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Show 16 INTBODUCTOEY. 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 probabilities 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 several thousand feet. In due time the St. Lawrence would sink its channel by the increasing corrasive power of its waters, and would drain in succession 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 thoroughly responsive to the former. Now, the grand truth which 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 geology, 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 |
