142 CEs DOLMAN 
deposition, the aggradational equivalent of the active erosion above. 
These slopes are the dominant feature of the arid landscape, each 
mountain range (or isolated hill under severe climatic conditions) 
appearing to stand on a symmetrical pedestal. The novelty of these 
features, and the prominent and distinct place they hold topographi- 
cally, have led to a repeated request for a formal name. In a former 
publication already referred to, I recommended the use of the term 
“slope,”’ describing each particular incline by the name of the moun- 
tain which gave it birth: viz., Tumamoc slope, Catalina slope, etc. 
The difficulty of preventing confusion between the detrital slopes 
and the rock slopes of the mountains brought out the necessity of a 
new name for this feature, for which the Spanish word bajada has 
been selected, local usage almost exactly corresponding to the technical 
meaning suggested. (3) The playa. Finally in the well-developed 
bolsons, there is a central flat, or flats in the irregular and larger 
bolsons, which is occasionally or even permanently occupied by a 
water sheet, the life of the temporary lake or pond depending on 
local climate conditions. In less perfect examples of the semi-bolson 
this central feature is lacking. In such a bolson, especially where 
large, the central portion may be a more or less irregular plain, com- 
posed of river, outwash, and other deposits, and the general name 
to be applied is ‘‘bolson plains.” 
ANALYSIS OF GEOLOGICAL PROCESSES IN ARID REGIONS 
Torrential precipitation.—Before attention is directed to the 
aggradational deposits of the bajadas and playas, a preliminary 
investigation is advisable regarding the action of geological processes 
under the stimulus of aridity. It is generally assumed that all the 
precipitation of the desert regions is of torrential character. This 
is the exaggerated recognition of the fact that there is a marked 
tendency in that direction, and even where a given shower has no 
greater density than one in humid region, the run off is more rapid, 
and torrential concentration accentuated. 
Assume a moderately arid region, with a marked tendency toward 
torrential concentration of precipitation, with both a large daily 
thermal swing (marked difference between day and night tempera- 
tures) and an active wind transportation and deposition. The coarse 
