482 COSMOS. 



to the formation of pumice. The sand-lakes of Dasar, 

 which lie about 6828 feet above the mean level of the sea, 

 are not covered with pumice, but with a layer of rapilli, 

 described as being obsidian-like, semi-vitrified fragments of 

 basalt. The cone of Vesuvius, which never emits pumice, 

 gave out from the 24th to the 28th October, 1822, a layer 

 18 inches thick of sand-like ashes, consisting of pulverised 

 trachytic-rapilli, which has never been mistaken for pumice. 

 The cavities and air-holes of obsidian in which crystals of 

 olivine, probably precipitated from vapours, have formed, as, 

 for example, in the Mexican Cerro del Jacal, are sometimes 

 ibund in both hemispheres to contain another kind of en- 

 closures, which seem to indicate the manner of their origin 

 and formation. In the wider portions of these long-extended, 

 and for the most part very regularly parallel cavities, frag- 

 ments of half-decomposed earthy trachyte are found embedded 

 Beyond these the cavity runs on in the form of a tail, as if a 

 gas-like elastic fluid had been developed by volcanic heat 

 in the still soft mass. This phenomenon particularly attrac- 

 ted the attention of Leopold von Buch when he visited the 

 Thomson collection of minerals at Naples in company with 

 Gay-Lussac and myself in the year 1805. 19 The inflation of 

 obsidian by the operation of fire, which did not escape atten- 

 tion in the early period of Grecian antiquity, 20 is certainly 

 caused by some such development of gas. According to 

 Abich, obsidians pass the more easily into cellular (not 

 parallel-porous) pumice, the poorer they are in silicic acid 

 and the richer they are in alkalies. It remains, however, very 

 uncertain, according to Rammelsberg's researches, 21 whether 

 the tumefaction is to be ascribed to the volatilisation of 

 potash or hydrochloric acid. It is probable that similar 

 phenomena of inflation in trachytes rich in obsidian and 

 sanidine, in porous basalts and amygdaloids in pitch-stone, 

 tourmaline, and that dark-brown flint which loses its colour, 

 may have very different causes in the different materials 



19 Leopold von Buch, in the Abkandl. der Akademie der Wiss. zu 

 Berlin, for the years 18121813 (Berlin, 1816), s. 128. 



20 Theopkrastus de lapidibus, s. 14 and 15 (Opera ed. Schneider, t. i, 

 1818, p. 689, t. ii, p. 426, and t. iv, p. 551), says this of the "liparian 

 stone" (Ai7Ttt()aio). 



21 Rammelsberg, in Poggend. Annal., Bd. Ixxx, 1850, s. 464, and 

 fourth supplement to his Chemische Handworterbuch, s. 169; compare 

 also Bischof. GeoL, Bd. ii, s. 2224, 2232, 2280. 



