in a very peculiar state of decomposition. And on the south-eastern side, our path and the outlet into the ravine. The whole area thus included, might be three or four hundred yards in length, and half that distance in breadth. The surface of the ground, not occupied by the ravine, was broken and strewed with fragments and masses of the porphyritic rock, for the most part so exceedingly decomposed as to be friable and to crumble on the smallest pressure. For some time I thought that this substance, which is perfectly white and in some instances exhibits an arrangement like crystals, was a peculiar mineral; but afterwards became convinced, that it was merely the porphyritic rock singularly altered, not by the action of the air or weather, but, as I conjecture, by a strong sulphurous or sulphuric acid vapour which is generated here, and which is probably driven more against one side by the eddy wind up the ravine, the breeze from any other quarter being shut out by the surrounding hills.[1]
Amidst the loose stones and fragments of decomposed rock are
- ↑ This peculiar decomposition of the surrounding rock has been frequently observed in similar situations, and under analogous circumstances, and has I find been accounted for by other persons in the same way: thus Dolomieu says, “La couleur blanche des
pierres de l'interieur de tous les cratères inflammés est due a une veritable alteration de la lave produite par les vapeurs acido-sulfureuses qui les pénètrent, et qui so combinent avec l'argile qui leur sert de base, y formant l'alun que l'on retire des matières volcaniques.” Voy. anx Isles de Lipari. p. 18.
And he afterwards adds, “ cette alteration des laves par les vapeurs acido-sulfureuses, est une espèce d'analyse que la nature fait elle meme des matiéres volcaniques. Il y a des laves sur lesquelles les vapours n'ont pas encore eu assez de tems d'agir pour les dénaturer entiérement, et alors on les voit dans différens états de decomposition que l'on reconnoit par la couleur.”
Alum is doubtless formed at this place, as well as elsewhere under similar circumstances: the potash necessary for the composition of this salt, being, as well as the argil, derived from the surrounding rock. See Vauquelin's Memoirs. Journ. des Mines, vol. X. p. 441.
