618 PHYSIOGRAPHY 



One can also imagine that a deep-seated, continuous folding may 

 express itself at the surface as a low, dome-like elevation. In any 

 case it is certain that we must not interpret all crustal movements 

 as the direct results of a single, universal process. One needs only 

 recall the close association of vertical crustal movements and the 

 areas of ancient glaciation, in order to have an illustration of the 

 fact that phenomena of elevation and subsidence may result from 

 physical changes in the uppermost layers of the earth's crust. One 

 must choose from among several explanations, and this can only 

 be done with certainty when both structure and diastrophic condi- 

 tions are clearly in mind, so that the whole geographic history of the 

 mountain system may be passed in review. 



We shall content ourselves with the foregoing suggestions. They 

 are meant to point out directions in which the genetic study of the 

 forms of the earth's surface may yield results of significance to geo- 

 physics and to the study of the earth's development, as soon as it is 

 founded upon a broad basis and brought into sympathy with all its 

 neighboring sciences. But without doubt the richest results are to be 

 looked for from the recognition and appreciation of the forces at work 

 upon the earth's surface, the more detailed study of which belongs to 

 the dynamic problems of physiogeography. 



All the movements which take place upon the surface of the earth, 

 the winds, rivers, surf, and glaciers, all stand in intimate relationship 

 with one another, all are dependent upon the forms of the earth's sur- 

 face and react upon them. This mutual reaction extends also to the 

 organic life of the globe, and although it is not the task of physio- 

 geography, but of biogeography, to investigate the distribution of 

 organic forms, yet a physiogeographic study cannot overlook the 

 influence which the association of biologic processes exerts upon 

 the forms of the earth's surface. Examples of this influence are found 

 in the protection of the earth's surface against erosion by the cover 

 of vegetation, and in the widespread cooperation of organisms for the 

 formation of sediment, e. g., the work of the reef-building corals. 

 It is, however, precisely these biologic communities of geomorphologic 

 significance, which combine with the peculiarities of surface and soil 

 and the activities of the water and the atmosphere, to determine the 

 physical expression of the lands. It is the general investigation of 

 these communities which teaches most clearly which portions of the 

 neighboring sciences belong also to physiogeography. The latter 

 must take from each enough to gain an understanding of the physical 

 features as a whole of the different countries in order to understand 

 the local interlocking of the different phenomena in their "causal 

 nexus." Physiography does not attempt to found laws within the 

 domain of these sciences, but takes already established laws from 

 these sciences and applies them. The result is the discovery of de- 



