60 VEGETATION OF A DESERT MOUNTAIN RANGE. 
of their ranges. It was impossible, nevertheless, to secure a set of 
soil samples in the humid mid-summer which would be representative 
of the maximum moisture conditions and at the same time comparable 
for the various altitudes. A set of samples taken at the same interval 
after a rain of the same amount, at each of the several elevations, 
would comply with the requirements. 
All samples of soil for moisture content were taken from a depth 
of 15cm. The conditions at this depth are of importance for ephemeral 
herbaceous plants and for some shrubs, but the trees and larger shrubs 
are, of course, dependent for their supplies on much more deep-seated 
bodies of soil. The rocky character of the substratum means that the 
largest perennial plants are dependent to a great extent upon the 
moisture contained in the soil which occupies the crevices of the rock 
in situ. Itis particularly noticeable that the lowest trees of the Encinal 
region grow in the uppermost part of talus slopes or along the bottoms 
of cliffs. In such situations it is doubtless possible for the roots of 
these trees to reach soil-filled crevices which are fed by gravity with 
the water of large veins of soil above. 
The samples of soil were secured by digging with a hand trowel and 
transferring quickly to bottles, which were tightly stopped, and then 
coated over the stopper with vaseline. The soils were dried in the 
original bottles by heating to 100° C. until they showed constant 
weight. The percentages of moisture have been calculated on the dry 
weight as unity. The physical texture of all samples taken was very 
similar, but there was a greater amount of humus in those from the 
higher elevations. 
Three series of soil samples were taken at various times to determine 
the conditions prevailing in the arid fore-summer. These samples 
were taken at 1,000-foot intervals, from the vicinity of the rainfall 
stations, and were secured in pairs, one sample being from a south 
slope and one from a north slope. The localities chosen for sampling 
were typical of the slopes at the several elevations, and in every case 
the pair of samples was secured in the midst of the dissimilar vegeta- 
tions which occupy the opposed slopes. 
On April 27 to 29, 1911, a series was secured from 3,000 feet to 7,000 
feet (see table 7). For the three months preceding the taking of these 
samples there had been only light and infrequent rains over the sur- 
rounding region, the rainfall of the mountains themselves for this period 
being unknown. At Tucson there was a rainfall of 0.28 inch on April 
2, and there was no appearance of rain on the mountains after that 
date. On June 9 to 11 another series of samples was secured at the 
same stations, together with a pair from the station at 8,000 feet. 
There had been no rain between the securing of the two sets of samples. 
A comparison of the percentages of moisture in April and in June 
shows them to be of about the same order of magnitude. The relative 
