96 
DYNAMICAL GEOLOGY 
Brosional Agencies under Varient Climatic Stimuli. It is the 
custom to treat the subject of general land-sculpturing not only 
independently of climatic considerations, but as if the molding of 
all landscape features were controlled by the same geologic laws. 
The fertility of suggestion growing out of the novel conception 
of a definite cycle of development through which all land-forms 
must pass tends to exaggerate the evolutionary aspects of the theme 
at the expense of the genetic means by which the physiographic 
changes are accomplished. Even the latest and most authoritative 
treatise on physical geography premises the same derivation of 
physiognomy for the profoundly glaciated Swiss Alps and the arid 
High Plateaus of western America, for the forest-clad Apj>ala- 
chians and the bare South African veldt, and for the jungle- 
matted eastern Andes and the desert Australian interior. Ordinary 
stream corrosion is made to account for all. Rain thus comes 
to be regarded as the universal and sole graving-tool in land- 
sculpture. 
These diverse and incongrous phenomena may be viewed from 
a quite different angle. The relative efficiencies of the different 
erosional agencies may be quantitatively measured under variant 
climatic stimuli. It may be shown that under the variant favor¬ 
able conditions imposed by aridity the wind, for example, may 
become a general erosive power in every way comparable to that 
of rain, wave or river. Circumstances may be pointed out under 
which eolic activity assumes an erosional ascendency over all other 
forms of gradation. Some of the most characteristic geologic 
processes and most typical geographic products may be described 
in regions where the wind displays its maximum efficiency as a 
general erosional agent. 
Most descriptions of the geologic work of the winds treat the 
geomorphic effects presented in moist and in dry climates together. 
There are grave objections to this practice. Under such genetically 
diverse conditions the resultant phenomena should be separately 
considered and strongly contrasted. In humid lands the erosive 
activity of the wind is all but completely dominated by that of the 
rain. Although this is the phase of the eolation which is really 
most trivial it is the one which receives usually main consideration. 
Keyes. 
