370 VIEWS OF NATURE. 



In the ordinary periodical manifestations of volcanic activity 

 a shower of ashes usually terminates each partial eruption. 

 The letter of the younger Pliny contains, moreover, a 

 passage which clearly shows that the dry ashes falling 

 from the air immediately attained a height of four or five 

 feet, independent of accumulation by drifts. " The court," 

 the narrative continues, " which led to the apartment in which 

 Pliny took his siesta, was so filled with ashes and pumice 

 that, had the sleeper tarried longer, he would have found the 

 passage wholly blocked up." Within the inclosed limits of 

 a court the wind cannot have exercised any very considerable 

 influence on the drifting of the ashes. 



I have interrupted my comparative view of volcanos by 

 different observations in relation to Vesuvius, partly on account 

 of the great interest excited by its recent eruption, and partly 

 because every great outpouring of ashes almost involun- 

 tarily recalls to mind the classic soil of Pompeii and Hercu- 

 laneum. In a note, not adapted to be read to the audience 

 to whom this lecture is addressed, I have collected all the 

 elements of the barometric measurements which I made 

 during the close of last year at Mount Vesuvius, and in the 

 Campi Phlegrsei. 



We have hitherto considered the form and effects of those 

 volcanos which are permanently connected, by means of a 

 crater, with the interior of the earth. The summits of such 

 volcanos are upheaved masses of trachyte and lava intersected 

 by numerous veins. The permanency of their effects indi- 

 cates a highly complex structure. They have, so to say, a 

 certain individuality of character, which remains unaltered 

 for long periods of time. Contiguous mountains generally 

 yield wholly different products; for instance: leucitic and 

 feldspathic lavas, obsidian with pumice, and basaltic masses 

 containing olivine. They belong to the more recent pheno- 

 mena of the earth, usually breaking through all the strata of 

 the floetz formation, and their lava currents and products are 



