ENVIRONMENTAL AND HISTORICAL FACTORS. S87 
cent of its dry weight.’ This factor, together with its shallowness and 
loose physical structure, allows rapid percolation and rapid evaporation 
after rain. It appears that after saturation has been attained a con- 
siderable amount of water is drained away along the caliche surfaces 
below. Thus it appears that almost the entire mass of this soil is nearly 
air-dry during the greater portion of the dry season. This dry condi- 
tion probably accounts for the absence here of almost all vegetation 
excepting the characteristic Larrea and a number of forms which are 
active only in the rainy seasons. Larrea appears to obtain its moisture 
from the small amount in the soil and perhaps from the still more meager 
supply in the crevices of the caliche. 
Attention should be called here to the fact that the soil of the Larrea 
slope receives more water than does that of the hill. The former is 
exposed at a lower angle, so that precipitation water does not flow off 
as rapidly as it does in many situations on the hill, and it is often flooded 
after a rain by the superficial run-off from the hill itself. Any advan- 
tage accruing from these conditions, however, is apparently more than 
counterbalanced by the shallowness of the slope soil and its drainage 
facilities, as well as by the ease with which it becomes desiccated. 
(3) The soils of the wash are sandy, varying from a coarse gravelly 
sand, in the present intermittent stream-channels, to a light loamy sand, 
in areas not flooded at all or flooded only when the wash is running full. 
The stream channels are somewhat lower than the loamy areas, and 
the latter have the character of miniature flood-plains. Practically no 
vegetation occurs on the coarse sand, these soils being exceedingly well 
drained and almost constantly dry to a depth of many centimeters. 
Their surface layers are indeed moist only for a few hours following the 
disappearance of flowing water in the wash, which is present only after 
the heavier showers. A light shower appears to have very little perma- 
nent effect in raising the moisture-content of this soil. 
The comparatively luxuriant vegetation of the wash is rooted in the 
more loamy deposits, and it is here, and on the river flood-plain, that 
the most mesophilous plant-forms of the region occur. ‘These soils are 
30 cm. or more in depth, usually overlying coarser sands. The loamy 
deposits have a water capacity of about 25 per cent of the dry weight 
of the soil, a capacity which is seen to be somewhat higher than that of 
the soils of the Larrea slope, notwithstanding the fact that the wash soil 
is much more sandy than the other. This difference is probably due to 
the absence of a large amount of broken-stone in the former soil and to the 
almost complete absence of such material in the wash. 
The loamy sands of the wash receive much more water than either 
type already discussed; the flow of the wash includes, besides direct 
precipitation, the run-off from a large drainage area, so that they become 

*Had the pebbles normally present in this soil been sifted out, of course the moisture- 
holding power would have been higher. 
