ugh the tawny wreck of the last year's
greenery. North of the Gulf of California the desert is known as
"Coahuila Valley," the most desolate region on the continent. At one
time in the geologic history of this country the Gulf of California
extended a long distance farther to the northwest, above the point where
the Colorado River now enters it; but this stream brought its mud from
the mountains and the hills above and poured it into the gulf and
gradually erected a vast dam across it, until the waters above were
separated from the waters below; then the Colorado cut a channel into
the lower gulf. The upper waters, being cut off from the sea, gradually
evaporated, and what is known as Coahuila Valley was the bottom of this
ancient upper gulf, and thus the land is now below the level of the sea.
Between Coahuila Valley and the river there are many low, ashen-gray
mountains standing in short ranges. The rainfall is so little that no
perennial streams are formed. When a great rain comes it washes the
mountain sides and gathers on its way a deluge of sand, which it spreads
over the plain below, for the streams do not carry the sediment to the
sea. So the mountains are washed down and the valleys are filled. On the
Arizona side of the river desert plains are interrupted by desert
mountains. Far to the eastward the country rises until the Sierra Madre
are reached in New Mexico, where these mountains divide the waters of
the Colorado from the Rio Grande del Norte. Here in New Mexico the Gila
River has its source. Some of its tributaries rise in the mountains to
the south, in the territory belonging to the republic of Mexico, but the
Gila gathers the greater part of its waters from a great plateau on the
northeast. Its sources are everywhere in pine-clad mountains and
plateaus, but all of the affluents quickly descend into the desert
valley below, through which the Gila winds its way westward to the
Colorado. In times of continued drought the bed of the Gila is dry, but
the region is subject to great and violent storms, and floods roll down
from the heights with marvelous precipitation, carrying devastation on
their way. Where the Colorado River forms the boundary between
California and Arizona it cuts through a number of volcanic rocks by
black, yawning canyons. Between these canyons the river has a low but
rather narrow flood plain, with cottonwood groves scattered here and
there, and a chaparral of mesquite bearing beans and