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The specimens which form the subject of this paper were selected for examination on account of their peculiar appearance, and because it was thought probable that they might afford some evidence of solfataric action on British rhyolites of considerable geological age. Through the kindness of my friend Mr. H. B. Woodward, F.R.S., I have been able to learn a few particulars concerning the geology of Dufton Pike. From these it appears that the central portion of the Pike consists of volcaninc rocks of the Borrowdale Series, bounded by four faults, those on the east and west being approximately parallel and running in a north-north-westerly direction. The rocks faulted against this central mass of volcanic rocks are of Lower Silurian age on the north, south, and east; while those on the west are Upper Silurian, consisting of Stockdale Shales and Coniston Flags.
The specimens about to be described were collected by the late Prof. A. H. Green, F.R.S., and Mr. J. G. Goodchild, F.G.S., and were evidently procured from the Borrowdale Volcanic Series which constitutes the central mass of Dufton Pike. The specimens were given to me many years ago, so that, never having visited that part of Westmorland, I am ignorant of the precise spots from which they were derived. The chief interest which attaches to them lies in the peculiar character of the alteration that they have undergone: an alteration which appears to me to have been probably due to solfataric action.
For the careful analyses which accompany this
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