ACADEMY OB- SCIENCES] WHEELER SURVEY 35 



which he applied them, but much more extensively; for by following certain physiographic 

 principles in the development of which Gilbert was much concerned, it has been discovered 

 that the present height of many if not most mountain systems is due largely to one or more 

 broad upheavals of relatively modern date, sometimes accompanied by strong faulting but 

 not by marked folding, while the pronounced folding of the mountain rocks must be ascribed 

 to horizontal compressive forces of date so ancient that the relief and height then given to the 

 writhing surface had been greatly reduced if not obliterated by long periods of erosion before 

 the later upheavals gave the worn-down mountain mass its present height and permitted its 

 present deep dissection; hence many mountains that we actually see as existing topographic 

 features seem after all to owe their visible height largely if not wholly to vertical forces. 



HOT SPRINGS AND DIASTROPHISM 



In spite of the confidence that Gilbert exhibited in the action of vertical forces, he under- 

 took a study of the distribution of hot springs in the United States with the object of deter- 

 mining whether their high temperature was due to the development of subterranean heat by 

 mountain corrugation under horizontal pressure, as "advocated by Messrs. Hunt, Mallet, 

 and Le Conte" (146) ; this citation being one of the few references to other geologists than those 

 who had worked in our western field. The result gained by charting nearly 150 hot springs 

 was that their distribution coincides "very exactly with that of corrugation" (148); but the 

 hot springs of the Appalachian region, where corrugation and eruption are things of the past, 

 are regarded as depending on mountain-making deformation only in so far as it furnished 

 fractures by which their waters might reach the surface; their temperature is ascribed, 

 following Rogers, entirely to the normal increase of earth-crust temperature with depth. 

 It is only in the West, where corrugation and eruption "have persisted to so late a period 

 that we have good reason to believe they have not ceased" (148) that heat due to rock crushing 

 under the action of mountain-making compressions is believed to control the occurrence of 

 hot springs; and the hottest ones are found in volcanic districts. 



Gilbert's views as to the diastrophic relations of the apparently unlike provinces of the 

 basin ranges and the plateaus is clearly expressed in the following extract. After pointing out 

 that in both provinces the deforming forces were deep-seated in position and nearly vertical in 

 direction, he goes on: 



A single short step brings us to the important conclusion that the forces were identical, (except in time 

 and distribution) ; that the whole phenomena belong to one great system of mountain formation, of which the 

 ranges exemplify the advanced, and the plateau faults the initial stages. If this be granted, as I think it must, 

 then it is impossible to over-estimate the value of this field for the study of what may be called the embryology 

 of mountain building. . . . The field is a broad one and its study has but begun; but with its progress I con- 

 ceive there will accrue to the science of orographic geology a more valuable body of geological data than has 

 been added since the Messrs. Rogers developed the structure of the Appalachians" (61). 



THE COLORADO PLATEAU AS A FIELD FOR GEOLOGICAL STUDY 



An excellent summary of Gilbert's ideas about the plateau province is to be found in an 

 article with the above title that was published after he had taken a position under Powell, 

 whose influence is repeatedly manifest, although the influence had not then gone so far as to 

 cut off the terminal -al from geological. Extracts from the article are presented here, but it 

 deserves attentive reading as a whole, so forcibly did it set forth a large body of new knowledge. 

 Both the climate and the drainage of this region favor its study. As to climate; the thinness of 

 the soil and the usual absence of trees facilitate observation to a degree unsuspected by workers 

 in a moister climate: 



From a commanding eminence one may see spread before him, like a chart to be read almost without effort, 

 the structure of many miles of country, and in a brief space of time may reach conclusions, which, in a humid 

 region, would reward only protracted and laborious observation and patient generalization. There is no need 

 to search for exposures where everything is exposed. 



As to drainage: 



The Colorado and its branches flow across the Plateaus in deeply carved, narrow canons. . . . Empowered 

 by the rapidity of its descent, each tributary river has carved a cation of its own, and so too has each branch 

 and creek tributary to a river, until the whole tract is divided by a labyrinth of ramifying canons. . . . Thus 

 does drainage conspire with aridity to prepare for the geologist a land of naked rock. 



