10 VEGETATION OF A DESERT MOUNTAIN RANGE. 
find root. There are many deep soil-filled crevices through which the 
roots of trees are able to penetrate to bodies of deep-seated soil of 
favorable moisture content. : 
The restricted areas of alluvial soil in the desert and lower mountain 
regions are of a fine sand or sandy loam and possess considerable humus, 
in contrast to the soils of the slopes. At the forested elevations the 
soil is similar to that of the evergreen oak region. The soil of the lower 
pine belt is scarcely superior in depth or humus content to that of the 
upper oak region. Above 7,500 feet, however, the amount of humus, 
as well as the amount of surface litter, increases with the increasing 
density of the stand of pines. On the north-facing slopes which are 
clothed with fir forest the soil is not much if at all deeper than in the 
heavy stands of pine, but is notably richer in organic matter. 
The alluvial slopes which immediately surround the mountain are 
so closely related to it in all of their physical and biological features 
that it will be necessary in the following pages to give some considera- 
tion to their vegetation. Throughout the arid southwest the long 
straight profiles presented by the outwash slopes of the hills and moun- 
tains form one of the characteristic features of the landscape. The 
distinct character of these slopes is to be attributed to the manner in 
which they have been laid down under conditions of torrential rainfall 
and of violent and intermittent stream flow, and their distinctness from 
the parabolic alluvial slopes of the humid regions has caused Tolman * 
to designate them technically by their popular Spanish name“‘bajada.’”’t 
The bajadas constitute almost the total area of all the intermontane 
valleys of southern Arizona. The only portions of the valleys topo- 
graphically separable from them are the stream beds, the flood-plains, 
and the ‘‘playas’’ or undrained areas into which one or more streams 
flow and deposit their load. To the student of vegetation there are 
marked differences between the upper and lower portions of all bajadas. 
The differences in the physical features presented by upper and lower 
bajadas of the same elevation have been only superficially investigated; 
the differences in their vegetation are very obvious, as will be described. 
The principal environmental features which appear to differentiate the 
high and low bajadas are the coarser character of the soil in the high 
bajadas, the possibility of higher soil moisture in them, at least at a 
depth of several feet, and the greater development of calcareous incrus- 
tations, or ‘‘caliche,”’ in the soil of the low bajadas. The layers of 
caliche lie near the surface in some places, while in others the upper- 
most ones have been covered by deposition; they extend downwards 
for a few feet in some cases, or more frequently recur to a depth of 
100 feet or more. 
The bajada of the southern face of the Santa Catalinas has been 
*Tolman, C.F. Erosion and Deposition in the Southern Arizona Bolson Region.* Jour. Geol., 
vol. 17, pp. 136-163, 1909. 
+ Pronounced bahada. 
