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ELIOT BLACKW ELDER 



position, and the progressive lowering of grade makes them only 

 more so. The present altitude of the remnants may therefore be 

 ascribed to movements in the lithosphere. Two possibilities sug- 

 gest themselves: {a) local warpings whereby small parts of the 

 peneplain were elevated or their surroundings were depressed; 

 and {h) a widespread uniform change of level which incited the 

 erosive agencies to excavate the soft rocks deeply, and so left only 

 remnants of the peneplain standing out in relief, where the rocks 

 were most resistant. 



Fig. 21. — Diagram of a part of the Wind River Range before and after the dis- 

 section of the peneplain. 



If the Wind River plateau be regarded as a local uplift, it may 

 be either a horst or an upwarp (Fig. 22). That it is not the former 

 is sufficiently indicated by the observed fact that it is not sur- 

 rounded by faults. If it were an upwarp the peripheral slopes 

 would be likely to preserve remnants of the peneplain bent down- 

 ward but otherwise showing the trunkation of the various struc- 

 tures. I have looked carefully for such features in the foothills 

 of the range, but with negative results. On the contrary, the 

 peneplain comes out to the west front of the range at a nearly 

 constant elevation and breaks off abruptly to the lowlands west 

 of it. These facts suggest that the present contrast between the 

 the isolated plateau and the surrounding plains is not due primarily 



