32 National Geographic Magazine. 



duced above that such a classification is objectionable unless tbe 

 greater geographic elements due to diastatic movements (in which 

 the constructive action is veritable but different in kind from 

 those in the other categories) be excluded, and this is impractica- 

 L)le without limiting the classification to subordinate phenomena. 

 Moreover it is illogical and useless to unite the constructive phe- 

 nomena of the remaining categories, since (1) the processes exem- 

 plify widely diverse laws, which must find expression in any 

 detailed classification whether genetic or not, and since (2) the 

 differences between the forms united are much greater than the 

 differences between the forms separated in such a classification — 

 e. g. the differences between a dune, a drumlin and a mesa (all 

 constructive forms) are far greater than the differences between 

 a fresh lava sheet and a deeply cut mesa, between a drumlin and 

 the smallest drift remnant, or between a dune and a Triassic 

 mound of circumdenudation ; and this is true whether the distinc- 

 tion be made on analogic, homologic, or genetic grounds. Indeed 

 it seems evident that while discrimination of constructive and 

 destructive forms is necessary and useful in each genetic cate- 

 gory, the use of this distinction as a primary basis of classifi- 

 cation is inexpedient. 



The classification of topographic forms proposed a few years 

 ago by Davis, who regards " special peculiarities of original 

 structure " as a primary, and " degTee of development by erosion " 

 a secondary basis, and Riclithofen's arrangement of categories 

 of surface forms as (l) tectonic mountains, (2) mountains of 

 abrasion, (3) eruptive mountains, (4) mountains of deposition, (5) 

 plains, and (6) mountains of erosion,* in addition to depressions of 

 the land (Die Hohlformen des Festlandes), are more accept- 

 able, since they are based in part on conditions of genesis. But 

 it is clearly recognized by modern students of dynamic geol- 

 ogy that waterways are the most persistent features of the 

 terrestrial surface ; and the most widely applicable systems of 

 classification of the surface configuration of the earth thus far 

 proposed have been based substantially on the agencies of grada- 

 tion. Thus Powell, Lowl and Richthofen classify valleys by the 

 conditions of their genesis ; Gilbert classifies drainage ; and 

 Phillipson, unduly magnifies the stability and genetic import- 

 ance of the water parting, classifies the hydrography through 



* (1) Tektonische Gebirge, (2) Rumpfgebirge oder Abrasionsgebirge, 

 (3) Ausbruchsgebirge, (4) Aufschiittungsgebirge, (5) Flacliboden, und 

 (6) Erosionsgebirge. 



