GEOLOGIC HISTORY. 73 
the geologic history of the Grand Canyon region. The succession of 
Tertiary and Quaternary events described by them may be sum- 
marized, in order from oldest to youngest, as follows: 
1. Eocene. A period of deposition during which the Plateau 
region was low. 
2. A period of elevation accompanied by folding, flexing, and 
extrusion of andesite. 
3. A second period of erosion — the pre-fault cycle. 
4. Earlier faulting, during which the great faults of the Plateau 
region, such as Hurricane and Grand Wash faults, were originally 
formed. 
5. Interfault cycle of erosion, during which the Mohave peneplain 
was formed. 
6. Later faulting and final uplift of the High Plateau. 
7. Erosion of Grand Canyon. 
The history thus developed corresponds in many ways with that 
of western Arizona, but differs notably in some respects. The early 
Tertiary erosion in western Arizona seems to correspond in time 
with the Eocene sedimentation of the plateaus, and eruptions of 
andesite marked the close of the period of both provinces. There 
is, furthermore, no disagreement regarding the time of uplift and 
faulting immediately preceding the erosion of Grand Canyon, but 
further investigation is necessary before the subdivision of middle 
and later Tertiary time in the two provinces can be harmonized. 
Apparently, the great masses of rhyolite, 3,000 feet thick in western 
Arizona and presumably extruded at the close of the Miocene, were 
not found in southwestern Utah, and the long period of erosion which 
followed their extrusion, presumably during Pliocene time, seems to 
be included by Huntington and Goldthwaite in the time during 
which the Mohave peneplain was being formed, since according t<> 
them this cycle of erosion ended with the uplift of the plateau and 
the origin of Grand Canyon. 
Since the Mohave peneplain is here used as a correlation horizon, 
it may be in place to inquire into its probable extent. As already 
stated, this peneplain, which in the Plateau region of southern Utah 
and northern Arizona is represented by the platform at the top of 
Grand Canyon, is here regarded as extending generally over north- 
western Arizona. A similar peneplain has been described by Lind- 
gren a in the Sierra region of eastern California, where he finds evi- 
dence of an extensive peneplain, formed presumably during the 
Miocene and covered in part by the younger auriferous gravels, and 
these in turn by rhyolites and andesites — a sequence which corre- 
sponds with that in northwestern Arizona. Other localities mighl 
oLindgron, Waldemar, Age of the auriferous gravels of the Sierra Nevada: Jour. Geology, \"i 55, 
1896, pp. 881-906. 
