SOUTHERN PACIFIC LINES 207 
beds of boulders underground, and at a later stage the river shifted 
to its present course north of these mountains. At this time it 
deposited the boulder beds that yield the underflow about Phoenix. 
These later gravel deposits lie in an old channel roughly parallel to 
the present one and excavated in the finer beds which were spread 
widely by overflows during the earlier period of accumulation. 
Care has to be taken in irrigation not to let the mineral contents 
of the water accumulate in the soil, especially some of the well 
waters, which are more highly mineralized than the river water. In 
some parts of the valley the soil has been poisoned in this way, but 
this can be avoided by suitable underdrainage to carry off water that 
otherwise would evaporate and leave its dissolved mineral matter. 
The Salt River Mountains, which rise abruptly from the desert 
plain a few miles south of Phoenix, consist of chloritic schist and 
fine-grained biotite granite. The granite is quarried to some extent 
as an ornamental stone. The Sacaton Mountains and many of the 
peaks and ridges on the east and south sides of the Salt River Valley 
are made up of granite, some of which is very coarse grained, with 
many of the feldspar crystals as much as 2 inches in length. A few 
miles north of Phoenix are the Phoenix Mountains, which consist 
largely of quartzite and other metamorphic rocks in massive beds, 
several thousand feet thick in all, tilted at high angles. Some of the 
mountains in the Salt River region are upthrust blocks; others are 
remnants of older ridges nearly buried by valley deposits. 
The climate of Phoenix is similar to that of most of the deserts of 
southwestern Arizona at elevations from 1,000 to 2,000 feet. Accord- 
ing to the records of the United States Weather Bureau, the mean 
annual precipitation is about 8 inches, most of which falls in mid- 
summer showers. The amount varies greatly from year to year, 
however, in some years being less than 5 inches and in others as much 
as 14.41 inches (1911). The mean annual temperature is 70°, and 
the summers are long and warm, but the summer heat is much less 
oppressive than in regions with more moisture in the atmosphere. 
The amount of sunshine, as compared with the greatest amount pos- 
sible, is 84 per cent. The mean temperature during the winter is 
about 40°, owing to cold nights, but most of the winter days are mild. 
Parts of the valley are free from killing frosts. (Continued on p- 218.) 
DETOUR BY THE APACHE TRAIL 
A most picturesque chapter is added to the transcontinental trip 
by the detour over the Apache Trail. (See fig. 51.) The distance is 
120 miles in all and requires about one day in time and certain extra 
expenses for bus fare and hotel stop. This additional time and ex- 
pense are well justified, however, by the superb scenery and the thrill- 
ing character of the trip. 
