Olservaiions on Volcanoes and their Lava. 263 



We read in the Journal de Physique for January, 1804, 

 uiider the title. On the cajise of Volcanoes, the following 

 assertions ; 



*' What is the nature of tlie matters which maintain 

 these subterranean fires? We have seen that ChinjboraQO, 

 all these enormous volcanoes of Peru, and the Peak of 

 Teneriife, are composed of porphyry. 



" The Puy-de Dome is also composed of porphyry, as 

 well a> the Mont-d'Or and the Cantal. 



*' -^tna, Solfatara, and Vesuvius, are also of the por- 

 phyry kind. 



*' These facts prove that the most considerable volcanoes 

 with which we are acquainted are of porphyry." 



This opinion, that the fires of volcanoes have their cen- 

 tres in such or such a rock, and that their lavas are pro- 

 duced from these rocks, has always appeared to me not to 

 be founded on any certain data. Opinions also on this 

 subject have varied ; some having placed the origin of lava in 

 horn rock, others in granite or schist, and at present it is 

 assigned to porphyry. 



Ihave always been of opinion that nothing certain could 

 be determined in regard to this point. It ever remains un- 

 certain whether the seat of the matters of which lava is 

 formed be in compact rocks, or in strata in the state of 

 softness, pulverulent, and muddy. 



Those who see lava issue from a volcano in its state of 

 fusion and incandescence, and in its cooling, are convinced 

 that the nature of every thing is changed, that it exhibits a 

 paste in which nothing can be known, except the sub- 

 stances which the volcanic fires have not reduced to fusion. 



But these substances contained in the paste of lava, 

 and those which are the most numerous, show us, that the 

 strata from which they proceed cannot be similar to those 

 exposed to the view, nor even to the most profound strata 

 to which we can penetrate. 



The schorl of volcanoes, which was named audits, and 

 then pyroxene, an octaedral prism with two biedral pyra- 

 mids, is not found in the strata with which we are ac- 

 quainted ; and the case is the same with the leucite or white 

 garnet, a crystallization of a round form, with twenty-four 

 trapezoidal faces. And these crystals, which are observed 

 perfectly insulated in lava, are found there also, united in 

 groups, which are likewise insulated, having no marks of 

 former adhesion. 



Here then we have two species of crystals exceedingly 



puinerous in several kinds of lava. Those of iEtna are 



R 4 hllcd 



