=o GUIDEBOOK OF THE WESTERN UNITED STATES. 
extensive plateaus. It contains also many great lava flows, some of 
which form the surface of the plateaus. Most of this province is more 
arid than the western part of the Great Plains. Toward the west, 
however, about Flagstaff, Ariz., and the Grand Canyon, where the 
altitude increases to 7,000 foot. or more, the precipitation is much 
greater and luxuriant pine forests cover a wide area. 
Still farther west this high plateau descends by a succession of great 
steps or westward-facing cliffs, and finally, near Colorado River, the 
traveler enters another, a very different province—the desert province 
of eastern California. This province consists of wide desert valleys, 
out of which rise long, narrow mountain ranges, most of them that 
lie north of the Santa Fe line trending north and south and some of 
them continuous for many miles. These mountains are particularly 
rocky and jagged and their meager vegetation is so scattered that 
they appear to be bare. The broad desert plains of gravel and 
sand likewise have but little soil and scant vegetation, for this 
-is the most arid province in the whole country. It extends halfway 
across southern California to the foot of the San Bernardino Moun- 
tains and includes the Mohave Desert, a part of the vast area known 
as the Great Basin, whose streams do not reach the ocean but are 
lost in the desert. The San Bernardino Mountains and associated 
ranges rise as a high barrier on the west side of this basin, intercept- 
ing ‘the moist air currents from the Pacific and thus causing the arid 
climate of the region to the east. These mountains are uplifted 
blocks of tilted rocks, largely granitic and metamorphic (altered). 
The coastal-plain part of southern California extends from the 
western foot of these ranges to the Pacific Ocean, a distance of about 
100 miles. Much of its surface is made up of coast and valley plains; 
its climate is mild, and although the precipitation is only moderate 
in amount the conditions for plant growth are so favorable that, 
with the help of irrigation from the streams that cross it and from 
water in the underlying sands and gravels, it has become one of the 
most productive agricultural districts in the United States. 
Note.—For the conveni f the traveler the sheets of the route nep in this bul- 
letin are so arranged that he can unfold them one by one and keep each one in view 
while he is reading the text relating to it. The contour lines , in brown, represent 
lines of equal elevation above sea level. Each line indicates the path that would 
be taken by one who walked over the country by a course always at the same level, 
at the vertical distances apart (‘contour interval’) stated on each map. Where 
close together they indicate a steep slope; where far apart, a gentle slope or plain. 
A reference is made in the text to each map at the place where it should be 
unfolded. The areas covered by these sheets are indicated on Plate I, and a list 
of the sheets and the other illustrations is given on pages 187-190. A glossary of 
geologic terms is given on pages 182-185, and an index of stations on pages 191-194. 
