56 RECONNAISSANCE OF PART OF WESTERN ARIZONA. 
petrographic character as those contained in the Temple Bar con- 
glomerate (see p. 17) and were supposedly erupted at about the same 
time. For these reasons the detrital filling is correlated tentatively 
with the Temple Bar conglomerate. 
GEOLOGIC HISTORY. 
INTRODUCTORY STATEMENTS. 
It is desirable to consider at this point the geologic history of the 
region — to review the facts that have been presented and to emphasize 
their significance. Certain considerations which depend upon the 
interrelations of features at various localities are also presented with- 
out reference to former descriptions. The reader should be reminded 
again that the observations were made during rapid reconnaissance 
trips and that large intervening areas were not examined. With the 
exception of the Paleozoic formations of the Plateau region, none of 
the sediments of northwestern Arizona are fossiliferous so far as ob- 
served, and the younger formations can not be traced continuously 
from place to place. The history of the region is largely one of de- 
structive processes, and the ordinary means of correlation are wanting. 
For these reasons, although the sequence of the events is apparently 
clear, their places in the geologic time scale can not now be definitely 
fixed. The observed facts show that the late geologic history of the 
region is separable into epochs comparable with those established in 
neighboring regions, but in the absence of adequate correlation data 
the intention is to emphasize the sequence of events, the provisional 
correlations being offered only as a working basis. 
PRE-CAMBRIAN CONDITIONS. 
Crystalline rocks. — Throughout northwestern Arizona the oldest 
rocks are coarsely crystalline granites and gneisses. In the plateau 
to the east the granites underlie the Cambrian sandstone and are inter- 
sected by less coarsely crystalline intrusives. 
Sedimentation. — The pre-Cambrian sediments so extensively devel- 
oped in the Grand Canyon region apparently once extended over west- 
ern Arizona, and remnants of them remain at the northern end of the 
region near Virgin Canyon and from Williams River southward in the 
southern part of the area examined. 
Base-leveling. — Previous to the deposition of the Cambrian sedi- 
ments there was a long period of erosion during which such sediments 
as may have been deposited in the eastern part of the region were re- 
moved by erosion and the underhung granites were base-leveled . The 
line representing this base-level may be followed in the cliffs from 
Grand Canyon to the southern end of the Juniper Mountains. 
