60 RECONNAISSANCE OF PART' OF WESTERN ARIZONA. 
mainly of angular fragments of rhyolite, but otherwise are similar in 
character and stratigraphic position to the Greggs breccia, which, as 
already stated, consists mainly of granitic debris. Both breccias 
antedate the present course of Colorado River, which has eroded into 
them. This correlation would tend to place the Greggs breccia in 
the Pliocene. On the other hand, the physiographic evidence does not 
seem to favor this correlation. The rhyolites, supposed to have been 
erupted at the close of the Miocene, rest upon the peneplain, a fact 
which tends to place the breccia in the Miocene. It is evident, there- 
fore, that the determination of the age of the Greggs breccia must 
await further investigation. 
Erosion of the Detrital-Sacramento Valley. — A long period of erosion 
apparently followed the rhyolitic eruptions and succeeding accumu- 
lation of breccia, during which the Detrital-Sacramento Valley was 
excavated. Measured from the original surface now represented by 
the mesas on either side, this valley is 5 to 15 miles wide and 3,000 
feet deep, including the gravel filling. It extends from southern 
Nevada southward to Williams River, where it is apparently inter- 
rupted by a lava-covered plateau, but its real continuity is shown by 
the gravel-filling beneath the lava (fig. 16, p. 54). From the plateau 
southward it is occupied by Colorado River for a distance of 75 miles, 
in the basin known as the Great Colorado Valley. Owing to circum- 
stances beyond the writer's control, the old valley was not traced 
south of .this basin. It may be pointed out, however, that at the 
southern end of the basin the river turns abruptly to the east and 
passes in a narrow rock canyon through the Chocolate Mountains. 
The general trend of the old valley suggests that it may continue 
through the hills west of Chocolate Canyon, or perhaps beneath the 
lavas, and connect with the Gulf of Calfornia (including at that time 
the Salton Sink), which lies southwest of the Chocolate Mountains. 
The origin of the Detrital-Sacramento Valley, although a matter 
of great interest, can not be fully described until more information 
is available. It is in a region of profound faulting and surface warp- 
ing, and may have originated as a succession of depressed areas. 
But whatever its origin it is believed to have been occupied and 
greatly modified, if not wholly formed, by a stream of considerable 
size. The reasons for this belief are as follows: 
The beds of volcanic tuff, breccia, and flow in the mesas on either 
side are composed of the same kinds of rock and are so disposed as 
to indicate that they may originally have been connected across the 
intervening space. The width of the valley, 5 to 30 miles, as com- 
pared with its length, about 375 miles (including the Great Colorado 
Valley), together with its somewhat sinuous course, accords more 
nearly with the conception of a moderately mature river valley than 
with a succession of downthrown areas. But perhaps the strongest 
