12 RECONNAISSANCE OF PART OF WESTERN ARIZONA. 
No definite line of separation can be drawn between the Desert and 
Mountain provinces in the area here described. In central and south- 
ern Arizona the distinction is well marked ; a but in western Arizona 
the Mountain region, so prominent in the central part of the Terri- 
tory, becomes a transition belt in which the descent of about 6,000 
feet from the high plains of the Colorado Plateau to the low-lying 
plains of the Desert region of southern Arizona is accomplished 
through several mountain groups with gradually diminishing eleva- 
tion. The transition becomes less marked northward until in north- 
ern Arizona the change in elevation is accomplished largely by fault 
scarps. 
North of the area described Gilbert, in the Wheeler reports (and 
other geologists following him), makes but two general provinces, the 
High Plateau and the Great Basin, but states b that on account of an 
interlocking of characteristics along the borders their common bound- 
ary is not easily located. This division into two topographic prov- 
inces is a more natural one for northwestern Arizona than the 
threefold division of central Arizona. The basin ranges and broad 
gravel-filled valleys of Nevada extend southward into Arizona and 
bear the same general relations to the High Plateau that they do in 
southern Nevada and Utah. 
TOPOGRAPHY. 
General character. — The Colorado Plateau here enters into consid- 
eration only in so far as it forms the boundary of the area described. 
This bulletin is concerned chiefly with the low-lying area to the west. 
In the Mountain region, south of the area described, the rocks have 
been deeply dissected by erosion, and great crustal movements 
have occurred. Near the escarpments bordering the plateau the 
mountains consist in some places of disturbed portions of the sedi- 
mentary formations of the plateau, let down to their present position 
by faulting. Mountains of this kind occur in the vicinity of Globe, 
Ariz., c in the Tonto basing and within the region described near 
the mouth of Grand Canyon. In many places, especially at a distance 
from the edge of the plateau, sedimentary rocks are not found, and 
it is difficult to determine whether mountains composed entirely of 
crystalline rock are due to local elevation or are simply masses left 
by erosion during the general degradation of the region. 
a Lee, W. T., Underground waters of the Salt River valley, Arizona: Water-Supply Paper U. S. 
Geol. Survey No. 136, 1905. 
b Gilbert, G. K., and Marvine, A. R.: U. S. Geog. Surv. W. 100th Mer., vol. 3, 1875, p. 57. 
c Ransome, F. L., Description of Globe district, Arizona: Geologic .Atlas U. S., folio 111, U. S. Geol. 
Survey, 1904. Also Geology of the Globe copper district, Arizona: Prof. Paper U. S. Geol. Survey 
No. 12, 1903. 
d Lee, W. T., Underground waters of the Salt River valley, Arizona: Water-Supply Paper U. S. Geol. 
Survey No. 136, 1905, p. 117. 
