304 Olservations upon the 



which w« have a well known example in aqua regia, com- 

 posed of nitrous and nuiriatic acid. 



To these facts, which prove most completely that tliese 

 crystallized bodies are anterior and foreign to the lava \\ liich 

 contains them, I shall add, as a superabundant proof, a sin- 

 gular accident, found in the ashes which covered Pompeia, 

 and which is in mv collection of volcanic matters. 



It is a Icucite isolated iVom three to four lines of diameter 

 through its whole crystallization, joined to a schorl, the 

 greatejit part of the prism of wiiich it embraces ; this s-chorl 

 is also in its perfect cr\ stullization, and e^ch of these cry- 

 stals has retained the colour which is pro-^er to it : we see 

 by the vestiges adhering to the schorl that these two crystals 

 have been enclosed in a reddish spongy lava. This is rvot 

 the on K' curiosity of that description I am in possession of; 

 I have another tVcmi tlie sa.rac place, not so clean as the 

 former, because it has retained more lava. It is also a leu- 

 cite of the same size, perfectly distinct, which embraces a 

 small group of schorl, one of which is greater than the two 

 ot-hers wliich are joined to it. 



Are these not similar examples to those \\ hicb frequently 

 happen to tlic crystals (;f the la-, ers formed by the humid 

 w;ay ? These green- schorls, or epidotes as they are called, 

 which we see enclosed in rock crystals; these micas, these 

 pyrites, also enclosed in this same species of crystal, and the 

 latter, iu its turn, in crj'stals of calcareous spar, — junctions 

 which indicate a succession of formations. The creen 

 schorlp, vnicas, and pyrites, have preceded the rock crystal, 

 and the rock crystal has preceded the calcareous spar. We 

 also fiud junctions of these three crystals in the same order. 

 Whence this- natural conclusion follows, that the pyroxene 

 schorl has preceded in its luruiation that of the leucite. 



I shall alio mention spongy lavas, which shew in their 

 fractures leucites partly isolatetl, and the greater number so- 

 litary, and others in groups, as it happens to crystals of 

 every kind. Is this the process ; is it what is shown by that 

 confused heap of crystallites of cooled glass in the. melting- 

 pots of glass-houses ? 



We 



