EROSION IN ARIZONA BOLSON. REGION 153 
We must assume that there are certain areas of the globe which 
have had a dominantly arid climate as long as the atmospheric and 
oceanic circulation and the shape and relief of the continents resemble 
that of today. Doubtless those tracts whose arid condition is due 
chiefly to the trade winds have had a more continuous and a more 
steady arid history, than the deserts of the northern regions due 
mainly to topography. We have had the curious anomaly in the 
development of a geological philosophy during the last twenty years 
permitting, on the one hand, the upthrust of a continent thousands 
of feet, on the evidence of a slightly river-scarred plain, and, on the 
other hand, an almost universal assumption of climatic constancy, 
that could not possibly exist in view of the movements involved. 
Fully as curious are the physics involved in the earth movements 
necessary to make the lakes hold water, and then discharge the same 
again, in which the shifting piedmont and outwash deposits (errone- 
ously interpreted as lacustrine) have been assumed to be laid down. 
In the bolson region it is believed that if only there is a general 
dominance of arid conditions, a very considerable swing from arid 
to humid and back again is permitted. The through drainage of 
the moister climates has had great difficulty in cutting through the 
great talus and rock slopes which aridity has carved and built up 
into undrained basins. 
While aridity depends primarily on deficiency of precipitation, 
variations in the same may take place along the following lines: 
TYPES DEPENDING UPON THE DISTRIBUTION OF PRECIPITATION 
Type t. Torrential concentration of rainfall: (a) with moderate 
rainfall; (6) with deficient rainfall. 
Type.2. A distributed and gentle rainfail, moderate in amount. 
Type 3. A distributed and gentle rainfall very deficient in amount. 
Torrential concentration.—The importance of torrential precipita- 
tion need not be investigated here. All writers are unanimous in 
giving it an important réle. The general assumption seems to be 
that torrential concentration increases with aridity. It should be 
emphasized, however, that we have not the observational data to 
properly gauge this factor. The weather bureau arranges its data 
along average lines. Average daily, weekly, monthly, yearly tables 
