32 RICHTHOFEN — 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 Voleanic Activity. 
It has been mentioned in this chapter, that the law of the periodical succession of 
voleanic rocks regards those outpourings of large volumes of matter not resulting 
from voleanic activity proper, and which we called massive eruptions. For conven- 
ience’ sake, we make use of the following expressions: propyliie epoch ; andesitic epoch ; 
trachytic epoch ; rhyolite epoch ; basaltie 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 voleano 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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