434 MR. G. BARROW ON THE MOJ.NE GNEISSES [Nov. I904, 



felted biotite-films in the Moine Gneisses, and along which the roek 

 readily splits. Close to the Quartzite, a few far more siliceous pink 

 and white bands occur, and in these rauscovite is more abundant and 

 there is less biotite. The intermediate portion is a thinly-banded, 

 fine-grained, brown or grey rock, obviously containing a great deal 

 of fine brown mica. This is the dominant constituent of the group, 

 and in a typical example (9797) the more siliceous bands are 

 composed of a singularly-even admixture of quartz-grains and 

 minute flakes of biotite, with, possibly, some water-clear felspar ; 

 the structure is essentially that of a fine biotite-granulite. The 

 more micaceous portion contains far less quartz or felspar, and is 

 largely composed of finely-felted brown mica, with which some larger 

 crystals of muscovite are associated, set athwart the foliation as in 

 a spangled gneiss. These micaceous films are peculiarly interesting, 

 for they have been met with over a large area, among others, on 

 the north-west side of Ben Yuroch. 1 



The Little Limestone, though still containing the typical dark 

 dust, differs from the tremolite-rock in the fact that the bulk of the 

 hornblende is now actinolite. A still more important difference 

 occurs in the schist forming the steep bank above the stream. It 

 is lighter in colour than the typical Dark Schist, and a series of 

 sections shows that it originally contained far less clastic chlorite 

 and fine dark dust ; still, the maximum amount of chlorite occurs 

 in the portion of the bed next the Little Limestone (9794, 9792, 

 9795, 9790). It is thus seen that, although we have here the full 

 sequence of the beds about the Little Limestone, each band differs 

 slightly in composition from the type-rocks of the section about 

 Auchallater. It places the true position of the Honestones, however, 

 beyond dispute, and is especially important because it will be seen 

 immediately that this is, so to speak, the most siliceous phase in 

 which the Honestoues are ever known to occur in this area accom- 

 panied by the full sequence. So soon as they become markedly more 

 siliceous, the Little Limestone and part of the Dark Schist appear to 

 be almost always missing in the area here described. 



This fact can be seen at once by ascending Glen Mohr. A little 

 above the junction with Glen Bheag, the stream flows along the 

 strike of the rocks. In the bank on one side we have the repeatedly- 

 folded margin of the Limestone, on the other the white edge of the 

 Quartzite : the bed of the stream being formed by the Honestones, 

 now somewhat more siliceous, but still unmistakable. In quite a 

 short distance, the Honestones pass into a small group of quartzite- 

 bands, with a patch in the centre in which the honestone-character 

 is still traceable. Of these quartzite-bands, the one nearest the 

 limestone is quite white and almost indistinguishable from the 

 margin of the Quartzite ; the other bands are pink and grey. When 

 the ground was first examined, the limestone was taken for the 

 Little Limestone ; for it appears to be very thin, has a bright pink 

 colour, and is exactly in the position where the Little Limestone 

 should be. The recent traverses, however, make it more probable 



1 See explanation of the Geological Survey 1-inch map, Sheet 55 (Scotland). 



