236 STEUCTTJEE AND MODE OF ACTION 



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 Euboea in the Lelantine Fields. Many 

 of the upheaved islands belong to this class of transitory 

 phenomena. 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. Yeins or dykes of basalt, dolerite, and por- 

 phyry, 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 sub- 

 stances 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 



