162 PROE. T. G. BONNEY ON THE LITHOLOGICAL 



before us a granite or a gneiss. Examples of this class are the 

 coarse gneisses of the Hebridean series, which underlie the Tor- 

 ridon Sandstone and many of the Malvernian rocks of England. At 

 the same time it must be remembered that now and then beds more 

 distinctly foliated also occur in this series. 



Naturally we should expect that as a rule the above distinctions 

 would have a certain chronological value, and thus we are justified 

 in using them, in default of other evidence and with due caution, 

 for purposes of classification. In the following notes I have, for 

 convenience of reference, followed the numbering which Dr. Hicks 

 had placed upon the specimens, but have added I., II., or III. ac- 

 cording to the type of which the specimen reminded me, inserting 

 a qualification where ueeded. 



1 (Glen Logan). Principal minerals, quartz, felspar a good deal 

 decomposed, but a considerable amount of plagioclase still discernible, 

 generally extinguishing at small angles with the twin plane. In 

 parts of the slide is a mineral which, at first sight, has exactly the 

 aspect of grains of olivine in process of conversion into a greenish 

 serpentinous mineral. Eurther examination, however, shows that 

 the former mineral is garnet. The latter appears in part isotropic, 

 in part shows filmy streaks of pale bluish light. I do not, however, 

 observe here either the transverse microcrystalline structure of the 

 " strings " or the opacite clotting so common in serpentine when 

 formed from olivine. Granules of an earthy-looking mineral are 

 probably formed by the aluminous constituent of the garnet, which, 

 as these are not abundant, was probably Bredbergite rather than 

 pyrope. So far as my experience goes, this replacement of garnet 

 by a serpentinous mineral is very rare* (III.). 



2 (north side of Glen Logan, eastern branch). Quartz, mica (both 

 white and brown, the latter partly altered into a green micaceous 

 mineral), some felspar (rather decomposed), and a good many 

 garnets almost colourless, but with many microlithic enclosures (II.). 



3 (same locality). Generally similar, but with numerous small 

 granules of a mineral which I take to be epidote. 



4 (same locality). Macroscopically appears to be a compact gneissic 

 rock ; microscopically it exhibits a ground-mass, consisting mainly of 

 quartz granules in which are scattered flakes of mica exhibiting a 

 certain foliation, and grains, generally somewhat irregular in outline, 

 of a rather decomposed felspar. The last look as if they might bear 

 record of original constituents ; but on the whole the rock appears 

 to me to agree best with those in II. and to be more highly 

 altered than is usual in I., the outlines of the quartz grains in the 

 ground-mass being very irregular. 



5 (south side of Loch Roshk). Macroscopically a not very fissile 

 mica-schist ; microscopically quartz, white and brown mica, with 

 one or two small garnets ; very typical example of II. 



6 (top of hill-road to L. Coulan from Achnashellach). A rather 

 fissile mica-schist, rich in a dark lead-coloured mica, very distinctly 

 foliated. Two micas, both, I suspect, more or less hydrous, are present, 



* See Eosenbusch, ' Mikroskopische Physiographic,' vol. i. p. 163. 



