ON THE ARCH.EAN ROCKS OF GREAT BRITAIN. 547 



marked that one observer has denied that the strata are more meta- 

 morphosed than many of the ' Lower Silurian flags in Wales.' I will 

 describe the lithological characters of the eastern gneiss along the above- 

 named line of section, as it is followed southwards. Specimens have been 

 examined from more than one point along this northern edge. Undoubtedly 

 some of the rocks have, macroscopically, a very flaggy, stratified, and but 

 slightly altered aspect ; all, however, under the microscope show that there 

 has been considerable change. ' They consist chiefly of quartz and a 

 micaceous mineral, with a fair amount of felspar, some epidote, &c. 

 Minute grains of quartz, as it were agglutinated together, compose the 

 greater part of the slide, with the micaceous mineral, both disseminated 

 and in wavy bands, parallel with the stratification. In this ground-mass 

 are scattered longer subangular grains, lying genei-ally lengthwise, with 

 the mica scales bending round them, so that they form, as it were, " eyes " 

 to the slide. Often most of these are felspar, many are plagioclase, 

 one or two probably microcline. The micaceous constituent is rather 

 fibrous, fairly dichroic, showing moderately bright colours with the two 

 nicols, and is probably a hydrous magnesia mica, but there may be more 

 than one mineral present ; there are many small grains of epidote, a fair 

 number of iron peroxide, probably h;ematite.' ' Calcite, chlorite (?), and 

 perhaps a few minute garnets occur in some cases. Thus the series is 

 metamorphic, but evidently (if we may trust the microscopic indications) 

 much more modern than the typical Hebridean rocks, and possibly even 

 formed of their debris. In this region (and in others whence I have had 

 specimens) this eastern gneiss reminds me, in many respects, of the 

 great uppermost zone of schists, so largely developed in the Alps (the 

 schistes lustrees of Lory, part of the Bundner schiefer of Von Hauer), of 

 which (to avoid ambiguity) we may take the schists in the Binnenthal, 

 below the village of iiinn (Canton Valais), as an excellent type. These 

 are so perfectly bedded that at a moderate distance it would be im- 

 possible to assert positively that they were metamorphic rocks, while, 

 on close examination, especially with the microscope, they are in- 

 dubitably much altered. The eastern gneiss, south of Loch Maree, in 

 the above-named line of section, is cut roughly across the strike by the 

 valley of Glen Docherty. The rocks exposed in its crags maintain 

 the same macroscopic characters, except that, as we proceed up it, the 

 metamorphism becomes rather more marked, macroscopically and micro- 

 scopically ; quartz-schists, fine-grained gneiss, and a lead-coloured mica- 

 schist with small garnets, being noted among others. In the lower 

 part of Ben Fyn, on the north side of the valley traversed by the Ding- 

 wall and Skye railway, we have flaggy quartz-mica-schists, or gneisses 

 poor in felspar, with red garnets ; in the upper part, mica-schists and fine- 

 grained micaceous gneisses, still inclined to be flaggy, but indubitably 

 highly altered. 



Now it cannot be denied that in this section there is some evidence, 

 even taking the microscopic character, in favour of the Murchisonian view, 

 that tins series overlies the quartzite-limestone group, and that there is a 

 progressively increasing metamorphism as we proceed southwards. The 

 elaborate paper, already mentioned, by Murchison and Geikie, brings 

 forward other instances where there is apparently a true succession 

 from the quartzite-limestone series into the eastern gneiss. 



1 Bonney, Q. J. Cf. S., vol.xyxvi. p. 102. 



s n - 



