﻿180 J. LeConte — Elevation of the Sierra Nevada. 



still later by recent corals found by Alex. Agassiz 2900 feet above 

 the same level.* It is evident then that the whole western side 

 of the two Americas participated in this upward movement. 

 Now so great and extensive an elevation must have had its 

 correlative in corresponding subsidence somewhere. Where 

 shall we look for it unless in the Pacific bottom ? The observa- 

 tions of Darwin and Dana on the phenomena of Coral reefs 

 have long been regarded as demonstrative proofs of such a sub- 

 sidence over an immense area (at least 10,000,000 square miles) 

 in mid-Pacific. It is true that the u Subsidence theory " of the 

 •origin of barrier reefs and atolls has very recently been seri- 

 ously questioned by Mr. Murrayf and others, but it must still 

 be regarded as the probable explanation. Barriers may cer- 

 tainly and atolls possibly be formed without subsidence, but 

 subsidence is probably the most common cause4 The fact of 

 the recent continental elevation on the Pacific side of the two 

 Americas is additional presumption in its favor. If so, then, 

 it is probable that the down-sinking of mid-Pacific floor, the 

 elevation of the whole western side of the two Americas, the 

 prodigious erosion and canon-cutting of the Plateau region, 

 the deep Y-shaped canons of the Sierra and the great N. and 

 S. fissures and normal faults of the Basin and Plateau regions 

 are all closety related to each other. Moreover the movements 

 which determined these phenomena were all in progress from 

 the beginning of the Tertiary, reached their greatest intensity 

 in early Quaternary and are still progressing. 



Cause of these movements. 



In these latter times there has been a tendency to regard ele- 

 vation and subsidence when unattended with plication, as a 

 simple matter of equilibrium of a floating crust. According to 

 this view, wherever abundant sedimentation is going on, there 

 the crust weighted down by the increasing mass subsides pari 

 passu, and wherever erosion is exceptionally active as in great 

 mountains and high plateaus, there the ever lightening crust 

 rises pari passu. Thus subsidence and elevation are caused by 

 weighting and lightening.§ Doubtless this is a real cause 

 which must not be neglected, but it cannot be the principal 

 cause. Doubtless the proposition is true, but the converse 

 proposition is much more true, viz : that subsidence is the 

 cause and necessary condition of sedimentation, and elevation 

 the cause of exceptional erosion. The plateau region, forexam- 



* Proc. Am. Acad. Sci., xi, p. 257. 1876. 

 f Nature, vol. xxii, p. 251, 1880, and vol. xxxiii, p. 202, 1885. 

 X Dana, this Journal, vol. xxx, p. 89 and 169, 1885. 



§ Nature, vol. xxvii, p. 523 ; vol. xxviii, pp. 323, 365, 388, 488-539, 587 ; 

 vol. xxix, p. 212. Geol. Magazine, vol. x, pp. 302 and 348, 1883. 



