38 GEOLOGY OF THE HIGH PLATEAUS. 



their general outlines or even their larger details along the structure lines 

 The same is true of the heart of the province. The evidence is clear and 

 irrefragable that at a comparatively recent epoch there has been a wide- 

 spread uplifting coming upon the country suddenly as it were after an im- 

 mense period of repose. Before its advent the streams had long remained 

 at the limiting levels where they could sink no more, and the slower pro- 

 cesses of decay, the recession of cliffs, the widening of valleys, the shrink- 

 age of mesas, the lateral expansion of canons, had been in progress long 

 enough to have produced very extensive results. As this uplifting came 

 upon the land the rivers were at once disturbed and resumed their occupa- 

 tion of deepening their channels, and sank them almost as fast as the coun- 

 try rose. But they remain to-day with walls but little affected hj lateral 

 waste. Every indication points to the conclusion that they are freshly cut 

 and are still cutting. 



Thus the study of the effect of erosion upon the uplifted sides of the 

 great displacements of the High Plateaus everywhere indicates relative re- 

 cency. The time during which these displaced edges have been subject to 

 the action of the elements is trifling when compared with the interval which 

 separates us from the Eocene. It is represented only by a work which is 

 relatively small and easy of accomplishment and performed under circum- 

 stances most favorable to rapidity and efficiency. But the general denuda- 

 tion which dates back to the Eocene is incomparably greater in amount, 

 considering only ecpial areas ; and represents in chief part the kind of 

 degradation which is relatively slow, performed under circumstances not 

 always favorable to rapidity. 



There is another point of view from which we arrive at the same con- 

 clusion, that the great displacements are very young. The volcanism of 

 the country has a history which we are able to unravel as to its broader 

 features. It began after the disappearance of the Eocene lake which cov- 

 ered the Plateau Province. How long after the desiccation we cannot 

 sav even relatively. The lake had withdrawn apparently from the High 

 Plateau District soon after the close of the Upper Green River epoch, which 

 represents a period in the latter part (but before the close) of the local 

 Eocene Resting uuconformably upon the Upper Green River beds is a 



