16 INTEODUCTORY. 



as the lower course of the Colorado from the mouth of the Virgin to the 

 Pacific. Whether the connection was at first elsewhere and at an early 

 epoch in Tertiary time shifted to this place may be doubtful, but the prob- 

 abilities at present are that the connection was southwestward along the 

 lower course of the present river. But after the desiccation of che lake 

 began in the latter part of the Eocene, the course of the Colorado was fixed 

 for the remainder of Tertiary time. In order to conceive the growth and 

 evolution of this river, let us endeavor to imagine what might happen if the 

 whole region of the Canadian lakes were to be progressively uplifted sev- 

 eral thousand feet. In due time the St. Lawrence would sink its channel 

 by the increasing corrasive power of its waters, and would drain in succes- 

 sion Ontario, Erie, Huron, and Superior, becoming a great river with many 

 branches, while the lakes would be emptied. Such was the early history 

 of the Colorado ; first a Hellespont, then a St. Lawrence, then a large river 

 heading in the interior of a continent. 



The relations of the Colorado to the strata through which it runs present 

 certain phenomena which, when rightly understood, become a master-key 

 in the solution of a whole category of problems of a most interesting and 

 instructive character. It would be difficult to point out an instance of a 

 river under conditions more favorable to stability in respect to the location 

 of its course than the Colorado and its principal tributaries. Since the 

 epoch when it commenced to flow it has been situated in a rising area. Its 

 springs and rills have been among the mountains, and throughout its history 

 its slope has been increasing. The relations of its tributaries in this respect 

 have been the same, and indeed the river and its tributaries constitute a 

 system and not merely an aggregate, the latter dependent upon and thor- 

 oughly responsive to the former. Now, the grand truth A\hicli meets us 

 everywhere in the Plateau Country, which stands out conspicuous and self- 

 evident, which is so utterly unmistakable, even by the merest tyro in geol- 

 ogy, is this : The river is older than the structural features of the country. 

 Since it began to run, mountains and plateaus have risen across its track 

 and those of its tributaries, and the present summits mark less than half the 

 total uplifts. The streams have cleft them to their foundations. Nothing 



