SHREVE: ACROSS THE SONORAN DESERT 285 
a cover of annual and perennial summer-growing grasses, with 
very infrequent individuals of Prosopis and Opuntia. The 
elevation is too high for Covillea, Parkinsonia and Carnegiea, 
although all of these species are found in the hills near the 
Boundary, at a slightly higher elevation. This anomaly, which 
is common in many other localities, may be explained by the 
greater insolation and warmer soil of the south-facing slopes of 
the hills. 
On reaching the foot of the plain at San Rafael Ranch the 
read follows the flood plain of the Rio Seco nearly all the way to 
Altar. The vegetation of this region is essentially identical with 
that of the flood plains near Tucson, and the new species that 
appear in it are not so striking as the new ecological habits ex- 
hibited by some of the familiar plants. For example, Encelia 
farinosa,a composite shrub which is confined to rocky slopes and 
coarse outwash in the Tucson region, always in company with 
other plants, may here be seen in extensive pure stands on 
alluvial soil. Two conspicuous cacti are first seen a few miles 
south of San Rafael, Lophocereus Schottit and Lemaireocereus 
Thurberi. These are both columnar forms with branches from 
4 to 6 inches in thickness, which rise in groups of 5 to 25 and 
attain a height of 4 to 10 feet. Both of these plants are found 
in Arizona, but their northernmost outposts are naturally to be 
found on south slopes in sheltered positions. Lophocereus is 
first seen in the Rio Seco valley on alluvial and outwash soil, 
and is most commonly found in such situations throughout its 
range. Lemaireocereus is first seen on the hills that skirt the 
valley and its occurrence in general is confined to rocky slopes 
and coarse outwash. 
The Rio Seco is a tributary of the Altar River, which it reaches 
in a number of small channels after spreading out into an alluvial 
delta 10 miles in width. The town of Altar is at one corner of 
the delta and the town of Pitiquito near the other. These old 
settlements are both on the Altar River, and between them this 
stream is joined by the Magdalena, or San Ignacio River, which 
drains an extensive area of hilly country in the District of 
Nogales. The narrow streets of these towns are lined with 
unbroken rows of stuccoed and tastefully tinted adobe houses, 
with no hint of lawn, fence or porch. The streets and the mud- 
walled gardens of pomegranates, oleanders, oranges and date 
