A. M. Green — Geology of Co. Donegal. o55 



wonld be impossible to tell from granite : some varieties are ex- 

 tremely coarsely grained, and would, I think, be certainly classed 

 with that rock ; indeed, the first instance that came before me I at 

 first took for a granite vein, and it was only after much doubting 

 that I felt sure it was truly interbedded with the mica-schist ; some 

 beds are finer in grain, and in them perhaps a rude foliation may be 

 seen, though for my own part I doubt if I should ever have sus- 

 pected the presence of this structure if I had not discovered that the 

 rock was bedded, and thought it my duty to call it gneiss. 



As we go eastward, these beds of granitic gneiss, as I decided in 

 the hope of pleasing both sides to call them, become thicker and 

 more numerous, and the interstratifications of mica-schist thinner 

 and fewer, till at last the latter disappear altogether, and we seem 

 to have reached a district of granite only, where, even supposing the 

 latter to be a metamorphic rock, metamorphism has effaced all traces 

 of bedding. Looking a little closer, however, we find we have been 

 too hasty, and even in the heart of the granite region we are able to 

 detect what seemed to me most unmistakably bedding. 



I would wish, then, to lay particular stress on the following facts. 

 The interstratification with mica-schist of beds of rock which can 

 hardly, if at all, be distinguished from granite ; the very gradual 

 passage from alternations of granitic gneiss and mica-schist into 

 granite alone ; and the marked traces of bedding and other signs of 

 stratification that appear in the last. 



In support of these statements I will now put forward one or two 

 detailed observations. 



On my first day, with Sir E. Griffith's map in hand, I started from 

 Dunlewey, in a north-easterly direction, along the road to Letter- 

 kenney, to fix the place of the boundary between the mica-schist and 

 granite which was laid down on that map. After passing over 

 country where mica-schist was well shown, an interval occurred 

 over which there was no section ; then about half a mile beyond 

 Sand Lough, I came to a crag of what I at first sight called granite. 

 I concluded, therefore, that I had crossed the boundary I was in 

 search of. The rock was coarse, and in parts had weathered down 

 into coarse sand after the manner of granite. I noticed, however, 

 planes of division in it, which looked like bedding, but which might 

 be joints. Their bearing, however, coincided with the general strike 

 of the district ; so I noted this fact, and waited to see what it was 

 worth. 



Leaving the road and striking eastward across the moor, it was 

 soon clear that no such boundary as that I was in search of existed, 

 and that no hard line could be drawn parting mica-schist from 

 granite. Though the greater part of the ground was occupied by 

 the latter rock, narrow bands of the former, with the usual strike of 

 the country, were crossed, evidently interbedded with the latter. 

 These bands became fewer and fewer as I went further east, and at 

 last disappeared altogether, and I reached ground wholly granitic. 

 But even liere the rock was traversed by divisional planes, which 

 might be joints, but which had a suspiciously bedded look about 



