Page:Aspects of nature in different lands and different climates; with scientific elucidations (IA b29329668 0002).pdf/243

 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 communication 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 individual 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 phænomena, 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