262 Dr. C. Keyes — Isostasy in the Rocky Mountains. 



Didymograptus hifidus Hall. 



Didymograptus stahilis Elles &; Wood. 



Didymograptus nanus Lapw. 

 From tlie EUergill Beds exposed in the lower part of EUer Gill 

 I have obtained Glyptograptus dentatus (Brongn.). Since these 

 fossils are characteristic of the Didymograpitus hifidus zone, it would 

 seem that the Milburn Group and the associated Ellergill Beds 

 belong to this zone, and that here, too, volcanic activity began 

 in Upper Arenig time. 



In conclusion, I desire to express my sincere thanks to Miss Elles 

 for having determined the graptolites and permitting me to see her 

 determinations, and to Professors Sibly and Marr and Dr. Woolacott 

 for advice. 



Isostatic Measure of the Rocky Mountains. 



By Charles Keyes, Des Moines, Iowa. 



AMONG the enisled high plateaux of Eastern Utah the brilliant 

 isostasy theory had its birth. When these arid mountains 

 first became the subject of special description, about 1880, no such 

 thing as a distinctive desert geology was entertained. Possibility 

 of a definite geographic cycle in land sculpture was one of the modern 

 earth conceptions yet undreamed. Competency of the wind as a 

 general erosive agency was not yet established. Operation of 

 epeirogenic movements was little understood. Omission of suclj. 

 basic considerations necessarily led to curious aberration in inter- 

 pretation of the phenomena presented by lands of little rain. 



Following closely the rather unscientific Powellian policy of 

 geological saisissement. Captain Button entirely missed the larger 

 physiographic significance of his wide observations on the prodigious 

 erosion which the Utah region had manifestly undergone in relatively 

 recent geological times. He readily fell into speculations along- other 

 lines. With him inductive reasoning took a tectonic rather than 

 a physiographic turn. 



Principal among the Duttonian meditations was an estimate 

 of the crustal effects of sedimental loading and unloading through 

 erosion. Reviving astronomer Herschel's well-known view that 

 areas of sedimentation should be regions of insinking of the earth's 

 crust, the endeavour was to show that the converse of this was also 

 true, and that the Utah and Great Basin regions, from which 

 enormous volumes of rock- waste had been so recently removed, 

 should display the evidences of continual and notable upraising. 

 The hypothesis of isostasy was just beginning to take form. 



With bent of mind tectonic, Button turned eyes to the westward 

 for substantiation of his hypothesis. In the Basin ranges he believed 

 that he had found satisfactory proofs. With Gilbert, Powell, and 

 Russell he fancied that those desert mountains were floating on the 

 earth's soft interior, much after the manner of ice-cakes in a river 



