218 
THE GEAND CASTON DISTEICT. 
a land-locked area like the Euxine. Its outlet would necessarily have been 
along the lower courses of the Colorado to the Gulf of California, or, per- 
haps, straight westward to the Pacific. 
Having thus obtained a consistent view of the manner in which the 
great Eocene lake of the Plateau Province may have originated, it now 
remains to follow out such changes as are indicated in its subsequent his- 
tory. It should seem that the passage from the brackish water to the fresh 
water condition was quite sudden, and as the same is true of widely ex- 
tended areas outside of this region, we are apparently obliged to assume that 
the movement of which this was a result afiected the entire western portion 
of the continent, and that it was one of elevation. A considerable number 
of large lakes being formed, the next process was the desiccation of these 
lakes and the evolution of river systems. So long as the region occupied a 
low altitude this process, we may infer, would be very protracted. Before a 
larfire lake can be drained its outlet must be cut down. But several causes in 
the present instance would combine to render this action very slow and 
feeble. The elevation being small, the declivity and consequent corrasive 
power at the outlet must be correspondingly small. Moreover, the waters 
issuing from a large lake contain little or no sediment ; and sediments — 
sand, grit, &c. — are the tools with which rivers chiefly work in corrading 
their beds. Corrasion by clear water is an exceedingly slow process. 
It is not surprising, therefore, to find that the lakes produced by the 
first action of the elevating forces persisted for a very long time. This per- 
sistence is a general feature of the Eocene lakes of the West. The Plateau 
lake seems to have been one of the largest and most enduring, for it did 
not wholly vanish until the close of the Eocene. The volume of sediment 
accumulated upon its bottom was very large, ranging from 1,200 to more 
than 5,000 feet in thickness, and these deposits represent Eocene time ex- 
clusively. Here we are confronted by the same paradoxes as those we en- 
countered in viewing the Cretaceous condition of the region : a tract which 
is rising yet sinking ; a basin which is shallow, which receives great thick- 
ness of deposits, and yet is never full. 
At length we detect evidence of the gradual cessation of deposit and 
of the progressive upheaval of the country. In the chapter which treats 
