208 Prof. Bonney — The Felsite of Bittadon, N. Devon. 



is eruptive." At the time when these words were written, it was 

 undoubtedly impossible, from the nature of the exposure, to decide 

 whether the rock was intrusive or interbedded ; but, on visiting the 

 locality in September, 1876. I found that a small quarry (for road 

 metal) had been opened in the felstone, and its relations to the sedi- 

 mentary rocks were pretty clearly exposed. 



Section in Stone Pit, near Bittadon. 



A, A Felsite. 



B Ibid., decomposed. 

 C Fine Slate. 



D Fine Slate, much crushed. 

 E Quartz vein. 

 F, F Soil, Ground rising above. 



The annexed diagram will, I think, render a long description 

 needless. The junction on the right-hand side is very clear, and the 

 slate a little below seems much, crushed ; on the left hand side 

 the exact line of junction is not quite so easy to trace, as the felstone 

 becomes much, decomposed near it, but it is very nearly represented 

 by the dotted line. The rock on this side is a hard grey rather 

 lustrous slate, apparently dipping towards the S.E., with cleavage 

 dipping at about 80° in the same direction, or perhaps rather more 

 to the south. The felstone then is evidently not interbedded, but 

 intrusive. 



I have examined the rock microscopically ; and as its structure is 

 a little unusual, may complete this note by a short account of it. 



Its ground-mass is compact in texture, and a dull greenish grey in 

 colour, rather thickly studded with small felspar ciystals, commonly 

 from 0-1 to 0-2 inch long, of a reddish white colour ; the majority 

 certainly being orthoclase. A few small grains of quartz are visible. 

 Here and there are brown ferruginous spots, apparently occupying 

 the place of some of the felspar, and the whole rock has a decom- 

 posed aspect. It is also affected slightly by cleavage. Thus, in a 

 hand specimen, it is not easily distinguished from a highly altered 

 volcanic ash, such as might be found in the Borrowdale series of 

 Cumberland. A slide, examined with a ^' objective, shows the 

 ground-mass to be rather variable ; in parts it is crowded with 

 indistinct colourless microliths, brownish and greenish granules, 

 and occasional black specks ; in others it is irregularly banded with 

 pale yellowish-green streaks. This last, on examination with both 

 Nicols, proves to be one of the minerals provisionally classed as 

 viridite ; that exhibiting an irregular aggregate structure, and being 

 feebly double-refracting. The colourless part of the ground-mass is 

 now seen to be almost wholly made up of irregular rather fibrous 

 microliths, bright-coloured with crossed Nicols. Here and there are 



