Dr. Nugent on the Souffriere of Montserrat, 187 



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 decom- 

 posed 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 sulphureous or sul- 

 phuric 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,* 



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 I'interieur de tous les craters inflammes est due a une veritable alteration 

 " de la lave produite par les vapeurs acido-sulfureuses qui les penetrent, et qui se com- 

 *' binent avec I'argile qui leur sert de base, y formant I'alun que Ton retire des matiere* 

 " volcaniques." Voy. aux Isles de Lipari. p. 18. 



And he afterwards adds, " cette alteration des laves par les vapeurs acido-sulfureuses, 

 *' est une espece d'analyse que la nature fait elle meme des matieres volcaniques. II y a 

 " des laves sur lesquelles les vapeurs n'ont pas encore eu assez de terns d'agir pour les 

 " denaturer entierement, et alors on les voit dans difterens etats de* decomposition que 

 <* Ton 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 yurrounding rock. See Vauquelin's Memoire. Journ. des Mines, vol. x. p. 441. 



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