TRUE VOLCANOES. 401 



Vitruvius lived in the tiine of Augustus,* and consequently 

 a full century before the eruption of Vesuvius at which the 

 elder Pliny met his death. The passage thus quoted, there- 

 fore, and the expression pumex Pompeianus (thus connecting 

 pumice-stone with Pompeii), present a special geological in- 

 terest in relation to the question raised as to whether, ac- 

 cording to the acute conjecture of Leopold von Buch,t 

 Pompeii was overwhelmed only by the pumiceous tufa-beds 

 thrown up on the first formation of Mount Somma ; these 

 beds, which are of submarine formation, covering in horizon- 

 tal layers the whole level between the Apennine range and 

 the west coast of Capua as far as Sorento, and from Nola to 

 the other side of Naples ; or whether Vesuvius itself, entire- 

 ly contrary to its present habit, ejected the pumice from its 

 interior. 



Both Carmine Lippi,J who (1816) describes the tufa cov- 

 ering of Pompeii as an aqueous deposit, and his ingenious op- 

 ponent Archangelo Scacchi,§ in the letter addressed to the 

 Cavaliere Francesco Avellino (1843), have directed attention 

 to the remarkable phenomenon that a portion of the pumice 

 of Pompeii and Mount Somma contains small fragments of 

 chalk which have not lost their carbonic acid, a circumstance 

 which, on the supposition that they have been exposed to a 

 great pressure during their igneous formation, can excite but 

 little surprise. I have myself had the opportunity of seeing 

 specimens of 'this pumice-Btone in the interesting geological 

 collections of my learned friend and academical colleague, 

 Dr. Ewald. The similarity of the mineralogical constitution 

 at two opposite points naturally gives rise to the question — 

 whether that which covers Pompeii has been thrown down, 

 as Leopold von Buch supposes, during the eruption of the 



* At all events, Vitruvius wrote earlier than the elder Pliny, as is 

 evident, not merely because he is three separate times cited by Pliny 

 in his list of authorities, so unjustly attacked by the English translator 

 Newton (lib. xvi., xxxv., and xxxvi.), but because in book xxxv., cap. 14, 

 s. 170-172, as has been distinctly proved by SiUig (vol. v., 1851, p. 277) 

 and Brunn {Diss, deauctorum indicihus Plinianis, Bonnae, 1856, p. 55-60), 

 a passage has actually been extracted from Vitruvius by Pliny himself. 

 See also Sillig's edition of Pliny, vol. v., p. 272. Hirt, in his Essay on 

 the Pantheon, places the date of Vitruvius's writings on architecture 

 between the years 16 and 14 of our era. 



t PoggendorfTs Annakn, bd. xxxviU, s. 175-180. 



X Carmine Lippi : Ftt iljuoca a I'acqua che sottero Pompei ed Ercola- 

 wof (1816), p. 10. 



§ Scacchi, Osservazioni critiche sulla maniera come fa seppellita VArX" 

 tica Pompei, 1843, p. 8-10. 



