SOUTHERN PACIFIC LINES 231 
mostly along the sides of the valleys. The water is available in some 
places in the valleys, but ordinarily it is only sufficient for domestic 
use or for a few cattle. Along the river flats there is a ground-water 
plane sustained by the streams and extending laterally for some dis- 
tance; this is the source of supply for many wells, some of which in the 
lower Gila Valley yield water for irrigation. In the lower part of the 
Salt River Valley also the underflow is extensive and in much of the 
area of ample volume. 
The desert landscape has many peculiarities. At first sight its wide 
gray plains and bare mountain slopes seem forbidding and monotonous. 
However, they bave a certain grandeur and present attractive varia- 
tions in light and shade during different portions of the day and from 
day to day. Some of the sunsets are particularly beautiful. Under 
the direct rays of the midsummer sun the heat is intense, but ordinarily 
the low humidity keeps the skin comfortable, and there is much less 
suffering from the heat than in a moist region at much lower tempera- 
ture. Mirage is frequent, especially the sort due to a film of vibrating 
hot air near the ground, which gives the illusion of distant lakes. 
In the higher mountains precipitation is greater than in the valleys, 
the temperatures are lower, and occasionally there is snow. LEvery- 
where the rains are followed by rapid growth of many flowers. The 
desert region of the southwest corner of the United States is a part of 
the Sonoran Desert, which extends north from the State of Sonora 
in Mexico and is very much of a unit in climate, vegetation, and gen- 
eral aspect. Rainfall, which ranges from 3 to 6 inches a year in the 
region west of Phoenix, comes mostly in widely separated heavy 
downpours in narrow streaks, many of them ‘‘cloudbursts,’”’ which 
give rise to local sudden freshets of great volume. One of these in 1930 
washed out a large part of Wellton. Some floods are not confined to a 
channel but extend widely over the valley floor. 
Sand storms occur occasionally on the deserts of New Mexico, 
Arizona, and southern California, but most popular accounts of them 
are greatly exaggerated. The following description (by C. P. Ross) 
will give some idea of a typical sandstorm. It followed showers in the 
mountains and came from the southeast, where at frequent intervals 
before, during, and after the blow there were sharp claps of thunder. 
At first there came bodies of flying sand in long, thin pillars reaching 
far upward and resembling waterspouts in shape and appearance 
but moving with much greater speed. These were followed by 
billowing clouds of sand, which, however, did not transport much 
material, and then came the main blow in dees waves and carrying a 
large percentage of fine sand. Where these waves struck the moun- 
tains they were shattered, and the sand was whirled high on the foot- 
hills, much like waves of water driven by a hurricane. In 10 to 15 
minutes from the coming of the first sand the storm diminished, espe- 
