34 RICHTHOFEN —- NATURAL SYSTEM 
that the same law of periodical succession which has been established in regard to 
massive eruptions, is true for volcanic action, particularly when this happened to 
assume such unusual intensity and dimensions, and has been of as long duration as 
was the case at that volcano.” 
Summing up these considerations on the correlation of the different orders of 
voleanic rocks in respect to the age of their emission through volcanic vents, we arrive 
at the following conclusion: The commencement of the activity of the volcanoes of 
each separate order has been nearly coincident with, though in every instance success- 
ive to, the main phase of the corresponding massive eruptions. Thence it has, by each 
separate vent, either continued emitting similar material to that first ejected, until its 
extinction, or it continued in the same way to the present day, or it has been subjected 
to a periodical change in regard to the nature of its lavas, and this change is analogous 
to that exhibited by the succession of massive eruptions. In this case, as in the for- 
mer, the voleano has either become extinct when in a certain phase, or it is still active. 
We are thus furnished with a natural cause of the fact, that most active voleanoes are 
emitting basaltic, a smaller number of them rhyolitic or trachytic rocks, while an- 
desitic lava is peculiar only to a few of them, especially to some of the prominent vol- 
canoes of South America (Chimborazo, Cotopaxi, Antisana, Tungurahua, also Popoca- 
tepetl, Colima, and Teneriffe) which appear never to have changed in mineral char- 
acter. It will, too, be self-evident, why generally no material change in the nature 
of their lava should have been observed in regard to those volcanoes which have orig- 
inally emitted basalt and constitute our order of basaltic volcanoes. 
A few more instances may here be mentioned in support of our propositions. 
The interest attaching to volcanoes has furnished us with a much greater number 
of facts in regard to voleanic rocks when occurring as lavas, than we possess in regard 
to the grander and more frequent instances when similar rocks have been produced 
by the comparatively neglected action of massive eruptions. Among those observa- 
tions, none will be better evidence than such as prove the abrupt succession, by ejec-— 
tion from the same volcano, of two rocks so dissimilar in composition as rhyolite and 
basalt. On the other hand, the nature of volcanic action will explain why we should 
meet among lavas, more frequently than among massive eruptions, with the fact of 
two successive epochs blending into each other by the alternation of the two kinds of 
rock peculiar to them separately, and it cannot be surprising if instances are occasion- 
ally observed exhibiting, at least partly, a reversed order of succession. 
Lassen’s Peak. It is probable that it indicates the existence of a fourth epoch in the activity of that voleano. The ejection 
of basalt has been so frequently connected with the opening of vents in the neighborhood of, but not coinciding with, channels 
through which its predecessors had ascended, that its local separation cannot be an argument against its belonging, in our 
case, to the system of Lassen’s Peak. 
13 The fact that trachytic lavas are frequently followed by such of basaltic character has been known since long 
time, and was till now the only law of succession observed. Mr. Scrope has suggested the hypothesis indorsed by Mr. Dar- 
win, Sir Charles Lyell, and other distinguished geologists, that in the subterranean reservoirs of voleanic matter, the heavier 
particles will occupy the lower part, and the lighter ones be nearer the earth’s crust. It will easily be seen how totally 
inadmissible this theory is in the ease of Lassen’s Peak. It is not less so in those cases where rhyolite was succeeded by 
basalt, since the process of liquation can certainly not be supposed to have produced an abrupt passage under ground from 
one mass to the other, and it would be much more natural to suppose a gradual transition to take place, there as well as in 
the succession of the rocks emitted to the surface, if liquation had really taken place. 
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