ON THE AllClKEAN UOCKS OV GKEAT BRITAIN. 



547 



marked that one observer has denied that the sti'ata are more meta- 

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

 describe the lithological chai'acters of the eastern gneiss along the above- 

 named line of section, as it is followed soatliwards. Speoiraons 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 (jnartz 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 generally lengthwise, with 

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

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

 one or two probably microcline. The micaceous constituent is rather 

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

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

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

 number of iron peroxide, probably hiematite.' ' Calcite, chlorite (?), and 

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

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

 much more modern than the t3'pical Hebridean rocks, and possibly even 

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

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

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

 sch'isles Juslrees of Lory, part of the Butuhier schiefur of Von Hauer), of 

 wh^''h (to avoid ambiguity) we may take the schists in the Binnenthal, 

 below tiie village of Jiinn (Canton Valais), as an excrllent type. These 

 ai'o so perfectly bedded that at a moderate distance it would l)e im- 

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

 on close examination, especially with the microsco{)e, they are in- 

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

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

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

 tlio' same macroscopic characters, except that, as wo proceed up it, the 

 rat'tamorphism becomes rather more marked, macroscopically and niicro- 

 RC(ipically ; quartz-sehists, fine-grained gneiss, and a lead-coloured mica- 

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

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

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

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

 jrraiiU'd micaceous gneisses, still inclined to be flaggy, but indubitably 

 liiglily altered. 



Niiw it caimot be denied that in this section tliere is some evidence, 

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

 tliattbis scries overlies tlie quartzite-limestone group, and that there is a 

 proL;irssively increasing metamoi-phism as we proceed southwards. The 

 ('lHb(ii;ite ])aper, already mentioned, by Mnrcliisun and Geikie, brings 

 forwurd other instances where there is apparently a true successio]! 

 liom the quartzite-limestone series into the ea, *;L'rn gneiss. 



' I'.iinmv. O.J. a. X. \c ]. xv.wi. p. \W1. 



U N li 



