Dr. C. Keyes — Isostasy in tJte Rocky Mountains. 263 



at time of the spring break-up. To this quartette the erosional 

 derivation of the Basin ranges seemed entirely out of question. The 

 potency of eolation in arid lands was little suspected. 



In the seventies of the last century Dutton was really unable 

 to turn his attention profitably to the Rocky Mountains rearing 

 their peaks to the east of him, in order to test his hyjDothesis. 

 Geologically the Cordillera was too little known. Its extent was too 

 vast to draw u23on in hasty conclusion. It required half a century 

 of uninterrupted investigation by many persons to dislodge the facts 

 that had critical bearing upon fantastic musings. Newer evidences 

 appear conclusive. 



The Rocky Mountains seem to be excej)tionally well suited 

 to determine the span of isostasy. They occupy a tract which from 

 the earliest geological times has undergone continuous diastrophic 

 oscillation. The area is one which has been repeatedly uplifted. 

 It is one which has suffered again and again extensive planation. 

 The orograj)hic change is never so great as entirely to obliterate 

 the salient features of the different movements. Later upraisings 

 and down-sinkings are especially well marked. The geologic 

 record of events is unusually full. 



Since Palaeozoic times the Southern Rockies have experienced no 

 less than four distinct and notable uplif tings. In the same time they 

 have suffered a like number of total effacements. Each of the 

 ancestral Rockies has appeared no less majestically than the 

 mountains of to-day. Each of the old levellings has been down to 

 the surface of the sea. Peneplanation has been as perfect as is 

 probably ever attained. Great planation periods were during 

 Comanchian, Laramian, Miocene, and Recent times. 



Comanchian peneplanation is particularly widespread. It afiects 

 the whole continental interior from the Rio Grande to Hudson 

 Bay and from the Sierra Nevada border to the Mississippi River. 

 Even to this day its essential characters are unusually well dis- 

 played. Throughout its broad expanse the great Dakotan sandstone 

 conspicuously marks its stratigraphical position and preserves its 

 features. The old plain is one of the smoothest known in all 

 geological history. Only in a few places do there exist any un- 

 consumed residuals standing out as low monadnocks above the 

 general level. 



According to the fundamental premise of the isostasy hypothesis, 

 mountains continually rise because of the removal of their summits 

 through erosion, thus lightening the local crustal loads. On the 

 other hand, according to Herschel, areas of sedimentation, or 

 loading, should be tracts of notable crustal insinking. Singularly, 

 the Jurassic ancestral Rockies do not appear to have grown higher 

 as their substance wasted away. They allowed themselves to be 

 worn down to the very level of the sea. 



After reaching sea-level, when all their positive volume had 

 vanished, the tract which the Rocky Mountains once occupied, 



