OCR Text |
Show 390 STRUCTURE, AND MODE OF ACTION with ashes which had been emitted from the everywhere opening ground. In the ordinary periodical manifestations of volcanic activity, on the contrary, the shower of ashes marks the termination of each particular eruption. There is a passage in the letter of the younger Pliny which shows clearly that, at a very early stage of the eruption, the dry ashes which had fallen had reached a thickness of four or five feet, without accumulation from drift or other extraneous cause. He writes, in the course of his narrative, "The court which had to be crossed, to reach the room in which Pliny was taking his noonday repose, was so filled with ashes and pumice, that, if he had longer delayed coming forth, he would have found the passage stopped." In an enclosed space like a court, the action of wind in drifting the ashes can scarcely have been very considerable. I have interrupted my general comparative view of volcanos by a notice of particular observations made on Vesuvius, partly on account of the great interest excited by the recent eruption, and partly on account of those recollections of the catastrophes of Pompeii and Herculaneum, which are almost involuntarily recalled to our minds by the occurrence of any considerable shower of ashes. I have 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 these 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. Neighboring 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 phenomena, 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 |