DETAILS OF VALLEYS. 49 
The rock exposed along the river is principally effusive, although 
granite and metamorphosed sediments occur in some places. The 
effusive rock, consisting of pink and 'white rhyolite in the form of ash, 
breccia, and flow, extends from the river level apparently to alti- 
tudes of several thousand feet, and is characterized by conspicuous 
cliffs, erratic forms of erosion, and rock slopes barren of vegetation. 
Physiographic changes similar to those at Bulls Head and else- 
where were noted in Chocolate Canyon. At Lighthouse Rock the 
river has apparently left its old channel and cut laterally into the 
rhyolite, leaving Lighthouse Rock standing in the water. A boring 
in the channel at this point failed to reach bed rock at a depth of 80 
feet. Farther down the river, near Picacho, several borings in the 
river bed failed to reach solid rock at a depth of 100 feet. 
Probably the best illustration of changes in the course of the river 
in this vicinity is to be found at the so-called Cacopah dam site of the 
Reclamation Service, near the southern end of Chocolate Canyon. 
At this point the Colorado flows close to the rhyolite hills on the west, 
with one rock island standing in the river and another on the eastern 
bank. (See fig. 13.) Between these islands and the hills to the east 
is a broad sand flat. t ; 
Borings made at the various places indicateain fig. 13 show that 
the channel now occupied by the river has been eroded to a depth of 
about 100 feet, and later filled with sediment, while the old channel 
to the east is deeper, no solid rock being found at a depth of 138 feet. 
A significant observation was made at this point by Homer Ham- 
lin," who noted that in one of the borings the drill penetrated through 
the rhyolite and entered a gravel bed. (See fig. 13.) The rhyolite is 
presumably of Tertiary origin, as are also the rhyolites farther north, 
more fully described elsewhere (p. 83). It is possible that the under- 
lying gravels may be equivalent in age to some of the tilted and con- 
solidated gravels farther north, which were referred by Newberry 
to the Tertiary, but which the present writer has provisionally 
correlated with the Temple Bar conglomerate and referred to the 
Quaternary. 
HUALPAI VALLEY. 
Location and character. — The Hualpai Valley, about 60 miles long 
and 25 miles wide, is located in the northeastern part of the region 
described. Except for a few miles at the northern end, it is an 
undrained basin, in the lowest part of which flood waters gather and 
remain until they either evaporate or sink into the valley filling. 
"Red Lake," thus formed, contains water for periods varying from 
a few days to several months. At other times it is a broad, barren 
mud flat. 
« Personal communication. 
49964— Bull. 352—08 4 
