1890.] Erosive Agents in the Arid Regions. 455 
EROSIVE AGENTS IN THE ARID REGIONS. 
BY RALPH S. TARR. 
A NONG the mountains in the western part of the United 
States the agents of erosion are not unlike those of the 
moister sections of the east. Rain, snow, ice and frost are the 
chief factors in the sculpturing of these mountain masses. 
On the plateau the conditions are different. Snow and ice are 
rarely seen; frost seldom appears, owing to the extreme dryness 
of the atmosphere, and rain falls under peculiar conditions. In 
all climates the action of animals and plants, of sun and air, are 
important factors in the wearing away of land masses; but in 
few places are the effects of these agents so important as in the 
western plateau region. Most if not all the agents of erosion 
which I shall describe have been noticed and mentioned by the 
various writers on the geology of the arid regions ; but their im- 
portance seems so great that it may be well to group them and 
call especial attention to them. 
Of the physical agents, as in all sections of the world, except 
the most arid, rain is the chief. For fully nine months in the 
year little or no rain falls upon the truly arid belt. When, during 
these months, rain falls at all, it comes simply as light showers or 
brief drizzling rain, producing almost no geological effect. It is 
absorbed by the soil, and is rarely in sufficient quantity to freshen 
the parched vegetation. 
During the three summer months the region is liable to ex- 
cessive local rains, often called “ cloud-bursts.” Several inches 
frequently fall in an hour during such a shower. Two-thirds of 
the annual precipitátion falls in a few such rains. The geological 
effect of so much water suddenly poured .upon the ground is 
intense. The whole soil is completely wetted for a depth of 
several feet, and even in this dry region torrents flow tumultously 
to the streams. The arid plateaux are chiefly regions of young 
drainage, on which there are large tracts without drainage arteries. 
