OF VOLCANIC ROCKS. 4] 
ereat 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 comparatively 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 Washoe, 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 voleanic 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 Paleozoic 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, 
Palwozoic 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 porphyritie 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 voleanic 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. 
Correlation 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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