418 PROP. T. G. BONNET ON A SERIES OP ROCKS 



100 (Loch Ailsh, north end, p. 387). Minute grains of quartz with 

 . mica (fragmental), epidote, and fine granular matter, probably result- 

 ing from the decomposition of felspar. Does not appear to be very 

 compressed. 



101 (Cnoc Chaorinie, p. 387). A quartzite. The structure of this is 

 rather peculiar ; the recognizable fragments are interspersed in a quart- 

 zose ground-mass, of which at any rate a large part seems to be chalce- 

 donic. Most of the larger fragments are, as it were, fringed with 

 a deposit of microcrystalline quartz. The slide is traversed by veins 

 filled by the same mineral. A little haematite and opacite is scat- 

 tered about the slide. 



102 (Stack of Glen Coul). This rock is not free from difficulty. It 

 consists of quartz and felspar, corresponding with those in the 

 Hebridean gneisses, separated by thin films of a micaceous mineral, 

 more or less dotted with opacite. The structure is undoubtedly 

 fragmental ; the rock has undergone great compression, the frag- 

 ments being crushed, flattened out, and " packed " together, as one 

 sees in slates. I believe it to belong to this newer group ; but I 

 could conceive it possible (as my opinion must be formed on a single 

 slide) that we had here a specimen of the older series crushed in an 

 exceptional manner. 



103 (below Kinloch Ailsh, pp. 359, 388) consists of minutely 

 fragmental quartz with a little derived mica, imbedded in a matrix 

 chiefly consisting of an earthy material, felsite, and minute scales of 

 the micaceous (or perhaps sometimes hornblendic) mineral already in- 

 clusively termed sericite, with a grain or two of magnetite (?). It is 

 in fact not more altered than many of the older Palaeozoic slates. 



104 (ibidem, pp. 359, 388). The " sericite " is rather abundant and 

 conspicuous, giving the rock a little more the appearance of a true 

 schist ; but the differences are hardly more than varietal. 



107 (Glen Coul, near house, p. 359) is a rock of the same general 

 character, but with the fragments rather larger, looking less com- 

 pressed, and altogether even less altered . The quartz fragments though 

 small (usually less than -005") are rather markedly angular. 



108 (Traligill Burn, Ben More, p. 359). A rock probably of the 

 same group as these last. Shows interbanding on a small scale, bands 

 in which quartz fragments predominate being irregularly interlami- 

 nated with finer zones composed chiefly of earthy material and seri- 

 cite. It is an interesting rock for study, as I think it may be held 

 to represent the first stage in the process of the conversion of an 

 ordinary sedimentary rock into a mica-schist. There are numerous 

 granules and clots of an iron-oxide, and, possibly some minute garnets. 



97 (Glen Coul, south side, p. 390), 99 (Ben Arnaboll, p. 403, note). 

 These are peculiar and interesting rocks, closely resembling one an- 

 other. They may be called quartzites ; for the mineral is chiefly 

 quartz ; but it is extremely difficult, especially in the former, to detect 

 with certainty the original fragments. Viewed with crossed nicols, 

 the slide appears to be composed of minute granules of quartz of chal- 

 cedonic aspect; among these are wavy, somewhat parallel bands, 

 which appear almost homogeneous, but break up like the rest as the 



