160 PEOE. T. G. BONNET ON THE LITHOLOGICAL 



detail only the points in which these specimens differed from those 

 already familiar to me. 



In examining these Highland rocks (and I might say others also) 

 I have observed three rather well-marked types, indicating stages of 

 metamorphism. In the first it is obvious that many of the con- 

 stituents noticed in the slide, especially those of larger size, with 

 most of, if not all, the felspar, are original. Still many of the 

 quartz-grains present somewhat irregular peripheries, and appear 

 clotted or agglutinated together, as in the most highly altered 

 quartzites, instead of presenting the definite outlines of ordinary 

 fragments ; sometimes the smaller grains have even a chalcedonic 

 aspect, as if of secondary origin. It is often doubtful whether the 

 larger flakes of mica are endogenous or not, and it is quite certain 

 that a minute flaky to fibrous mineral has been produced subse- 

 quently to the deposition of the rock. This is probably, in part at 

 least, a hydrous potash- or soda-mica (very like that which has been 

 recognized as sericite), and in part perhaps is nearer to actinolite. 

 Epidote and occasionally small garnets of secondary origin are 

 present. Of this stage of metamorphism the rocks forming the 

 northern escarpment of the " Newer Gneiss," in the neighbourhood 

 of the head of Loch Maree, furnish excellent examples. 



In the second stage of metamorphism, while, when we regard the 

 rock in the field, we can have no doubt of its sedimentary origin, 

 bedding being often well marked and foliation distinct, yet, under 

 the microscope, it is extremely difficult to identify any of the con- 

 stituents, in their present condition, as of clastic origin. The 

 quartz presents the appearances above described yet more markedly. 

 One might have hoped to have recognized, as has been done in 

 quartzites, original grains as nuclei; but in this I have not yet 

 succeeded, though I have examined a fair number of specimens. 

 The mica, garnets, &c. are almost certainly endogenous ; and this 

 appears also to be the case (though on this point I speak hesitatingly) 

 with the felspar where present. Of this stage of metamorphism, 

 which, so far as one can tell, is as complete as can be, the rocks of 

 the southern face of Ben Fyn furnish excellent examples. In Eng- 

 land I may cite as instances the schists of the Lizard peninsula, and 

 a considerable number of those in Anglesey, though some which 

 have been sent to me from that island belong rather to the former 

 type. 



Between these types intermediate instances will of course be 

 found. The nature of the constituents and the mode in which the 

 agents of metamorphism have operated must bring about varieties 

 of results, and it would be extremely rash to attempt in every case 

 a classification by microscopic evidence alone ; cases there are and 

 will be where " noscitur a sociis " will be true, and we must then 

 decide mainly by evidence obtainable by field-work. I do not 

 mean by this to say that the two methods of investigation will pro- 

 duce contradictory results, but that the microscopist, while certain 

 that he has before him a distinctly metamorphic rock, will not 

 venture to say to what extent alteration has taken place. I have 



