610 PHYSIOGRAPHY 



Davis happily terms it and of man in particular on the one side, 

 and the sciences which treat of the earth-whole in general on the 

 other. Its problem reveals its scope. To the extent that the inter- 

 action of the earth's surface and the forces at work upon it grows 

 clearer, so the old study of the forms of the earth's surface, which 

 was long scarcely more than descriptive in character, rises into that 

 important branch of physical geography called geomorphology. Topo- 

 graphy and cartography form necessary members of this branch, for 

 it is impossible to separate the descriptive treatment of a portion 

 of the observational material from the science. Physical geography 

 is the descriptive, genetic, and dynamic study of the earth's surface, 

 and is most intimately related both to geodesy and geophysics, which 

 treat of the whole earth, as well as to the sciences of geology, ocean- 

 ography, and meteorology, which treat respectively of those portions 

 of the earth called the lithosphere, hydrosphere, and atmosphere. 

 The character of this relationship becomes clearer if we consider the 

 special problems of this science. 



However manifold in form the physical surface of the earth may 

 appear to the eye of the observer, yet geometrically it is rather simply 

 modeled. Aside from unimportant and very rare exceptions, this 

 surface consists of combinations of slopes having various angles, but 

 yet all dipping towards one and the same basal surface. This surface 

 is that sensibly level one presented by the surface of the ocean, which 

 geodesy considers the surface of the earth. The close touch between 

 geodesy and physical geography is due to the fact that they both 

 start from the same surface of reference, although seeking different 

 ends. The geodesist endeavors to determine the form of the geoid 

 of sea-level under the continents, while the geographer concerns 

 himself with the variations from that surface which the firm crust 

 of the earth presents. Both workers employ the same set of . coor- 

 dinates in their studies. Of these coordinates, those of latitude and 

 longitude have long been considered as geographical, while the co- 

 ordinate of elevation above sea-level, which is indispensable to the 

 physiographer, has but rather recently been added to the others. 

 Geography first assumed its present character only after the intro- 

 duction of this third coordinate as a geographical measure. But 

 while it suffices for the geodesist to determine with very great accu- 

 racy the geographical coordinates of relatively few fixed points, the 

 geographer needs to know the positions of all the points of the earth's 

 surface. This would be impossible were it not for the aid of the geo- 

 graphical map, by means of which the geographer can graphically 

 represent the surface of the earth and show, exactly, the relative 

 positions of places (or points) according to their differences in longi- 

 tude, latitude, and elevation. The preparation of the geographic 

 map is the purpose of geographic surveys, while the purpose of geo- 



