Vol. 57.] TUFACEOUS RHTOLITIC EOCKS PEOM DUFT02f PIKE. 31 



3. On some Tupaceous Ehyolitic Rocks from Dxjeton Pike (West- 

 morland). By Feank Rutley, Esq., F.G.S. WitJi Analyses by 

 Philip Holland, Esq., P.I.C., E.C.S. (Eead November 21st, 



1900.) 



[Plate I.] 



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 evi- 

 dence of solfataric action on British rhy elites of considerable 

 geological age. Through the kindness of my friend Mr. H. B. 

 Woodward, E.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 volcanic 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 ou 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 con- 

 stitutes 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 paper I am 

 indebted to the kindness of my friend Mr. Philip Holland, F.I.C., 

 F.C.S., and for some admirable photographs to Mr. F. Chapman, 

 A.L.S., F.R.M.S. The rock from the northern end of Dufton Pike 

 appeared at first sight to be very like one previously described by 

 Mr. Harker, but further examination leads to the belief that it 

 differs therefrom considerably in some respects. 



The following is a description of these rocks : — 



No. 1. Northern end of Dufton Pike. — A pale brownish 

 rock, with some darker brown specks and diminutive colourless 

 crystals which have a vitreous lustre. The cut surface shows 

 minute greyish-white specks and a few small irregular veinings. 



A section of this rock, when examined in ordinary transmitted 

 light under the microscope, shows a nearly colourless to brownish- 

 yellow groundmass, containing numerous porphyritic crystals and 

 fragments of felspar, most of them in a more or less altered 

 condition, and crystals which for the most part give square or 

 approximately square sections. The latter are generally opaque 

 and of quite microscopic dimensions, so that when examined in 



