250 



NATURE 



{Jan. 16, 1879 



deposited, a large portion, and perhaps the whole of this 

 region became, for a time, land, and the uplifting was at- 

 tended by considerable dislocation and flexing of the strata. 

 In numerous localities the Cretaceous strata are seen to 

 be denuded, and the lowest Tertiary beds lie across the 

 bevelled edges. This uplifting took place after the depo- 

 sition of a group of beds which in part, at least, are the 

 equivalents of those which King and Hayden have 

 named the Laramie Group. I accept the verdict of 

 Marsh, Meek, King, and Powell, that these beds belong 

 to the local Cretaceous series, and reject the decision of 

 Hayden, that they are Tertiary. Thus the close of the 

 Cretaceous is marked by a physical break separating it 

 from the local Tertiary series by widely distributed un- 

 conformities. 



After an unknown interval of denudation immediately 

 following the close of the Cretaceous period the region was 

 again submerged, and then began the deposition of that re- 

 markable series of Eocene beds which form such a striking 

 feature in the stratigraphy of the peripheral parts of the 

 Plateau Country. Around the southern flanks of the Uintas 

 their aggregate thickness exceeds 4,000 feet, but south- 

 westward the upper members at length disappear, and 

 seventy miles north of the Grand Caiion only the lower 

 portion of the local Eocene (the Bitter Creek of Powell or 

 Vermilion Creek of King) remain ; indeed, in the latter 

 locality no later beds than the Bitter Creek were de- 

 posited. The evidence is now conclusive that the Bitter 

 Creek series stretched more than a hundred miles across 

 the Plateau Country, covering, doubtless, its entire extent, 

 while the middle and later Eocene covered smaller areas 

 to the northward. Only marginal remnants of these 

 huge deposits now remain. The heart of them has been 

 eroded and swept away. Just at the commencement of 

 the Tertiary periods the Plateau Country was covered with 

 brackish water, having perhaps an analogy to the Baltic 

 or Euxine, but after the accumulation of a few hundred 

 feet of deposits the region became a vast inland lake. 

 Its northern shore was along the base of the Uintas, 

 which had then apparently gained their first elevation. 

 Its north-western shore, by a coincidence which can 

 hardly be accidental, lay along the identical boundary 

 which now sharply separates the Plateau Country from 

 the Great Basin, and the latter was one of the mainlands 

 which furnished the sediments of the lake. From the 

 angle where the Uintas join the Wasatch it is possible to 

 trace this shore line more than 300 miles south-westward 

 into Arizona with certainty, and to point out its principal 

 bays and headlands, and even to locate the sites of some 

 of the ancient river channels through which the lower 

 Eocene sediments were brought down. The eastern, 

 south-eastern, and southern margins, and the remainder 

 of the south-western margin, remain to be determined by 

 future exploration. At length, after one-third to one-half 

 of the lacustrine beds had been laid down, there began a 

 series of events which has developed the physical features 

 of the Plateau Country, and which has pursued an un- 

 broken course to the present time, and which even yet 

 may not have culminated. Then began that uplifting 

 which has raised the Plateau Country more than 13,000 

 feet on an average. Then, too, began a marvellous 

 erosion which has cut down the mean level about one- 

 talf that amount, leaving the present mean altitude 

 nearly 6,500 feet. At the inception of this process the 

 lake began to dry up, the south-western portion now 

 drained by the Lower Colorado being the first to emerge. 

 Gradually through the succeeding periods the lake con- 

 tracted its area, withdrawing northward to the Uinta 

 Mountains, where, at the close of the Eocene, it dis- 

 appeared. 



We are now in a position to trace the origin, growth, 

 and history of the Colorado River, if not from the be- 

 ginning, at least from an epoch near its beginning. Its 

 creation was not the event of oae epoch, but a gradual 



process extending through several periods. The lower 

 course, extending from the mouth of the Virgin to the 

 Pacific, is the oldest portion, and makes its appearance in 

 geological history a little before, but very near, the middle 

 Eocene. Whether it existed before this epoch is not beyond 

 doubt, but probably it did. But earlier than the Tertiary 

 periods it cannot go ; for it is certain that up to the close 

 of Cretaceous times the ocean flowed over its track. When 

 the Plateau Country was first isolated from the ocean it 

 became a brackish Euxine, and may be presumed to have 

 had a Hellespont somewhere. It soon after became an 

 inland lake and must have had a St. Lawrence to keep 

 its waters fresh. There can be little doubt that in the 

 middle Eocene the outlet was the lower course of the 

 Colorado. Whether the lake prior to that had some 

 other outlet which it abandoned for this one is an open 

 question,. with the probabilities (on general principles) in 

 favour of the negative. But the question is of no great 

 importance. 



The growth of the Colorado may be illustrated by 

 considering what might happen to the St. Lawrence if 

 the whole region of the Canadian lakes were uplifted two 

 thousand feet. In no great length of time Ontario would 

 be drained by the St. Lawrence, lowering its channel, 

 and that river would become one with the Niagara. 

 The same process would be repeated at Erie, Huron, and 

 Superior, the lakes vanishing and leaving only a great 

 river with many branches. Such was the origin of the 

 Colorado ; first a Hellespont, then a St. Lawrence, then 

 a common but rather large river heading in the interior 

 of a continent. Its principal branch, the Green River, 

 cuts through the Uinta mountains by the Flaming Gorge 

 and Caiion of Lodore. A second lake, apparently coeval 

 with the one we have just discussed, lay to the north of 

 that range and poured its waters through these gateways 

 into the southern lake. What other bodies of fresh water 

 may have been connected with either of these it is impos- 

 sible to say at present. 



At the epoch when the desiccation was completed it is 

 not probable that the caiions had any existence, for the 

 indications are that the elevation of the country at the 

 commencement of the Miocene period was not great. 

 Conditions favourable to caiion cutting are highly excep- 

 tional, and there is no evidence that this exceptional 

 combination of conditions existed at that time, while 

 there is much evidence that it did not. That the con- 

 ditions, however, were favourable to a rapid rate of 

 erosion is highly probable. But the forms which it would 

 produce might be more nearly analogous to those which 

 may be observed in eastern Ohio and western Pennsyl- 

 vania. That the climate was moist and sub-tropical is 

 rendered probable by the vegetable remains found in the 

 surrounding regions, and it is only rational to suppose 

 that such a climate in a moderately elevated region 

 would yield such results as may be seen in countries 

 similarly conditioned. Whether the valleys were broad 

 or narrow, abruptly walled or gently sloped, matters 

 little. It is almost certain that they were not deep. The 

 great caiions which we now see had not even been com- 

 menced, although they were foreshadowed, and the train 

 of events which was to produce them at a later period 

 had started into activity. 



The history of the Colorado and its drainage system 

 during Miocene time must be spoken of only in general 

 terms. In truth during this great age there is no evi- 

 dence of the occurrence of any critical event aside from 

 the general process of uplifting and erosion which affected 

 the region as a whole. The vast erosion of this region 

 has swept away so much of its mass that most of the 

 evidence as to the details has vanished with its rocks. 

 But the more important features of the work, its general 

 plan in outline, have left well-marked traces and these 

 can be unravelled. It was a period of slow upliftmg, 

 reaching a great amount in the aggregate, and it was also 



