72 RECONNAISSANCE OF PART OF WESTERN ARIZONA. 
According to Arnold, the great time breaks occurred: (a) At the 
close of the Eocene, the break in this case being indicated by a distinct 
change of fauna; (b) before the close of the Miocene; and (c) some- 
time during the Quaternary, the latter two being erosion breaks. 
It seems probable that the long periods of sedimentation along the 
Pacific coast may be equivalent in a general way to the periods of 
quiet erosion described in western Arizona, and that the great and 
widely recognizable unconformities of the coast may correspond to 
the periods of crustal disturbances and volcanic activity of the 
interior. Thus the long period of erosion resulting in the formation 
of the Mohave peneplain may correspond to the deposition of the 
Vaqueros and Monterey formations, and the eruptions of rhyolite in 
Arizona to the unconformity between the San Pablo and the Mon- 
terey, which is associated with the volcanic eruptions of the coast 
region. 
The similarity in succession of Quaternary events in the two 
regions is not striking. Pleistocene time is represented on the coast 
by thick sediments and Recent time by some of the raised sea beeches. 
The thickness of the Pleistocene sediments renders it probable that 
the time required for their deposition is comparable with the long 
period of erosion during early Quaternary in the Arizona region, and 
the relative position of these epochs in the time scale apparently war- 
rants provisional correlation. The terraces of the coast may corre- 
spond in time with the various epochs of erosion and deposition of 
the Arizona region. 
Comparison with the Plateau region. — In the Plateau region the sub- 
division of Tertiary and Quaternary time is based mainly on physio- 
graphic evidence. Davis ° has shown that there were two main 
cycles of erosion, designated the plateau cycle and the canyon cycle. 
During the first or plateau cycle a large area in northern Arizona and 
southern Utah was reduced to a peneplain. The original surface of 
this peneplain is preserved in places by lava flows, as on the Shiwitz 
Plateau in northern Arizona and near Toquerville, Utah, b but has 
generally been dissected by later erosion. The peneplain will per- 
haps be best recognized— if due allowance be made for subsequent 
erosion — as the surface of the plateau in which Grand Canyon is 
eroded in northern Arizona. During the second or canyon cycle 
Grand Canyon was carved by the Colorado, and the peneplain, now 
elevated, was dissected to a less extent by the smaller streams. 
Following Davis, Huntington and Goldthwaite in their description 
of the Toquerville region of southwestern Utah emphasize the impor- 
tance of these cycles and add many details which aid in studying 
a Davis, W. M., An excursion to the Grand Canyon of the Colorado: Bull. Mus. Comp. Zool. Harvard 
Coll., vol. 38, 1901, pp. 107-201. 
b Huntington, Ellsworth, and Goldthwaite, J. W., The Hurricane fault in the Toquerville district, 
Utah: BulL Mus. Comp. Zool. Harvard Coll., vol. 42, 1904, p. 231. 
