390 STRUCTURE, AND MODE OF ACTION 



with ashes which had been emitted from the everywhere opening 

 ground. In the ordinary periodical manifestations of volcanic ac- 

 tivity, 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 thick- 

 ness 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 ac- 

 count of the great interest excited by th recent eruption, and 

 partly on account of those recollections of the catastrophes of Pom- 

 peii 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 my- 

 self 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. 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 



