OLDEB KOCKS OF BKITTAlTr. 



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which unite their waters at the lower end of the town. The 

 dominant rock, for about half a league to the north and rather 

 more than double that distance to the south, is a strong gneiss 

 (mica-schiste granitique of Barrois), a broad zone of which extends 

 across the country in a general W.N.W. direction. The commonest 

 variety may be described as a moderately coarse, rather felspathic 

 gneiss, consisting of quartz, reddish felspar, and dark green mica, 

 the last-named mineral occurring in thin irregular bands, with a 

 generally parallel arrangement, so as to produce a somewhat foliated 

 structure, though the rock is not markedly fissile in this direction. 

 Macroscopically the rock recalls to mind some of the Hebridean 

 gneisses of Scotland, and of the Laurentian gneisses of Canada. 

 Varieties may be noticed. A quarry near the river and the viaduct, 

 just below the town, affords a good exposure of the commonest type. 

 Higher up, just beyond the last house, we find a more micaceous 

 variety, while, still higher up, just within the town, a quarry shows 

 a rock with but little mica, more resembling a fine-grained vein- 

 granite. Microscopic examination of the first-named rock shows it 

 to be composed of quartz, felspar (orthoclase and plagioclase) a little 

 decomposed, and a dark olive-brown mica. The wavy outlines of 

 the felspar, the way in which it encloses rounded grains of quartz, 

 and the general association of the two minerals and of the mica, 

 recall to mind a structure which, whatever be its significance, 

 characterizes, so far as I know, the Laurentian gneisses. 



The finer-grained rock mentioned above (quarry inside the town) 

 differs, under the microscope, from an ordinary vein-granite, and 

 exhibits the same arrangement of the quartz and felspar which 

 characterizes the gneiss just described. The only variations are 

 that all the constituents are slightly smaller in size, there is less 

 mica, less approach to a foliated structure, and perhaps a slightly 

 greater proportion of quartz to felspar. 



These rocks, then, like many of the older gneisses, give us no 

 dominant indication of cleavage-foliation, and we may venture, I 

 think, to say that, whatever be their genetic history, they belong to 

 the Laurentian gneisses, though they may subsequently have under- 

 gone some modification. Foliation at the first quarry strikes a little 

 to the east of N.N.E. ; the dip is towards the western side, and is 

 low, not exceeding 15°. This structure, however, may be, in the 

 main, a record of a later disturbance. 



Following the road to Bannalec in a general north-west direction 

 we pass at first outcrops of the above-named gneiss. Then we find a 

 quarry in a fairly well-preserved felspathic granite with two micas, 

 the black predominating, and about a furlong beyond we pass a vein of 

 rotten felspathic granite, intrusive in a micaceous gneiss, the foliation 

 of which I should regard as mainly the result of cleavage, and as 

 prior to the intrusion of the granite. At a distance of about 2-4 

 kilometres from the town is a quarry in a rotten, rather fine-grained 

 felspathic granite (the granulite of Barrois) ; then comes a schistose 

 felspathic gneiss (granulite schisteuse of Barrois); the foliation ih. 

 the latter appears to me due to mechanical causes. The strike in a 



