Marcu 14, 1884.] 
This period closed amid important disturbances ; 
as, in neighboring districts, the succeeding beds 
are occasionally found to rest directly on the 
Jura-trias, or basset edges of the cretaceous. 
As a result of these movements, a great eocene 
lake was formed, which appears to have been 
not far above the sea-level, and to have out- 
flowed southward. ‘The change from salt to 
fresh water conditions is quite sudden. During 
the eocene period this lake gradually shrank 
back, and finally disappeared to the north, 
near the base of the Uinta Mountains, where 
alone the later eocene deposits are found. 
The author finds a little difficulty in explain- 
ing the great horizontal similarity in character 
of the materials laid down during the later 
periods in a wide expanse of water so persist- 
ently shallow. It is suggested, however, that 
the very shallowness of the sea or lake may, 
by insuring a constant sectional area, have re- 
sulted in maintaining the velocity, and conse- 
quent transporting-power, of any currents which 
existed. The continuance of shallow-water 
conditions also proves that the profound sub- 
sidence required by the volume of the sedi- 
ments was not acquired suddenly, but that the 
two increased pari passw. 
With the eocene the immensely prolonged 
period of subsidence came to an end, and ele- 
vation — shown to have been somewhat spas- 
modic — and consequent erosion began. It 
is found by reducing the faults and flexures 
of the region, and taking into consideration 
the thickness of the beds, that at this time the 
earboniferous must have lain over considera- 
ble areas, at a depth of from ten thousand to 
twelve thousand feet below the level of the sea ; 
and it is interesting to remark in passing, that 
no great degree of alteration appears to have 
resulted from this deep burial in the earth’s 
crust. The present altitudes of the plateaus 
mark the difference between the amount of the 
succeeding uplift and that of denudation; and 
it is shown that the total movement in eleva- 
tion has been in different places from twelve 
thousand to eighteen thousand feet. 
The ‘ great erosion,’ as Capt. Dutton names 
the second period of ‘the history, began at the 
time of the drainage of the eocene lake, of 
which there is good reason to believe the 
Colorado still marks the position of the deep- 
est portion. The immutable permanence in 
position of the channels of the Colorado and 
its main tributaries, and the fact that they 
have been able thus to maintain their original 
courses in opposition to the superimposed north- 
ward dip of the beds, is one of the most strik- 
ing facts brought out in the study of this 
SCIENCE. 
329 
district, being, in fact, that which has been most 
influential in impressing it with its peculiar 
features. In the latter part of the eocene, and 
in the miocene, great progress was doubtless 
made inthe removal of the mesozoic strata. 
The process of elevation continued, and rapid 
corrasion by numerous streams made steady 
progress ; the Colorado, at this time, probably 
flowing in a canon walled by these mesozoic 
formations, the escarpments of which have now 
retreated to the terrace district, fifty miles or 
more to the northward. Through all this lapse 
of time we are, however, without any very 
precise data as to the progress of the erosion ; 
and it is not till a date approximately referred 
to the close of the miocene that any measure 
of the waste accomplished can be arrived at. 
The elevation of the district was then for a 
time arrested; and the streams reached what 
the author, following Major Powell, calls a 
‘ base level of erosion,’ in which, with the pro- 
duction of a uniform light gradient, the wear 
of their channels clased, and denudation acted 
only in reducing the probably rough and 
irregular features of the neighboring country 
to an approximate level. ‘The Permian strata 
apparently at this time constituted the actual 
surface. 
About the time at which the Colorado began 
to cut into the carboniferous rocks, a climatic 
change occurred, which resulted in producing 
very arid conditions, and dried up the smaller 
streams to their sources. ‘This, from what is 
elsewhere known of the western tertiary, is 
presumed, with great probability, to be syn- 
chronous with the close of the miocene and 
beginning of the pliocene. Nearly contem- 
poraneous with these events was an uplift of 
two thousand to three thousand feet, and the 
outpouring of the earlier basalts, which, form- 
ing protective cappings, have preserved por- 
tions of the Permian surface above alluded to. 
The great faults, also, about this epoch first 
betray their existence ; though it is by no means 
certain that all were then formed, and the evi- 
dence is clear that their throw subsequently 
continued to increase gradually. Corrasion, 
or the wear by the rivers of these beds, now 
again became active, but only in the case of 
the larger streams, which, by reason of their 
origin in high, well-watered uplands beyond the 
cahon district, had been enabled to survive. 
A base level was soon again reached: and the 
Colorado remained during the greater part of 
the pliocene at the level of what Capt. Dutton 
calls the esplanade, or wide upper valley of 
the present canon; which valley continued to 
increase laterally, but not in depth, till the 
