446 REACTION OF THE INTERIOR OF THE EARTH 



24th to the 28th of October 1822, sand-like ashes, 

 triturated trachytic rapilli, forming a layer or stratum 

 19 inches thick, which have never been confounded 

 with pumice. 



The hollows and vesicular cavities in obsidians in 

 which (as, for example, at the Mexican Cerro del Jacal) 

 crystals of olivine have formed, probably from precipi- 

 tated vapours, are occasionally found in both hemi- 

 spheres to contain another kind of enclosures, which 

 seem to point to the mode of their origin and forma- 

 tion. In the wider portions of these long-extended and 

 mostly very regularly parallel cavities, there are morsels 

 of half-decomposed earthy trachyte. The cavity has a 

 narrower tail-like prolongation, as if, under the influ- 

 ence of volcanic heat, a gaseous elastic fluid had deve- 

 loped itself in the still soft mass. This phenomenon 

 had particularly attracted the attention of Leopold von 

 Buch in 1805, when he, Gray-Lussac, and I visited 

 Thomson's collection of minerals at Naples. ( 643 ) The 

 puffing up of obsidians by fire, which had not escaped 

 observation in Grecian antiquity ( 644 ), is certainly caused 

 by a similar development of gas. According to Abich, 

 obsidians submitted to fusion pass so much the more 

 easily into cellular pumice, in which the fibres are not 

 parallel to each other, the poorer they are in silicic acid 

 and the richer they are in alkalies. But whether the 

 inflation is due solely to the flying off of potassa or 

 hydrochloric acid remains, according to Eammelberg's 

 investigations ( 645 ), very uncertain. Apparently similar 

 phenomena of inflation in trachytes rich in obsidian and 

 sanidine, in porous basalts and amygdaloids, in pitch- 

 stone, tourmaline, and the dark-brown flint which 



