Page:Aspects of nature in different lands and different climates; with scientific elucidations (IA b29329668 0002).pdf/244

 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 again, 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 Eubœa in the Lelantine Fields. Many of the upheaved islands belong to this class of transitory phænomena. In all these cases the communication 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 characterise 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 resembling superimposed strata. The volcanic products or rocks transmitted 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