The Classification of Geographic Forms hy Genesis. 29 



"been in operation throughout geologic time, though they perhaps 

 represent a diminishing series ; they have added materially to the 

 superficial crust of the earth ; and it is fair to suppose that they 

 have modified the geoid not only by additions to the surface but 

 by corresponding displacements in their vicinity. The category 

 may be tentatively (but rather improperly) called vuloanism, and 

 the antagonistic classes of movements constituting it are extra- 

 vasation and its antithesis. The vibratory movements of seis- 

 mism probably result from both deformation and vulcanism under 

 certain conditions. 



The second subordinate category of processes is closely linked 

 with all of the others. It comprises the various chemic and 

 chemico-mechanical alterations in constitution and structure of 

 the materials of the earth's crust. The processes have affected 

 the rocks ever since the solidification of the planet, though prob- 

 ably in a progressively diminishing degree ; and they have ma- 

 terially (but indirectly rather than directly) modified the internal 

 constitution and external configuration of the earth. The pro- 

 cesses may be collectively called altei-ation; and the antagonistic 

 classes into which the category is divisible are lithifactioti and 

 decomposition in their various phases, or roek-formation and 

 rock-destruction. 



The third subordinate category of processes, viz : glaciation, 

 is related to the second principal category ; but since (l) it is, 

 probable if not actually demonstrable that under certain circum- 

 stances glacial grinding tends to accentuate preexisting irregu- 

 larities of surface, and since (2) it is well known that glacial de- 

 position sometimes gives great irregularity of surface, it is evi- 

 dent that glaciation is not a simple process of gradation, but 

 must be clearly distinguished therefrom. A considerable portion 

 of the earth's surface has been modified by glaciation during 

 later geologic times. The general process compi'ises glacial con- 

 struction and glacial destruction. 



There is a fourth subordinate category of processes, which is 

 also allied to gradation, viz : wind-action, which may be made to 

 include the action of waves and wind-born currents ; but since 

 the winds scoop out basins and heap up dunes, while the waves 

 excavate submerged purgatories and build bars, it is evident that 

 this category, too, must be set apart. The processes are only 

 locally important as modifiers of the land surface of the globe. 

 They comprise constructive action and destructive action. 



