EROSION IN ARIZONA BOLSON REGION 155 
decrease in inclination, and also are protected against removal by 
grasses.'. Here then the factor of increasing importance is the pro- 
tective sheet of vegetation. 
Effect of temperature-—Here again the extreme dazly difference 
in temperature at the naked rock surface is desired, and the abundant 
data compiled from observations taken under shelters, etc., are of 
little value. Further, in the attempt to discover those factors of 
primary importance, all changes of longer period may be overlooked. 
Where the freezing-point is crossed daily, we probably have one of 
the maximum points in temperature change action, and where the 
rock surface remains above or below freezing for more than a day, 
the effect is diminished.? Here again there is a chance to gather 
further information, at moderate outlay of trouble, by placing self- 
recording thermometers on exposed rock surfaces, at different alti- 
tudes and times of the year in desert regions. It is evident that 
daily temperature is a variable, at least partially independent of 
the distribution of the rainfall, and after data have been collected 
its separate analysis as a climate factor should be attempted. Sum- 
ming up we find that the following varieties of desert climate are 
important: 
Type 1. Torrential Precipitation Dominant 
\ A—extreme |] saree Race a) extreme 
Torrential distribution.. . ee with daily temp. dif. 
or 
b) large 
Type 2. Protective Plant Covering Controls 
A moderate and distributed rainfall with no great extremes of daily thermal 
change. 
Type 3. Wind Action Important 
\ a) extreme 
and daily temp. difference or 
A thin distributed rainfall grading 
b) large 
to no rainfall. 
DEPOSITS OF THE BOLSON 
The Bajada Outwash. Inclination.—In the moderate torrential con- 
ditions of southern Arizona, the middle portion of the slopes varies from 
5oto 300feet permile. The grade of this middle portion appears remark- 
t Johnson, op. cit., p. 625. 
2 See Barrell, op. cit., p. 172, for a summary not entirely in accord with the above 
and for a list of authorities; also C. and C., Geology, Vol. I, pp. 44-48, and MacDougal, 
op. cit. pp. 77-79. 
