OF VOLCANIC ROCKS. 31 



rise to the establishment of the above mentioned law of the periodical succession of 

 volcanic rocks. n 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 volcanic 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 in age. 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 

 propvlite and S3 ? enite, 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 lengtb 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 

 bisin encircled to the west by gra litic walls several thousuil fe3t in hsight. 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 



" Kiclitliofcn, loc. cit. 



(69) 



