Br. C. Keyes — Isostasy in tlie Rocky Mountains. 265 



and vitally demands. The isostatic hypothesis fails utterly to 

 sustain itself before such an array of critical geological testimony. 



Turning to the arid Great Basin, from which the chief advocates 

 of the isostatic hypothesis drew their main inspiration, we find that 

 the theory is everywhere rendered nugatory. The hard-rocked 

 mountains do not appear to be growing higher through extensive 

 unloading, but only relatively so because of the fact that erosion 

 peculiar to the desert is going on much faster over the soft inter- 

 montane plains. The tectonic structures are mainly not recent 

 but comparatively ancient. The entire surface of the region seems 

 to have been epeirogenically uplifted long prior to the formation of 

 the mountains. In the arid regions the evidences are therefore 

 really against isostasy. 



The mountain-tops of to-day probably mark approximately the 

 level of the old uplifted peneplain surface. Genesis of the positive 

 features of desert landscape does not appear to be tectonic but 

 deflative. 



The isostatic hypothesis receives strong support from mathematics. 

 Certainly mathematics is a mill which grinds exceeding fine. But, 

 as many thinkers have remarked, what comes out of the mill depends 

 very largely upon what is fed into it. So that, concerning earth 

 problems, there is often Kttle to choose between the rough guess of 

 the geologist and the painstaking calculations of the mathematician. 



In view, then, of the recent observations in the Kockies, bearing 

 critically, it seems, upon the theme of isostasy, it appears that the 

 mathematical equations will have to be radically modified in order 

 projDcrly to satisfy the necessary consequences lately derived from 

 the field, or else they will have to be given up as pure fictions. 



So far as we are able now to discern, it seems quite manifest 

 that in the hypothesis of isostasy effects are mistaken for causes. 

 At the time, a hundred years ago, when Herschel advanced the idea 

 of crustal insinking, because of sedimental loading, the accepted 

 notion concerning the earth's interior was that it was in a highly 

 plastic condition. Some such conception also pervaded the minds 

 of those who were instrumental in formulating the hypothesis in its 

 later aspects. Since that time, now nearly half a century ago, 

 the phenomena which were so earnestly cited in suj)port are not 

 only readil}^ explained in far simpler ways, but the observations 

 themselves are found to be curiously distorted on account of fancied 

 conditions. 



It is not at all strange that the Rocky Cordillera should, on the 

 one hand, so strongly belie the verity of the isostatic hypothesis, 

 and, on the other hand, point so conclusively to the strictly telluric 

 nature of its genesis, and of orogeny in general. Referring 

 characteristic mountain tectonics with which we are acquainted 

 to the secular diminution in rate of the earth's rotation, all the 

 principal orographic features are satisfactorily reproduced in the 

 laboratory. 



