OF VOLCANIC ROCKS. I 1 



great extent in the structure of that mountain range, cannot possibly have been 

 ejected prior to the Jurassic epoch. The texture of these rocks is, notwithstanding 

 this comparative]}- recent origin, that of all true granite, and the prominent varieties 

 cannot be distinguished from some European kinds of granite, as for instance those of 

 the Adamello and the Cima d'Asta in the Southern Alps, which are among the most 

 recent in age on that continent. Volcanic rocks are widely distributed in the Sierra 

 Nevada, and are of the same or similar age as in Europe. Quartzose porphyry occurs 

 to some extent in AVashoe, under circumstances which make the exact determination 

 of its age difficult, but render it certain that it is intermediate in this respect between 

 granitic and volcanic rocks. These relations would appear to be an exact counterpart 

 of those observed in Europe, with the one prominent difference, that the commencement 

 of the eruptive action was much later in America. Very recently, however, additional 

 observations have been made, which give a somewhat different aspect to these relations. 

 Mr. Clarence King observed granite, covered by Palaeozoic rocks and antecedent to 

 them in age, near the Colorado River ; while Prof. Whitney and myself discovered true 

 quartzose porphyry in the county of Plumas, in northern California, associated with 

 rocks proved by the former to be of Triassic and Liassic age, in such way as to leave 

 little doubt about its cotemporaneous origin. Farther east, in the Great Basin, 

 Palaeozoic granite is of no rare occurrence, and it is among the prominent features in 

 the geology of the Rocky Mountains ; while the discovery of porphyritic rocks may 

 have to await further examination, they having been in most countries the last erup- 

 tive rocks to be detected. 



However these facts may affect the theoretical conclusions in regard to the 

 origin and mutual relations of granitic and porphyritic rocks, which had been made on 

 the strength of former observations, they appear to confirm the separation, from a geolo- 

 gical point of view, of both classes of rocks. There has been in the Sierra Nevada and 

 adjacent countries, it appears, an ancient granitic era corresponding to that of Europe, 

 followed by a porphyritic era which was nearly or quite coincident with the European. 

 But, while the manifestations of subterranean agencies almost ceased in Europe during 

 the following ages, they recommenced with great intensity on the western coast of 

 North America, and gave rise to a second granitic, followed by a second porphyritic era. 

 The volcanic era commenced in both countries in the Tertiary epoch, but it appears to 

 have been in an advanced stage in Europe while it was still in its birth-throes in 

 America. 



( orrelation of Age and Composition. 



This point of view is not inferior in interest to the foregoing. The most 

 noteworthy fact is this, that quartzose, and in general highly silicious rocks prevail 

 among those of ancient origin, basic compounds among those of later ages. Granite 

 and syenite are overwhelmingly predominant among ancient eruptive rocks. Diorite 

 and diabase are generally associated with them, but remain always quite subordinate 

 in bulk. The relative proportion is different with porphyritic rocks. So little 

 attention has been paid to these, outside of Europe, that general conclusions in 



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