OF VOLCANIC ROCKS. 31 
rise to the establishment of the above mentioned law of the periodical succession of 
voleanic rocks." It has since been corroborated by observations made in other coun- 
tries far remote from the Carpathians. Near Nangasaki, in Japan, andesite preceded 
trachyte in age, and of the former there are two varieties represented, one augitic 
and one hornblendic, of which the former is of more recent origin than the latter: 
thus, even the more minute relations observed in the Carpathians are repeated in 
other countries. The islands of Java and Luzon are too intensely voleanic and 
covered with lava to aid in establishing the laws in respect to massive eruptions 
without very close observation. But another region offering copious evidence is that 
of the Sierra Nevada, together with the adjoining parts of the Great Basin. Observa- 
tions in these countries are still limited as regards our present subject. The great 
part, however, which volcanic rocks take in their composition, as well as in that of the 
highlands of Mexico, and of the entire range of the Andes, promises to make the 
Western Coast of America the most prolific source of observations necessary for the 
definite establishment of geological laws of which, at the present day, we can only 
trace the first foundation. 
In Washoe, a country adjoining the Sierra Nevada immediately to the east, propy- 
lite forms extensively the foundation for all other volcanic rocks, which fact proves 
clearly its priority inage. It composes the plateau of Virginia City and Gold Hill, and 
derives a practical interest from the fact that the Comstock vein is enclosed between 
propylite and syenite, though in some parts of it both walls consist of the former rock. 
Andesite is Insignificant in bulk in that region. It composes a few small hillocks on 
the propylitic plateau, and in some cuts and tunnels andesitic dykes may be seen, which 
appear to have been the feeding channels of the surface accumulations. Trachyte, on 
the contrary, is among the prominent rocks of Washoe. It forms a high and rugged 
crest, encircling the plateau of Virginia and Gold Hill to the east, and extending for 
miles to the north, while to the south it reappears across the Carson River. No evi- 
dence is afforded, in Washoe, for the establishment of the mutual relations of andesite 
and trachyte, while it is conspicuous that the latter was of later origin than propylite. 
Besides the evidence offered by intersection and superposition, another fact may be 
noticed which is suggestive for the length of the period that elapsed between the 
eruptions of both rocks. It is this, that propylitic sediments occur, at least, at one 
thousand feet more elevation than those consisting of trachytic matter, which fact 
appears to indicate that the former have been deposited at a much earlier part of the 
period marked by the gradual subsidence of the inland seas of the Great Basin than 
has been the case with trachytic sediments. 
At Silver Mountain, propylite, of the augitic variety, fills the bottom of a deep 
basin encircled to the west by granitic walls several thousand feet in height. Its 
massive accumulations are intersected by andesitic and trachytic rocks, which latter 
appear to compose the summit of the high peak of Silver Mountain itself, while rhyo- 
lite occurs in such a position as to make it probable that it has arrived at the surface 
last of all eruptive rocks. Basalt appears to occur only to a limited extent at Silver 
11 Richthofen, loc. cit. 
(69 ) 
