dottoh.J GENERAL METHODS OF EROSION. 63 



This theorem is of great utility in the study of the Plateau Province, for 

 it throws light upon many problems which would otherwise be obscure. 

 The course of a river is the index of the slope, and, to a great extent, 

 the configuration of the primitive unmodified surface of a tract. It be- 

 trays the amount of tilting or flexing which the strata have undergone, 

 and also conveys information as to the amount of strata which have 

 been denuded. This information, however, is in many cases incomplete, 

 but when placed in relation with other facts it frequently becomes con- 

 clusive. The application of the theorem to the San Rafael district is a 

 beautiful instance of its validity. 



Across the San Rafael Swell extend two river channels, one crossing 

 it near the northern and the other near the southern end. They head 

 in the High Plateaus, and pass through the successive terraces in deep 

 canons; then crossing the swell, enter the high cliffs on the eastern side 

 and How on. The northern stream— the San Rafael River— ultimately 

 joins the Green River; the southern one— Curtis Creek— enters the Fre- 

 mont River, a tributary of the Colorado. A glance at the map and an 

 interpretation of the topography as expressed by the contours will 

 quickly show that these streams are quite independent of the existing 

 topography and could not have had their situations determined by it. 

 They must have been laid out upon some ancient surface differing 

 widely from the present. To find that surface is not difficult. It must 

 have had a continuous descent, though doubtless of slight declivity, 

 from the western margin of the province to the line of the Green and 

 Colorado Rivers. We shall obtain precisely that surface configuration 

 by reducing or bending back the flexures, and depressing the tilted 

 strata until the Cretaceous beds are everywhere horizontal, and then 

 filling up the gaps made in the continuity of the strata by erosion. 

 Thus we shall reach, by argument from the persistence of rivers, the 

 same conclusion which we reached by studying the effects of the reces. 

 sion of cliffs, and by the independent study of the displacements. 



The example of erosion thus given by the San Rafael Swell illustrates, 

 as a sharply defined type, the denudation of the Plateau Province. The 

 thickness of the strata removed varies greatly in different portions. In 

 the High Plateaus it has amounted to only a few hundred feet. In large 

 areas it amounts to two or three thousand feet and in others of consid- 

 erable extent it reaches more than 10,000 feet. Preliminary compari- 

 sons of known facts derived from nearly the entire extent of the southern 

 province lead to the conclusion that on the average 5,500 to 0,000 feet of 

 strata have been removed from its entire expanse. Our knowledge of 

 the geology of some portions of it is at present very imperfect. Still, 

 enough is known to justify us in believing that this summary estimate 

 will not be much affected by future investigation. 



We may for special purposes of convenience regard the province as 

 consisting of districts or spots of maximum erosion separated from each 

 other by high mesas or dividing platforms where erosion has been at its 



