264 Observations on Volcanoes and their Lava. 



filled with schorl ; and those of ^'esuvius, particularly the 

 antient, contiain schorl and leucitcs in <rreat numbers*. 



I shall make no mention of other substances, such 

 as chrysolites and olivins, because their form is not 

 Eufiiciently determined to enable us to decide whether they 

 are found or not in the exterior strata. 



It is not the kva of Vesuvius and ^^tna alone which 

 contains one or other of these crystals, or both of them to- 

 gether. Most of the lavas of the antient volcanoes in the 

 neighbourhood of Rome are filled with mvriads of leucites. 

 Several of the lavas of the Brisgau contain schorls in great 

 quantity. The gravel of the volcanic lake of Andcrnaeh is 

 filled with them. Tlicy are found in the basaltes of the circle 

 of Lcwiomeritz in Bohemia, and in the scoriae of the crater of 

 Puv-de-la-Vache in Auvergne. I mention onlv the lavas 

 of which I possess specimens, most of them collected by 

 myself on the spot, or which were sent to me by my bro- 

 ther, who collecLcd them in his excursions to the old volca- 

 noes of Germanv. 



Are these two crystals so numerous in lava, the schorls 

 of volcanoes, and leucites, lound in any porphvrv, granite, 

 or horn rock? They are not found there: the question 

 then is decided ; lavas do not derive their origin from por- 

 phyry, nor from the two other rocks. 



What, in all probability, has led to the contrary opinion, 

 is the appearance of several kinds of lava, whicti, by the in- 

 sulated substancts they contain, have a porphyroid appear- 

 ance, though they are not porphyritic. ' 



Leucite is said to have been found : — Is this crystal, of a 

 round form, with twenty-iour trapezoidal faces, really that 

 substance ? If it is, in what kind of rock was it found ? Is 

 it found there by myriads, as in lava? Were this the case, 

 must it not have been long since known? And if it be 

 found only rarely, it is only an exception of very little con- 

 sequence, compared with the grand fact presented by lava. 



I hav£ said that it is uncertain whether lava proceeds from 

 solid rocks, or strata still in the state of softness, pulveru- 



* The biedral pyramids of schorl are subject to severLl varietier, but 

 never to that of tiie prism, which has rdwp.ys eight faces: these faces vary 

 in their si/e hke tlio-c of "rock crystal. Some are frequently seen which have 

 two opposite faces broader than the rest ; a variety which is observed 

 also in tJie prisms of rock crystah These perhaps are modifications which 

 have made these prisms be considered bv Doiomieu and Spallanzani as hcx- 

 ai-dra : they are certainly as much octacdral as rock crystal is he5;aedral, and 

 the rose feld-spar of Baveao tetracJrai. 



1 have in my possession a Icucile which exhibits a verv singular accident. 

 It is united to a schorl, one part of the length of which it embraces. This 

 union has produced an elongation cf the leucite to embrace the schorl. 



lent. 



