Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/199

 Rh 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.

Amidst the loose stones and fragments of decomposed rock are