STRUCTURE AND ACTION OF VOLCANOS. 369 



as the whole quantity which has fallen since volcanic pheno- 

 mena have been observed with attention in Italy. A stratum 

 from 16 to 19 inches in thickness does certainly, at first sight, 

 seem very inconsiderable, when compared with the mass with 

 which we find Pompeii covered. But, without taking into 

 account the heavy rains and the inundations which must 

 have increased the bulk of this stratum in the course of ages, 

 and without reviving the animated contention maintained 

 with much scepticism on the other side of the Alps, regarding 

 the causes of the destruction of the Campanian cities, it may, 

 at any rate, be here observed that the eruptions of a vol- 

 cano, at widely remote epochs, cannot be compared with 

 respect to their intensity. All conclusions must be insufficient 

 that are based on mere analogies of quantitative relations of 

 the lava and ashes, the height of the column of smoke, and 

 the intensity of the explosions. 



We learn from the geographical description of Strabo, and 

 from the opinion expressed by Vitruvius on the volcanic origin 

 of pumice, that, until the year of Vespasian's death, that is to 

 say, until the eruption which buried Pompeii, Vesuvius 

 appeared more like an extinct volcano than a Solfatara. 

 When, after a long-continued repose, subterranean forces sud- 

 denly opened for themselves new channels, penetrating through 

 strata of primitive rock and trachyte, effects must have been 

 produced to which no analogy is afforded by those of subsequent 

 occurrence. We clearly learn from the well-known letter in 

 which Pliny the younger informs Tacitus of the death of his 

 uncle, that the renewal of the eruptions, or, one might almost 

 say, the revival of the slumbering volcano, began with an 

 outbreak of ashes. The same phenomenon was observed at 

 Xorullo, when the new volcano, in the month of September, 

 1759, breaking through strata of syenite and trachyte, was 

 suddenly upheaved in the plain. The country people fled in 

 terror on finding their cottages covered with ashes thrown 

 up from the earth, which was bursting in every direction. 



SB 



