La. vis — On the Structure of Rocks. 151 



M. Bourgeois 1 accounts for the crystals of pyroxene in leucite 

 to be the crystallization of the glass cavities. This is obviously 

 not the case, for the following reasons : — In the leucites of Rocca- 

 monfina and "Vesuvius the crystals of pyroxene entirely traverse, 

 project their ends on each side, whilst the leucite material is 

 accurately moulded on the crystal facets of the pyroxene, which 

 form leucite could not give to a glass space. Besides, many 

 pyroxene crystals bear no relation whatever, either in size or 

 position, to the remaining cavities, which themselves do not show 

 such crystallization. Their crystals are often imbedded in the 

 leucite mass, and project into a glass cavity, the latter portion 

 being no thicker than the former, which was entirely enveloped in 

 the leucite mass. Where much growth of crystals in glass cavities 

 take place, that portion surrounded by the vitreous paste of the 

 glass cavity should have increased in size. That the artificial 

 conditions employed in the laboratory fairly represents the natural 

 ones in the production of leucite there exists little doubt ; the varia- 

 tions in temperature were just such as we meet with in the forma- 

 tion of that mineral at Vesuvius. Besides, the two minerals were 

 identical in crystallographic characters, both externally and inter- 

 nally, as seen by polarized light, and also the great resemblance 

 as exhibited in the strata of glass cavities. 



That leucite may separate or any rate increase in size, after 

 expulsion of lava, seems to be demonstrated by the observation of 

 Scacchi, 2 that the scoria of the lava of 1855 did not contain large 

 crystals, and that in the lava the distribution of them was irregular, 

 which seems to show that recuit at least increased their size. 



In describing leucite I have considerably erred from the direct 

 road, led on by the train of argument, based principally on the 

 physical and chemical properties of this interesting mineral. 



Biotite, though commonly met with in volcanic rocks, could 

 not be obtained as a distinct form by Messrs. Fouque and M. 

 Levy. In lavas we generally meet with this mineral in large, 

 well-formed crystals, as also in pumices. In some basic pumices 

 of Monte Somma (Phase III.) very beautiful hexagonal micro- 



1 Encycl. Chim., vol. ii., Metalloids, I er Appendice. Reprod. Artif. des Roches, 

 p. 212. 



3 Guarini, Palmieri, Scacchi. Mem. Sul. Incend. Vesuv., 1855, p. 152. 



