286 The American Geologist. November, 1003. 
a period of little or no deposition till Cretaceous time. The 
Laramie was then deposited after which the sea retreated for- 
ever from the region. As soon as the sea was expelled erosion 
began and as the country was being rapidly elevated great riv- 
ers chiselled out deep canyons in the strata. This period of in- 
cision was of long duration for in many places at least 3000 
feet of strata were removed. It continued from the Laramie 
epoch till the seismic disturbances of Tertiary time blocked 
the water courses and formed lake basins on a grand scale. 
Then a period of deposition began which was continuous till 
near the close of the Quaternary when a period of denudation 
again set in, which has continued almost without break to the 
present time. 
GEOGRAPHY AND PHYSIOGRAPHY. 
Mountains. 
The Ellison Dome. — -After the deposition of the Palaeozoic 
strata the region hi the vicinity of Ellison became land and so 
far as the evidence goes no more sediments were deposited in 
the region till in the Tertiary and Quaternary periods. At the 
beginning of the interval of non-deposition the area was domed. 
During the period of elevation base-leveling processes began 
and continued till they removed nearly all the elastics from the 
Archaean centre, removing from 4000 to 6000 feet in vertical 
section. On the domed area the streams flowed in all directions 
towards its rim, as the dry canyons of Tonto, Chevelon, Car- 
rixo, Dry Fork, and Cibicu indicate. Although the upland sur- 
face was reduced to a fairly even plain there still remain rem- 
nants of an old upland overlooking the general level of the 
domed area ; in other words, the topography suggests the devel- 
opment of a peneplain which was subjected to further move- 
ments before the base-leveling was complete. Then followed 
additional doming, accompanied by faulting in such a manner 
as to increase the rock dips both on the east and on the west 
slopes of the original dome. The west series forms the Sierra 
Ancha. the eastern the Canyon Creek escarpment Avhich is con- 
tinuous with the Plateau escarpment south of Salt river. The 
• south part of the dome was also faulted off. Since faulting 
streams have incised channels along- the fault lines in the domed 
area, and besides removing much of the remaining sediments, 
they have given off side branches one of which (Salt river) 
