duti-on.) EROSION OF THE SAN RAFAEL DISTRICT. 57 



cast and west through the Wasatch Plateau to the San Rafael Swell. It 

 will be observed that the lower Tertiary is found only on the summit and 

 western flank of the plateau. The Cretaceous extends further out, but 

 is at last cut off in turn ; and as lower and lower beds are exposed t o day- 

 light, they too are similarly cut off until the summit of the Carboniferous 

 is nearly or quite exposed within the swell itself. The approximate length 

 of the section here given is about 55 miles, and the thickness of the strata 

 from the summit of the Carboniferous to the top of the Lower Tertiary is 

 nearly 11,000 feet. 



The geologist who becomes aware through observation of the general 

 facts thus set forth quickly reaches the following conclusion : The beds 

 which are successively terminated in the terrace cliffs once reached fur- 

 ther eastward, and in all probability every one of them extended in full 

 volume and without a break entirely over the locus of the swell to 

 regions far beyond it. Upon the eastern side of the swell, and at vary- 

 ing distances from it, the missing strata reappear in inverse order, 

 with terminal cliffs facing the westward. From the intervening space 

 they have been swept away by erosion. 



In restricted localities of a few square miles, in a river valley, in the 

 open glades of a hill-country, the most unscientific observer maybe easily 

 convinced that the waste of thousands of years has broken the continuity 

 of the strata and quarried away large masses of rock. But in the wide 

 expanse before us even the mind of the geologist may falter before 

 accepting a conclusion so portentous. The magnitude of the work is 

 oppressive, and cautious philosophers are reluctant to take up and carry 

 the burden of unusually large figures. They prefer to cast about in order 

 to see whether some easier conclusion may not be discovered. The one 

 already stated is to the effect that a body of strata more than 10,000 feet 

 thick and more than 500 square miles in area have been swept off from 

 the surface of the swell ; that nearly 9,000 feet have been removed from a 

 much larger annular space around it; 7,000 feet from a still larger and 

 remoter space; and so on with expanding annuli, from which succes- 

 sively decreasing amounts have been denuded. It is needless to define 

 iust here the limits of the denuded region, even if it were possible. It 

 is sufficient to say that its extent is much more than 10,001) square miles, 

 and that the thickness of the strata removed varies from a few hundreds 

 to more than 10,000 feet. Xor does the conclusion stop here. The San 

 Rafael region is only one of a considerable number of the subdivisions of 

 the Plateau Province where the same enormous extent of erosion has 

 t aken place. It is not the largest of those subdivisions, nor is the thick- 

 ness of the removed strata the greatest there. It is merely an example, 

 and whatsoever it reveals in regard to erosion is but a group of events 

 common to the entire southern province with its vast area of nearly 

 100,000 square miles. I have selected it for discussion because its array 

 of facts in evidence is more easily handled and can be more lucidly pre- 

 sented than those of the other subdivisions. Let us, then, examine in 



