56 



GROVE KARL GILBERT— DAVIS 



[Memoirs National 

 [Vol. XXI, 



Colorado River report, and by Gilbert himself in his block diagrams of Powell's Uinta Moun- 

 tains and of his own Henry Mountains, was not here employed. 



Gilbert's published reports may be next examined. After introducing the term, basin 

 ranges, for " all that system of short ridges separated by trough-like valleys which lie west of 



the Plateau system, without reference to 



its drainage conditions," and thus indi- 

 cating that this "orographic province, 

 which has its type in the Great Basin, 

 ... is not coincident with it" (22), he 

 turns, before describing individual ranges, 

 to a general consideration of their age or 

 epoch of upheaval. This must lie for 

 each range between "the age of the new- 

 est rocks uplifted with it, and the age 

 of the oldest rocks which rest unconform- 



Fig. 5. 



-Ideal diagram of Confusion, House, and Fish-spring Ranges; from Gil- 

 bert's notebook, October 29, 1872 



ably upon them " (24) . Following this rule, the age of many ranges can be only vaguely denned. 

 For example, those of southwestern Arizona consist of highly crystalline schists unconformably 

 overlaid by Quaternary gravels ; hence for these "only the most indefinite idea of the epoch of up- 

 heaval" is reached. Certain ranges standing near the plateaus and including deformed Paleozoic 

 strata that rest unconformably upon Archaean rocks, are said " to have been first upheaved at some 

 time anterior to the Carboniferous, and again at some time subsequent to the Carboniferous"; 

 but this statement was misleading, for it is not to the ranges but to their rocks that it properly 

 applies. However, in other ranges which include a fuller stratigraphic series, it was presumed that 

 " their principal elevation was coeval with the first and chief elevation of the Wasatch Mountains 

 and the Sierra Nevada, proved by Whitney and King to have occurred at the close of the Jurassic 

 period" (24). It thus appears that no basin ranges are treated here as having a later epoch 

 of upheaval than mid-Mesozoic; for while post-Eocene disturbances were detected in the Wa- 

 satch Range and the plateaus farther east, nothing definite could be predicated "from the 

 stratigraphical data" in hand concerning the extension of these relatively modern disturbances 

 into the basin-range province. 



The structure of nearly a score of ranges — frequently called ridges — is then concisely de- 

 scribed in less than as many pages, after which a one-page summary states that " the great majority 

 of the ranges . . . exhibit in cross section but a single direction of dip," with or without fault- 

 ing; these are called "simple and compound monoclinals" (40). But "anticlinals and synclinals 

 also occur as subsidiary features within some ranges." Monoclinal escarpments or outcrop 

 faces are mentioned, but without emphasizing their greater steepness than that of the opposite 

 or dip slope of the mountain side. "A few most remarkable ranges present escarpments on both 

 faces" (41). It is to be regretted that more details were not given. 



BEARING OF RANGE FORM ON RANGE ORIGIN 



The effort is then made to discover the origin of the ranges, and it is here that the physio- 

 graphic relation of structure, erosion, and form appears to have been utilized with most novel 

 results. The form of the ranges is first used to show that they can not have been produced by 

 the erosion of broadly corrugated structures. As Gilbert was already familiar with the features 

 of the Pennsylvania Alleghenies, he wrote: 



I entered the field with the expectation of finding in the ridges of Nevada a like structure, and it was only 

 with the accumulation of difficulties that I reluctantly abandoned the idea (41). 



The morphological principles latent in that highly significant confession deserved fuller 

 exposition, for it suggests that Gilbert had more or less clearly in mind the method of threefold 

 analysis above mentioned as essential to a proper explanation of land forms. In more expanded 

 phrasing, his meaning must have been: The nearly level depositional surface of the Paleozoic 

 strata in Pennsylvania before their folding was deformed into a series of great folds or corruga- 

 tions by compression; but the deformation took place so long ago that contemporary and sub- 



