ON THE CRYSTALS IN LAVAS. 



181 



litary leucite of three or four lines diameter throughout its 

 whole crystallization, united to a schoerl, the greater part of 

 the prism of which it embraces. This schoerl too is per- 

 fectly crystallized, and each of these crystals retains its pro- 

 per colour. It appears by some vestiges still adhering to 

 the schoerl, that these two crystals were enveloped in a red- 

 dish spongy lava. 



This is not the only singularity I possess. I have another ^ Jeuclte en- 

 that came from the same place, though not so well defined, schoerls. 

 because it has retained more of the lava. This too is a 

 leucite of the same size, perfectly distinct, and including a 

 small groupe of schoerls, one of which is larger than the 

 other two united with it. 



Are not these instances similar to those that frequently Similar to roc3« 

 occur to crystals of strata formed in the humid way? to ^°'^"^^^'°'^** 

 those green schoerls, or epidotes, we see incbided in rock 

 crystals ; those micas, those pyrites, included in the sam.e 

 kind of crystal ; and this in its turn enveloped in ciystals 

 of calcareous spar: unions that indicate a succession of for- 

 mations. The green schoerls, micas, and pyrites, have pre- 

 ceded the rock crystal ; and the rock crystal the calcareous 

 spar. We find also combinations of these three crystals in- 

 cluding each other in the same order, whence this natural 

 consequence follows, that the pyroxene schoerl preceded 

 the leucite in formation. 



I would likewise remark, that spongy lavas exhibit in Leucitesinth 

 their fissures leucites in part isolated, the greater part soli- ""^"f^sof la- 

 tary, but some in gi'oups, as happens with crystals of all 

 kinds. Is this the course, is this the appearance, of these 

 confused heaps of crystallites of glass cooled in the glass- 

 houses ? 



We are not acquainted with any lava of Etna that con- No leucites in 



tains leucites ; or any of Vesuvius that encloses those I!*® '^^^* °^ 



. . •' . . Etna; and no 



whitish crystalline laminae, which are so abundant in the crystalline la- 

 lavas of Etna. This is a fact, to which the naturalist, that "''";'= '" '^°^^ 



of Vesuvius; 



supposes these crystals to be formed in the lava, ought to yet both con- 

 pay some attention. If the leucites were really formed in *^'" pyoxenes 



• 111, n -n • I M 1 ^"'^ chryso. 



It, why do the lavas or Etna contam none, while they are mes. 

 filled with pyroxene schoerls and chiysolites, which they 

 possess in common with the lavas of V'^esuvius ? Is not this 



difference 



