32 R1CIITHOFEN NATURAL SYSTEM 



Mountain. But this rock is largely distributed in the regions adjoining the Sierra 

 Nevada to the east, and bears evidence of its recent origin. It is the only volcanic 

 rock which covers in places the sand of the deserts, and the belts distinguished by its 

 eruptions are still marked by the occurrence of hot springs and other post-vol- 

 canic phenomena. Most of .these are contiguous to those places where basalt and 

 granite are in close contact, as is very conspicuously the case at Steamboat Springs, 

 near Washoe, and in the Coso Mountains. No phenomena of similar nature appear 

 to be connected at the present time with any other volcanic rock east of the Sierra 

 Nevada. There are not a few instances where basalt may be seen covering propylite 

 or andesite ; but I met with only one case where it comes in contact with rhyolite, 

 close enough to establish their mutual relations. This is in Esmeralda, on the eastern 

 slope of the Sierra Nevada, a country of unusual interest for the study of volcanic 

 rocks in general. Propylite encloses the silver-bearing veins of that place. It is 

 overlain by trachyte and rhyolite, both of which occur in very great variety. To 

 the east of the place, basalt has not only flowed over rhyolite, but contains numerous 

 fragments of it enclosed, which fact confirms also for this country the more recent 

 origin of basalt. 



Many other examples might be added to this short list, partly of positive obser- 

 vations made in the countries already mentioned, and partly of facts described in 

 treatises on the geology of other countries, such as Armenia, the Caucasus, Central 

 France, the Eifel, Bolivia, Mexico. As these descriptions, however, have not directly 

 in view the illustration of our subject, great care should be used in drawing from 

 them conclusions in regard to it. Let it suffice to remark, that every observation on 

 record which bears on our subject, appears to confirm the proposed law, while none 

 can be found giving evidence against it. This may justify the assumption that the 

 periodical succession of volcanic rocks, in the order above mentioned, is a general law, 

 true for all parts of our planet. 



Laws regarding the Mutual Relations of Massive Eruptions and Volcanic Activity. 



It has been mentioned in this chapter, that the law of the periodical succession of 

 volcanic rocks regards those outpourings of large volumes of matter not resulting 

 from volcanic activity proper, and which we called massive eruptions. For conven- 

 ience' sake, we make use of the following expressions : propylitic epoch; andesitic epoch ; 

 trachytic epoch ; rhyolitic epoch ; basaltic epoch designating thereby those epochs 

 in which the massive eruptions of rocks belonging to each of these orders, have taken 

 place in every different country. If we now direct our attention to the other mode 

 of manifestation of subterranean energy, the volcanic eruptions, a cursory review of 

 active volcanoes in regard to the nature of the rocks which they eject, shows that the 

 same law is no longer true for them, as their lavas belong to several different orders 

 of volcanic rocks. It is, however, known that each volcano ejects lava or scoria 

 belonging petrographically only to one distinct order, and the examination of the 

 material accumulated by former activity will show that with most volcanoes the 

 nature of the rocks ejected has never materially varied, while with some of the 



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