STATE OF THE VALLEYS 273 



broken by isolated eminences of a mountainous nature, it not infre- 

 quently happens that low peaks protrude above the generally even sur- 

 face and occasionally a considerable mountain range breaks their slopes. 

 Here and there, especially when the valleys are of the widest form, vol- 

 canic peaks occur, commonly as degraded stocks — that is, the remnants 

 of cones. In fact these structures, though not common, are among the 

 characteristic features of these broad vales They are irregularly distrib- 

 uted, sometimes lying in the central part of the troughs; again, perhaps 

 most commonly, near their borders or even in the marginal parts of the 

 mountain ranges that constitute the divides between the drainage basins- 



Noting the evident likeness of these broad valleys to those of rivers in 

 a country brought near to its baselevel of erosion, the observer is at first 

 led to the supposition that they are but extreme instances of widened 

 drainage troughs, wherein, as usual, the cutting streams in their wind- 

 ings have excavated very wide vales. A little observation shows that 

 this explanation is insufficient for the reason that nowhere are the water- 

 courses now at work on the hard bed rocks of the country, access to them 

 being made impossible by the long detrital slopes on either side. These 

 slopes, though they are made of rather incoherent detritus, are never cut 

 into escarpments of any considerable height. They control the drainage 

 and are not controlled by it. Finding, as we 'do, many instances of val- 

 leys 10 miles or more in width and 50 or more in length, with their rivers 

 everywhere defended by detritus from their ordinary incising work, we 

 are forced to conclude that these vales are not widening by the swing- 

 ings of their drainage channels and the successive excavations which are 

 thus induced. 



The next working hypothesis which may be adduced is that these 

 broad valleys, after having been excavated to a much greater depth than 

 they now exhibit, have, by some process of warping or tilting of the 

 region in which they lie, been converted into lakes, and that their slop- 

 ing central parts are made up of lacustrine deposits. That such is not 

 generally the case is indicated, as will be shown further on, by the char- 

 acter of the deposits, but even mor6 clearly by the topographic shape of 

 the materials. Save in such instances as are afforded by the extinct 

 lakes of the Bonneville group, where there.are characteristic shorelines, 

 the lacustrine hypothesis, thoujjh frequently applied, is usually inad- 

 missible. This is indicated by the facts now to be set forth. 



Where we trace the margins of the slopes or benches to their contact 

 with the bordering ranges we nowhere find distinct beach terraces such 

 as are so well exhibited about the Utah lakes. It can not be assumed 

 that such terraces once existed, but have been destroyed, for such ruin- 

 ing action would have greatly changed the surface of the slopes, which 



