ABSTRACT OF THE MONOGRAPH. 
7 
country was then upheaved, enormously eroded and again submerged. 
Upon the denuded surface the Carboniferous was deposited unconformably, 
and deposition continued without notable interruption until the close of the 
Mesozoic. In that long succession of ages from 12,000 to 10,000 feet of 
strata accumulated over the entire Plateau Province. The beds are remark- 
able for their homogeneity and constancy of character over vast areas, 
though the different formations vary greatly in lithological character; that 
is to say, homogeneity in horizontal range, with great heterogeneity in ver- 
tical range. The Carboniferous system may have accumulated in waters 
of moderate depth, but the Mesozoic beds are all shallow-water deposits 
The surface of deposition remained throughout Permian and Mesozoic 
time very near sea level, which is equivalent to saying that the beds sank 
as rapidly as they accumulated. Near the close of the Cretaceous signs of 
the coming revolution make their appearance. The waters became brack- 
ish, indicating a restricted access of the ocean. At the close of the Creta- 
ceous important disturbances took place, and portions of the province were 
uplifted and denuded. These were again submerged, but the new con- 
ditions differed from the old, for the new deposits (Eocene) laid down un- 
conformably upon the Cretaceous and Jurassic are of fresh-water origin, 
indicating that a great lake was formed. The extent of this lake corre- 
sponds very nearly with that of the Southern Plateau Province itself, but 
not exactly. Near the middle Eocene began that slow action which has 
gradually elevated the western portion of the continent, and which has 
prevailed until a recent epoch. It does not appear to have progressed at a 
constant rate, but rather by maxima and minima, or still more probably 
through alternating periods of activity and repose. 
The Tertiary history of the region is a great chapter of erosion. Many 
thousands of feet of strata have been swept away. The thickness removed 
from some large areas amounts to about 10,000 feet, while from others a 
much less thickness has been denuded. In the Grand Canon district we 
find the largest area of maximum erosion. Much the greater part of this 
denudation was probably accomplished by the close of the Miocene. 
The Colorado River appears to have originated in very early Tertiary 
time as the outlet of the great Eocene lake, and has persisted in its course 
