SOUTHERN PACIFIC LINES 3 
western Arizona after leaving the canyons in which they cross the 
Arizona Plateau. The mountains of the Southwest are rocky and 
jagged, and the meager vegetation is so scattered that they appear to 
be bare. The broad desert plains of gravel and sand between the 
mountains likewise sustain only scant vegetation, for this is the most 
arid province in the United States. Parts of it, however, that have 
water for irrigation are highly productive. 
The San Bernardino, San Jacinto, and associated mountain ranges in 
California form high barriers on the north and west sides of the Salton 
Basin which intercept the moist air currents from the Pacific and thus 
cause the aridity that prevails over a wide area to the east. These 
mountains are uplifted blocks, made up largely of granitic and meta- 
morphic rocks. 
In southern California lowlands extend from the mountain slopes 
east of Redlands to the Pacific Ocean, a distance of about 120 miles. 
The surface slopes mostly to the west and south and is diversified by 
scattered rocky ridges. The climate is mild, and although the precipi- 
tation is only moderate in amount, water is available for irrigation 
and wide areas are under cultivation for citrus fruits, grapes, nuts, 
and many other valuable crops. 
In these days of wide culture it is hardly necessary to point out the 
practical utility of geologic knowledge and the relation that exists 
between geology and the occurrence of nearly all materials of economic 
value. Soils are derived by geologic processes from rocks of various 
formations. Ores, minerals, oil, coal, and water all have close rela- 
tions to the structure and history of the geologic formations in which 
they occur. Some igneous rocks carry or have been the source of 
ores, and their history and relations have much to do with mining. 
The order and general succession of the strata making up the rocky 
shell of the earth are shown in the table on page m1. The oldest 
rocks now seen at the earth’s surface include some granites and other 
crystalline rocks, partly of igneous origin and partly of other types 
that have become crystalline through the agency of heat and pressure 
within the earth and have later been exposed by erosion. These are 
overlain by a great succession of sedimentary strata (laid down by 
water), consisting of sandstone, limestone, and shale, which have a 
thickness of many thousand feet. Some of these later rocks have also 
been altered by heat and pressure into schist, marble, and quartzite. 
In many areas there are lavas, ash, and tuff extruded by volcanic 
action at various times, some of it recent. Seas, lakes, and rivers 
have been the principal agents in depositing sand, clay, and limy 
sediments, which have later become sandstone, shale, and limestone. 
In general, sands were deposited mainly on the shores and by streams, 
clays in quieter waters, and limestones in deeper waters, so that 
these various materials indicate the geologic conditions at the time of 
