete denudation of the region, followed by more recent reele- 

 vation, yet in certain cases, as has already been demonstrated for 

 a number of mountain ranges, this interpretation does hold good. 1 

 The Alps, which have heretofore served as an example of a mount- 

 ain range which has originated by horizontal compression, may be 

 shown to have last undergone a vertical elevation in their western 

 portion. 



In our opinion the foldings, which are so important for the tectonic 

 structure of the crust, play a considerably smaller part in determining 

 the physiognomy of the earth's surface than do the vertical move- 

 ments of the crust. In building the great highlands of the earth, the 

 latter agencies have decidedly the chief r61e. It is true that this fact 

 has been proven, so far, only for the highlands of North America. 

 In the Colorado Plateau of Arizona great fault blocks of nearly 

 horizontal strata predominate. The great elevated masses have 



1 Davis, Bailey Willis, and de Martonne brought a number of such cases to 

 the attention of the eighth International Geographical Congress, Washington 

 session. 



