ON ITS EXTERIOK. VOLCANOES. 409 



according to the investigations of Bockh and Hirt, there 

 can no longer remain any doubt as to Vitruvius having 

 lived under Augustus ( 559 ), therefore fully a century 

 before the eruption of Vesuvius in which the elder 

 Pliny perished, the passage which has been quoted 

 with the expression " pumex Pompeianus " (connecting 

 pumice with Pompeii) presents peculiar geological in- 

 terest in relation to the debated question; whether, ac- 

 cording to Leopold von Buch's ingenious conjecture ( 56 ), 

 Pompeii was covered by beds of tufa containing pu- 

 mice, of submarine formation, spread in horizontal strata 

 over the whole surface between the Apennine moun- 

 tains and the western coast from Capua to Sorrento 

 and from Nola to beyond Naples, and pushed up at 

 the first formation of the Somma; or whether Vesu- 

 vius itself, quite contrary to its present habits, sent 

 forth the pumice from its own interior ? 



Carmine Lippi ( 561 ), who (1816) ascribes the tufa 

 which overspread Pompeii to water, as well as his 

 acute opponent, Archangelo Scacchi ( 562 ), in the letter 

 addressed by him to the Cavaliere Francesco Avellino 

 (1843), have called attention to the remarkable circum- 

 stance, that a portion of the pumices of Pompeii and 

 of the Somma enclose small pieces of lime retaining 

 their carbonic acid, which, if they were exposed to 

 great pressure in the course of an igneous formation, is 

 not very surprising. I have myself had the opportunity 

 of seeing specimens of these pumices in the interesting 

 geological collections of my learned friend and academi- 

 cal colleague, Dr. Ewald. This similarity of mineralo- 

 gical constitution at two opposite points must give 

 occasion to the question, whether the covering which 



