SHREVE: ACROSS THE SONORAN DESERT 289 
Indians and used in weaving the colored designs of baskets. 
Another abundant shrub in which the branches dwindle rapidly 
in diameter is the terrote blanco, Jatropha cinerea, but the 
basal diameter is rarely more than 2 in. Jatropha cordata is 
abundant here, together with the similar but smaller leaved 
J. cardiophylla. An undescribed Opuntia of the series Clavatae 
is abundant in several localities, its erect branches giving the 
largest plants a resemblance to the cylindropuntias. The 
platyopuntias, or prickly pears, become very much less abundant 
in this region than they are at Tucson, both as to the number of 
species and individuals. This type of cactus appears to be 
unable to resist the adverse moisture conditions of the driest 
portions of the Sonoran Desert, as it is poorly represented here 
and also in Arizona west of the longitude of Phoenix. 
The last one of the low passes in the Sierra Picu is 20 miles 
from the Gulf coast, and its elevation is approximately 1750 ft. 
The very gradual descent to sea-level is made over a detrital 
slope on which the vegetation is at first very heavy, then rather 
open, and finally very scanty and low. The vegetation of the 
heavily covered portion of the slope presents many features 
which distinguish it from similar situations to the east and 
north. The giant Pachycereus Pringlei here reaches such a size 
that many of the individuals must weigh 20 tons, as the largest 
plants of the much less massive Carnegiea are known to weigh 
half that much. Lemaireocereus Schottii, Opuntia fulgida and 
O. gosseliniana are also abundant. The principal distinctive 
features of the area, however, are the great number of small 
- trees and the abundance and variety of shrubs and shrub-like 
perennials. The commonest of the trees are Elaphrium micro- 
phyllum, Olneya tesota, Parkinsonia microphylla, Prosopis 
glandulosa and Parkinsonia Torreyana. Prosopis velutina, char- 
acteristic of the higher portions of the Sonoran Desert, was not 
detected here. The shrubby vegetation not only includes the 
compact type, branching freely from the ground, but numerous 
individuals of MaWwaceous, and Solanaceous shrubs, with a single 
erect stem and the form of a small tree. Some of these are not 
- woody, and their presence is a strong indication that frosts are 
either absent or very light in this locality. Among the shrubs 
and suffrutescent perennials characteristic of the upper seaward 
slope are Jatropha spathulata, Covillea tridentata, Encelia farinosa, 
