Grove Karl Gilbert. 073 



origin of which Gilbert proposed an altogether new inter- 

 pretation, is regrettably brief; bnt it is fortunately 

 recorded that the Great Basin was entered with the 

 expectation of finding the hard rocks standing in relief, 

 and the weak rocks worn down in valleys and low- 

 lands, as he knew them to be in the Appalachians ; and 

 on discovering that the Basin ranges "occupy loci of 

 upheaval and are not mere residua of denudation' ' — to 

 quote his classico-mathematical phrase — he was greatly 

 surprised. "The valleys of the system [i. e. the broad 

 intermont depressions] are not valleys of erosion but 

 mere intervals between lines of maximum uplift. Within 

 the ranges there are indeed eroded valleys, and the 

 details of relief show the inequalities of erosion due to 

 unequal resistance ; but there is not on a grand scale that 

 close dependence of form on durability that must main- 

 tain where the great features of the country are carved 

 by denuding agents." The ridges were found to be more 

 persistent than the structures ; one was instanced across 

 which an anticline runs obliquely. The excavation of 

 the broad intermediate depressions by erosion, while the 

 ranges remained in bold relief, was seen to be impossible. 

 The valleys were, therefore, explained as belts of relative 

 depression, and the ranges as belts of uplift. Thus began 

 a long discussion which is not yet closed to the satis- 

 faction of all concerned. The geologists of the Fortieth 

 Parallel Survey had, before Gilbert had entered the field, 

 interpreted the Basin ranges as prevailingly of anticlinal 

 structure between broad and deep synclinal valleys ; but 

 Gilbert's theory was afterward adopted to the extent of 

 adding vertical displacements by faults to the earlier 

 deformation by folding, yet without going so far as to 

 give to the faults the dominant value in producing the 

 existing relief that Gilbert had attributed to them. 



Unfortunately the leading chapter in Gilbert's report 

 concerning the Basin ranges occupied only twenty-two 

 pages, and of these only a few at its end were devoted to 

 theoretical discussion. This was by no means sufficient 

 space for a clear exposition of his novel views ; indeed it 

 is not possible to ascertain from his text alone how fully 

 he had worked out the "consequents" of the fault-block 

 theory of mountain formation. The important physi- 

 ographic principle that is involved in demonstrating the 

 presence of a great fault by the truncation of diverse 

 rock structures in simple alignment along the mountain 



