190 
THE GRAND CAHON DISTRICT. 
the Miocene age and througli at least two-thirds of the Eocene, and though 
we may not reckon this period in terms of centuries we cannot doubt that 
it was a vast one, and sufficient, under favorable conditions, for an enor- 
mous amount of waste. The work of removing nearly 10,000 feet of strata 
from a great area is a formidable thing to contemplate, but under the given 
conditions the time-factor will no doubt be regarded as being commensu- 
rate. What aspect the country presented during this great stretch of time 
we have no means of judging. All records of even the broader details 
have vanished with the strata. We only infer that not far from the close 
of the Miocene the great mass of Mesozoic beds had been in chief part 
denuded ; that the resulting platform of the Grrand Canon district was at a 
much lower level than at present, and probably not very far above the sea- 
level. The canon of the Colorado either did not exist at all at that time or 
was at most only just begun. 
At this epoch the climate gradually became more and more arid. 
Although the evidence of this is not found in the district itself, it appears 
conclusively in the regions adjoining it. It is a remarkable fact that the 
Pliocene lake-basins, both north and south erf the Grrand Canon district, 
exhibit saline deposits, while the earlier lake beds show nothing of the kind. 
A saline lake-deposit means an arid climate — small feeble streams flow- 
ing into an inclosed basin, which they cannot fill to overflowing, because 
evaporation is so rapid that it keeps pace with the influx. And as the ad- 
joining regions were arid, so also we conclude was the Grand Cation district. 
This inference is further sustained by the fact that those traces of Pliocene 
erosion which are here and there preserved have the characteristics which 
belong to the sculptural forms of an arid region. These are ancient canons 
with abrupt walls in the more adamantine strata which have survived the 
ravage of later periods. Thus, thi-ough the Pliocene the conditions were 
unfavorable to a very rapid rate of degradation. During the glacial period, 
however, this rate must have been vastly greater ; but, if we are to attach 
any value to current estimates of the relative duration of geological periods, 
this episode was comparatively brief. At its close the climate lapsed back 
to its former aridity . 
