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. 18 



Summing up these considerations on the correlation of the different orders of 

 volcanic 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 volcano 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 volcanoes 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 TeneritFe) 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 volcanic 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 frequentl}' 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. 



Lissen's Peak. It is probable that it indicates the existence of a fourth epich in the activity of that volcano. 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 Reparation cannot l>e an argument against its belonging, in our 

 ca-e, 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 volcanic 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 case 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, nnd 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. 

 (72) 



