ON THE CRYSTALS IN LAVAS. ]yQ 



The form of the leucites and volcanic schoerk is perfectly Their figure 



determinate; there is nothioi^ in it confused, but all jg, well marked, 

 ^ , and colour re- 



precise and well mavked. The leucice is constantly of a tained. 



round tigure, cut with twenty-four trapezoid faces, and of 



a gray white colour. The volcanic or pyroxene schoerl is an 



octaedral prism with two diedral pyramids, of a deep olive 



colour, and sometimes black. The chrysolite has its peridot 



colour, and its three crystals are found in the cellular and 



spongy lava, as well as in the compact. 



The schce 1 is strongly adherent to the lava, so that it can-* The ?choerl 

 not be dccdched, and appear with its faces polished and ""'^"^-^ ^° ^h* 

 angles entire, but by a chemical operation, the effect of 

 the sulphurous acid fumes of the volcano. The leucite is Leucites more 

 more easily separable, leaving impressed on the lava its ^^^''^ separa- 

 round form, with the shapes of its facets as clearly marked their impre*s- 

 as they are on the leucite itself. Its impressions in the lava sion on the 

 may be compared to those left by garnets, cubic martial 

 pyrites, and sevei'al other crystallized substances, on the 

 rocks that include them ; with this difference, that the im- 

 pressions of the leucite were made on a substance in fusion, 

 and those of the garnet a.d pyrites on a rock that was soft 

 from humidity. 



Hence we may draw this inference, that the leucites were Formed before 

 no more produced in the lava at the time of its cooline, " therefore, as 



11 1 • p n /• 1 1 ^^^ crystals 



than the garnets and pyrites were lormed from the substance found inrocfcs. 



of the I'ock, which encloses them now it is dried and hardened. 

 Both are equally foreign to the matter that contains them, 

 and existed before it; the leucites before the lava, and the 

 garnets and pyrites before the rock in which they are im- 

 bedded. Leucites are also found separate, and in great 

 numbers, among volcanic ashes. 



In this exact statement of facts, can we perceive any re- No analogy- 

 semblance, any analogy, between the crystallised substances ancTthe^crys'- 

 included in lava, and those confused heaps of vitreous cr}'s- ^'s oi glasr. 

 tallites formed of the substance of the glass in the pots in. 

 glass-houses? or between those fantastic forms of cooled 

 glass, and the crystals in the strata of our mountains, all 

 of a const mt and regular form,~each in its kind? 



The pyroxene schoerls too are found separate, and some- Loose pyrox*- 



times in multitudes innumerable. The crater that opened ®"^^ '^ ^■■^^-^^ 

 , T- ^ '^ . numbers oa 



N 2 in £tna: 



