14 RECONNAISSANCE OF PART OF WESTERN ARIZONA. 
The fourth group of mountains, situated south of Williams River, 
does not correspond with the other three groups in geographic posi- 
tion or in regularity of arrangement. In place of the parallel arrange- 
ment of north-south ridges just described, this group is made up 
of three large east- west ranges and three small north-south ranges. 
The Harcuvar and Harquahala mountains are about equal in altitude 
and are the loftiest and most massive mountains in central western 
Arizona. The other groups are less conspicuous, though the Buck- 
skin Mountains are moderately prominent, particularly at their east- 
ern extremity. 
Valleys and plains. — Three distinct kinds of lowland occur: First, 
the valleys and canyons now containing running water, such as the 
Colorado and Williams valleys, including Big Sandy Wash and Santa 
Maria Canyon; second, old debris-filled valleys, such as the Detrital- 
Sacramento, Hualpai, and Big Sandy valleys ; and third, plains eroded 
from hard rock or built up by flows of igneous rock, such as the Trux- 
ton Plateau. Extensive detrital plains, such as Cactus Plain, Rane- 
gras Plain, etc., which occur south of Williams River, are probably 
debris-filled valleys similar to those farther north, but their character 
is not certainly known. 
DESCRIPTIVE GEOLOGY. 
ROCK FORMATIONS. 
The rocks of western Arizona are mainly pre-Cambrian crystallines, 
Tertiary eruptives. and probably Quaternary sediments. Sedimen- 
tary formations of Paleozoic age occur only at the eastern margin of 
the area described. The igneous rocks are described by Albert Johann- 
sen (see pp. 81-92), but many of the details upon which this general 
description is based may be found in the following sections. 
PRE-CAMBRIAN. 
The oldest rocks of the region consist mainly of granites and 
gneisses. (See p. 81.) Coarse-grained granitic rock giving little or 
no evidence of gneissoid structure is found in many places throughout 
northwestern Arizona. It occurs beneath the Cambrian sandstone 
at the base of the cliffs in the eastern part of the region, forms the 
central core of the Black Mountains, and constitutes the main mass 
of the Cerbat and Hualpai mountains. Gneiss occurs in many places, 
either in extensive masses, as at the southern end of the Hualpai 
Mountains and in the Aubrey Hills, or in restricted areas where local 
movements have taken place, as near Hualpai Wash, at the northern 
end of the region. At the mouth of the Grand Canyon the granite, 
which had obviously been exposed at the surface prior to the depo- 
sition of the Cambrian sediments, is prevailingly red to a depth of 
