﻿178 J. LeConte — Elevation of the Sierra Nevada. 



Relation of the Post- Tertiary movement to the great fissures 

 and normal faults of the West. 



In the Basin region, Mr. Kussell and others have drawn at- 

 tention to the entire distinctness in time and in character, of 

 the forces which crumpled the strata (mountain-making forces) 

 and those which produced the fractures and faults, and there- 

 fore which determined largely the present orographic structure ; 

 the latter being much more recent. The former took place at 

 the end of the Jurassic, the latter probably at the end of the 

 Tertiary. So also in the Sierra range, we must distinguish be- 

 twixt the orogenic forces which crumpled the Carboniferous, Tri- 

 assic and Jurassic strata lying on its flanks and that much later 

 movement which greatly increased its height and determined 

 its present slopes and which caused the rivers to cut the present 

 canons so deep. The former, as Whitney has shown, took 

 place at the end of the Jurassic, the latter as we have seen, at 

 the end of the Tertiaty. The Sierra as we now know it, both 

 in height and in scenic configuration, is largely due to this 

 latter movement. 



Furthermore, it seems probable, nay, almost certain, that 

 those great fissures which break the crust of the earth into 

 blocks which by unequal settling give rise to normal faults, 

 were produced not at the time of orogenic crumpling, but sub- 

 sequently. Mountain ranges are in the first instance formed 

 by horizontal pressure crumpling and upswelling thick sedi- 

 ments on marginal sea-bottoms;* but these sediments being 

 comparatively soft, yield easily, and ina}^ be mashed together 

 with little fracturing ; or if there be fractures, these give rise 

 to reverse faults, i. e. a pushing of the hanging wall over the 

 foot wall, the extreme forms of which are the over-thrusts 

 and horizontal shearings described by Geikie and Peach in the 

 Scottish highlands.! But after the crust has again become 

 rigid, bodily movements of a somewhat different kind, and the 

 cause of which we still very imperfectly understand, give rise to 

 fractures which by gravilative settling produce normal faults so 

 common everj^where. It is certain that the great normal faults 

 which characterize the Basin region, among which must be 

 counted the Eastern Sierra fault and the Western Wasatch 

 fault, were produced in this way long after the orogenic crump- 

 ling had ceased. It is certain, also, that the prodigious faults 

 which characterize the Plateau region, where there was no oro- 

 genic crumpling at all, were formed in the same way. Is it 

 not probable then that the fissures of the Sierra also, which by 

 filling with mineral matter have formed its metalliferous veins, 



* Theory of formation of great features of Earth Surface, this Jour., vol. iv, p. 

 345 and p. 460, 1872. 



f Xature. vol. xxxi, p. 29, 1884. 



