794 ME. AV. BAELOW 02s^ THE HOEIZOIfTAL 



that large tracts of the earth's crust have in this district experienced 

 related movements, and points to a liquid or very plastic state of 

 the underljing rocks at the time. The probability that separation 

 by strain has in many cases initiated the diversion of rivers in the 

 Western States has been recognized by some American geologists*. 

 Other explanations of the want of conformity referred to appear 

 to me to involve very serious difficulties. Take the supposition that 

 the Plateau-region has been elevated so slowly that the corrasion of 

 the Colorado Eiver has kept pace with it, and that, in this way, the 

 position of this river has remained constant, while the ground- 

 surface has been changed and new structural features created by the 

 movement of the rocks. To this there is the following important 

 objection. However slow the rate of elevation of an uplift, there 

 must be some effect from the lessening of the gradients of the 

 watercourses where they are approaching the uplift, and from the 

 steepening of the gradients where they are leaving it, and con- 

 sequently, in the case referred to, diminished erosion should be found 

 on one side of the uplift and increased erosion on the other, and 

 the modification of the bed and banks of the stream resulting from 

 this distributio]! of force should be apparent. 



Now whatever uplift has taken place has, in the main, taken 

 place without materially affecting the horizontahty of the strata, 

 consequently it is only where the river leaves the elevated tableland 

 that any increase whatever in the fall of the river can be supposed 

 to have been directly produced by the uplift, and the extreme erosion 

 should, it would seem, if this were the explanation, be confined to 

 this end of the river. 



The facts that the river has sunk its bed deep into the strata 

 throughout the whole length of the elevated tract and that steep 

 gradients are not at all confined to its lower end prove, I submit, 

 that, whatever the explanation, it is not this. 



If when the uplift, which has raised the Plateau-region to its 

 present altitude, began, the Colorado river was peacefully meandering 

 along a nearly level surface of horizontal strata since cut away by 

 denudation, it appears to me that any such elevation as that which 

 has taken place must, however slowly it occurred, have diminished 

 the rate of flow of the river and have converted it into a succession 

 of sluggish pools, and finally have dammed it back and obliged it to 

 take a new course f. 



If, on the other hand, the course of the river was marked out by 

 the opening of a fissure by horizontal strain, one can see how the 

 weakening of the rocks bordering the precipices by the creeping 

 movements to which I have called attention would pave the way for 

 rapid erosion+. 



* Eeport Geological and Geograpliieal Survey of Colorado, 1874, pp. 105, 

 193, 201, 220, and 227 ; and Report Idaho and Wyoming, 1877, p. 65. 



t A very slight movement of the rocks is often sufficient to change the course 

 of a river. It requires a movement of a few feet only to change the outlets of 

 Lakes Michigan, Huron, and Superior from Illinois Eiver to the St. Clair. 



I See ' Geology of the High Plateaus of Utah,' p. 37. 



