OF VOLCANOS. 391 



with the interior of the earth. They often continue for centuries 

 in a state of repose, are then suddenly rekindled, and end by 

 becoming Solfataras, emitting aqueous vapors, gases, and acids; 

 sometimes, however, as in the case of the Peak of Teneriffe, we find 

 that their summit has already become a laboratory of regenerated 

 sulphur ; while from the sides of the mountain there still issue large 

 torrents of lava, basaltic in the lower part, but towards the upper 

 part, where the pressure is less, ( 2 ) presenting the form of obsidian 

 with pumice. 



Distinct from these volcanos provided with permanent craters, 

 there is another class of volcanic phenomena more rarely observed, 

 but particularly instructive to the geologist, as they recall the 

 Ancient World, or the earliest geological revolutions of our planet. 

 Trachytic mountains open suddenly, emit lava and ashes, and close 

 gain, perhaps never to reopen. Thus it was with the gigantic 

 mountain of Antisana in the chain of the Andes, and with the 

 Monte Epomeo in Ischia in 1302. Sometimes such an outbreak 

 has even taken place in plains : as in the high plateau of Quito, in 

 Iceland, at a' distance from Mount Hecla, and in Euboea, in the 

 Lelantine Fields. Many of the upheaved islands belong to this 

 class of transitory phenomena. In all these cases, the communica- 

 tion with the interior of the earth is not permanent, and the action 

 ceases as soon as the cleft or fissure forming a temporary channel 

 closes again. Veins or dykes of basalt, dolerite, and porphyry, 

 which in different parts of the earth traverse almost all formations, 

 and masses of syenite, augitic porphyry, and amygdaloid, which 

 characterize the recent transition and oldest sedimentary rocks, have 

 probably been formed in a similar manner. In the youth of our 

 planet, the substances of the interior being still fluid, penetrated 

 through the everywhere fissured crust of the globe, sometimes 

 becoming solidified in the form of rocky veins or dykes of granular 

 texture, and sometimes spreading out in broad sheets, and resem- 

 bling superimposed strata. The volcanic products or rocks trans- 

 mitted to us from the earlier ages of our planet have not flowed in 

 narrow bands like the lavas of the isolated conical volcanos of the 

 present time. The mixtures of augite, titaniferous iron, feldspar, 

 and hornblende, may have been the same at different epochs, some- 



