40

LIFE OF COLONEL MACERONI.

ejected the tuffo in the form of an eruption and flow of mudlava , as Humboldt very appositely terms it, whereas thepuzzolana was produced from the matter ejected explosivelyupwards, which being divided and suspended in its passagethrough the all-surrounding waters, settled in the pulverulentdivided state in which we find it. Both these varieties ofvolcanic produce are similar in their constituent composi-tions ; the main portion being alumine, with pumice, mica,sharl, and melanite or lucite. The basaltic lavas of Etna ,Vesuvius , Ecla, Fingal’s Cave &c., are of the same aluminousbase ; they only differ so much in texture by reason of theirhaving flown direct from tire great internal incandescentmass of our globe, without being so projected through andadmixed .with gases and the superincumbent water, as tocomminute and fix it in a state of separation, as in puzzo-lana ; or form a muddy mass, which flowed from the cavityof the volcano. I shall treat more amply on the volcanicphenomena at a future period ; but to begin, such is theformation of tuffo and puzzolana. I will in this place onlyobserve, that the deposits of the latter have enough solidityto admit of shafts, and galleries, and chambers being dugwithin it; for such are the extensive catacombs of PortaSt. Sebastiano at Home. Dangerous and sometimes fatalfallings-in of the puzzolana, have frequently occurred inthese excavations.

In company with Rochow, Echardstein, and the PrincesCarolat, I perambulated a vast extent of these catacombs,the galleries of which contain the tombs, one over the other,on the sides, just like the berths in a ship. Each berthcontains only one body, and is quite hermetically closed upwith large flat tiles and cement. We broke open severalscores of these receptacles, in the hope of finding somerings or other objects of curiosity, but found nothing butbones in a soft state which, on being pressed by the fingers,