Vol. 57.] ROCKS FROM DCFTON PIKE. 37 



The Brogger diagrams constructed from the molecular ratios of 

 these rocks correspond so closely with those of rhyolite (=Dufton 

 Pike No. 2) and soda-rhyolite ( = Dufton Pike No.l) that it is need- 

 less to reproduce them, since such diagrams accompanj' the jDaper by 

 Prof. W. H. Hobbs, entitled ' Suggestions regarding the Classification 

 of the Igneous Kocks,' nublished in the Chicago ' Journal of Geology/ 

 vol. viii (1900) p. 1. 



It is singular that so tufaceous a rock as Dufton Pike No. 2 should 

 be practically identical in chemical composition with a rhyolite ; but 

 this coincidence may no doubt be, in a large measure, accounted for 

 by the almost exclusively felspathic nature of the lapilli and dust 

 which constitute so large a proportion of the rock. I have been 

 unable, so far, to locate the small amount of baryta ( = 0-11 per 

 cent.) shown by analysis to be present in this rock. It may be 

 present among the felspathic constituents in the form of hyalophane, 

 or it may exist as witherite, barytocalcite, or barytes ; but the last 

 surmise appears improbable, since the presence of sulphur is not 

 indicated in the analysis. 



EXPLANATION OF PLATE I. 



Fig. 1. Dufton Pike No. 1. — Altered tufaceous rb5-olite of Lower Silurian age 

 showing very numerous spots in the partly devitrified groundinass. 

 The spots are in inost cases augitic, in others apparently felspathic 

 fragments of extremely small dimensions. Tlie partly corroded 

 crystals and fragments of much larger size are felspar, sometimes 

 orthoelase, at others oligoclase or andesine. In the centre of the 

 figure is a ci'ystal of orthoelase mainly altered into muscovite. X 30. 

 Nicols crossed. (See p. 32.) 



2. Dufton Pike No. 1. — Fragments of corroded felspar, that in the riglit 



upper quadrant being in a position of extinction. X 380. Nicols 

 crossed. (See p. 32.) 



3. Dufton Pike No. L — Vestiges of much corroded and altered felspars. 



All of these patches, with the exception of that just appearing on the 

 right edge of the figure, undergo simultaneous extinction, so that they 

 are parts of one crystal. X HO. Nicols crossed. (See p. 32.) 



4. Dufton Pike No. 2. — Altered and highly tufaceous rhyolite. Small 



fragments of felspar in a partly vitreous groundmass : that on the 

 right being represented by small rods of unaltered felspar which 

 extinguish simultaneously ; that on the left shows A'ery sliglit traces- 

 of such erosion. X 140. Nicols crossed. (See p. 35.) 



5. Dufton Pike No. 2.— Similar but larger fragments of felspar in the same 



section as that represented in fig. 4. X 140. Nicols crossed. Such 

 fi-agments constitiite fully one half of the rock. (See p. 35.) 



The following figures are drawn from microphotographs 

 made by Mr. F. Chapman, A.L.S., F.R.M.S. :— 



Fig. 6. Dufton Pike No. 1. — Showing a spotted part of the same section as that 

 represented in fig. L Here some of the diminutive fragments, mostly 

 pyroxenic or felspathic, are seen to constitute the nuclei of the spots. 

 X 200. Ordinary transmitted light. (See p. 32.) 

 7, Dufton Pike No. 2, — Showing portion of a small fragment of unattacked 

 felspar, whence proceed small rods of precisely the same character 

 as the mesh-like fragments (fig. 5) which constitute a very large pro- 

 portion of this rock. The photograph was taken in the position of 

 maximum illumination between crossed nicols. On rotation, the 

 fragment of felspar and the mesh of rods proceeding from it undergo 

 simultaneous extinction. X 200. (See p. 35.) 



