OF VOLCANOS. 235 



recorded in a note the measurements of height made by 

 myself and others on Vesuvius and in its vicinity. 



We have hitherto been considering the structure and 

 mode of action of those volcanos which have a permanent com- 

 munication with the interior of the Earth by craters. The 

 summits of such volcanos consist of masses of trachyte and 

 lava upheaved by elastic forces and traversed by veins. The 

 permanency of their action gives us reason to infer great 

 complexity of structure. They have, so to speak, an indi- 

 vidual character which remains unaltered for long periods 

 of time. Neighbouring mountains often present the greatest 

 differences in their products : leucitic and feldspathic 

 lavas, obsidian with pumice, and masses of basalt containing 

 olivine. They belong to the most recent terrestrial phse- 

 nomena, breaking through almost all the sedimentary strata, 

 and their products and lava currents are of later origin than 

 our valleys. Their life, if I may permit myself to employ 

 this figurative mode of expression, depends on the manner 

 and permanence of their communications 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 vapours, 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 



