STRUCTURE AND ACTION OF VOLCANOS. 371 



of subsequent origin to our valleys. Their life, if I may be 

 permitted to use a figurative expression, depends upon the 

 mode and the duration of their connection with the interior 

 of the earth. After continuing for centuries in a state of 

 repose, their activity is often suddenly revived, and they then 

 become converted into Solfataras, emitting aqueous vapours, 

 gases, and acids. Occasionally, as at the Peak of Tcneriffe, 

 their summits have already become a laboratory of regene- 

 rated sulphur, while considerable lava currents, being basaltic 

 near the base, and mixed with obsidian and pumice at greater 

 elevations, where the pressure is less, continue to flow from 

 the sides of the mountain (2). 



Besides volcanos which have permanent craters, there is 

 another kind of volcanic phenomena less frequently observed 

 than the former, but especially instructive to the geologist, 

 as they remind us of the primitive world, that is, of the 

 earliest revolutions of our planet. Trachytic mountains 

 suddenly open, and after throwing up ashes and lava, close 

 again never perhaps to re-open. Such has been the case with 

 the mighty volcano of Antisana in the chain of the Andes, 

 and with Mount Epomceus in Ischia, in the year 1302. 

 Occasionally such an eruption has occurred even in the 

 plains, as on the table-land of Quito, in Iceland at a dis- 

 tance from Hecla, and in the Lelantine plains of Eubcea. 

 Many upheaved islands belong to this class of transitory 

 phenomena. In these cases, the connection with the inte- 

 rior of the earth is not permanent, the action ceasing as 

 soon as the fissure, or channel of communication, is again 

 closed. Veins of basalt, dolerite, and porphyry, which tra- 

 verse almost all formations in different parts of the earth; 

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

 which characterise the most recent strata of transition rock, 

 and the oldest stratum of the floetz formation ; have all probably 

 been formed in a similar manner. In the youthful period of our 

 planet, the substances that had continued in a fluid condition 



2 b 2 



