ANALYSIS OF FAULT HYPOTHESIS 2G5 



Analyzing Mr Gilbert's reasons for the propounding of the hypothesis, 

 moreover, we find them almost wholly ph3^siographic. The frequentlj^- 

 abrupt ascent of the mountains from the level desert and the proximity 

 of the great fault-scarps of the Colorado plateau seem to have suggested 

 the idea. They may be epitomized as follows : 



(1) The ranges are principally monoclinal blocks. (2) These blocks 

 are limited on one or both sides by scarps. (3) These blocks and scarps 

 can not have been formed by erosion, and must therefore be fault-scarps. 



On this foundation there rests a great superstructure of theory and 

 deduction. 



The refutation of these points may be summarized from the foregoing 

 pages : 



1. According to the accumulated record of observation, ranges consist- 

 ing essentially of a single monoclinal ridge are exceedingly rare. The 

 writer does not remember a single instance where such a structure was 

 persistent when followed sufficiently far along the strike. The appar- 

 ent monocline almost invariably merges into a synclinal or anticlinal 

 fold, of vj^hich it constitutes one of the limbs. 



2. The writer has undertaken to show that the mountain fronts studied 

 are, in general, not marked by great faults, and, conversely, that the as- 

 certainable faults are very rarely attended by simple fault-scarps. 



3. In regard to the impossibility of erosion producing the desert 

 ranges as we have them, especially the monoclinal ridges, the writer 

 finds chiefly the most normal type of erosion in the Great basin. In 

 this point, however, he is glad to call the opinion of Mr Clarence King 

 to his aid, who has shown that in the Fortieth Parallel region the mono- 

 clinal ridges are in most cases parts of anticlinal or synclinal folds, and 

 can be traced into them longitudinally.^ 



Summary 



k 



The conclusion is that the topographic forms of the Great basin, as 

 we see them, are the net results of cdmpoui^d erosion active since Jurassic 

 times, operating on rocks upheaved by compound earth movements 

 which have been probably also continuous during the same period. 

 Only the more recent faults or folds find direct expression in the to- 

 pography ; the older ones are mastered by erosion. In general, in this 

 region, as in most others, deformation lags behind erosion. This is true 

 chiefly of the local movements, which produce Taulting and folding, and 

 not of vast general upheavals and depressions, which seem relatively 



* Geological Explorations of the Fortieth Parallel, vol. i, p. 737 

 XXXVIII— BuM..,GEor,. Soc. Am., Vol. 12, 1900 



