DETAILS OF VALLEYS. 29 
group is greatly eroded, the peaks being separated by comparatively 
low passes. The western part is composed of irregular hills formed 
by the dissection of a plateau of igneous rock, the general surface of 
which has an altitude of about 2,400 feet. The topographic features 
of the basalt-covered plateau near the mouth of Williams River are 
especially conspicuous (PI. VI, B). The basalt of the surface is very 
resistant, and the underlying sand and gravel (see p. 54) is easily 
eroded, resulting in the formation of precipitous cliffs nearly 2,000 
feet high. 
Rock masses. — The rocks of the Buckskin Mountains consist of 
biotite granite, granitic gneiss and schist, metamorphic sediments, and 
effusive rocks. The gneisses and schists are conspicuously exposed 
in Williams Canyon, where they are horizontally laminated and 
fissile, suggesting the name Banded Canyon, by which Williams 
Canyon is locally known. 
The crystalline rocks are overlain by quartzites, slates, and black 
metamorphic limestones. These are faulted and intersected by 
intrusive rocks, and contain deposits of copper and iron ore. A bed 
of hematite about 150 feet thick was observed near Planet. 
PALOMAS AND DOME ROCK RANGES. 
Location. — The Palomas and Dome Rock Mountains, in the south- 
western part of the region, w r ere not visited by the writer. Their 
form and location are taken from the railway-location map made by 
the Santa Fe engineers in locating the Arizona and California Rail- 
road, furnished for this bulletin by the chief engineer, W. A. Drake. 
Little is definitely knowm of their topographic features or rock masses. 
GEOGRAPHIC AND GEOLOGIC DETAILS OF VALLEYS. 
COLORADO VALLEY. 
GENERAL STATEMENT. 
Colorado River was traversed by boat from the mouth of Grand 
Canyon southward to Yuma, a distance of about 325 miles. Through- 
out this course the rocks in which the valley is eroded are well ex- 
posed and geologic features of great variety are well exhibited. For 
convenience of description the valley is divided into sections, the 
important features being taken up in order from the mouth of the 
Grand Canyon downstream. Several diagrammatic sections across 
the valley are included with the descriptions in order to illustrate 
relations not adequately shown in the more generalized sections on 
PL V. 
In the published accounts of Major Powell's explorations of the 
Colorado little is said of the river west of the mouth of Grand Canyon. 
