106 PKOF. T. G. bonnet's PETEOLOGICAL NOTES ON THE 



minute belonites, as well as enclosures similar to the last in less 

 number, and microliths of hornblende (?). There are clustered grains 

 of a mineral rather resembling sphene, which also occurs in separate 

 crystals*. There is also quartz, with hornblende (?) microliths, plagio- 

 clase felspar, and possibly a little zircon. The slide, of necessity, 

 has been cut rather thick, so it is difficult to be certain of some of 

 the microliths. Prom the aboYe description it will appear that the 

 rock might be named a hornblende- eel ogite. 



The bearing of the evidence of this district upon general questions 

 of metamorphism is highly interesting and important. Above the 

 comparatively unaltered Torridon Sandstones we have, as all allow, 

 the highly altered quartzite. Above the seemingly unaltered lime- 

 stone series we have the true schists of the upper group, which also 

 become more highly metamorphosed as they are traced towards the 

 south-east. The possibility of the latter change will probably be more 

 readily admitted, as it might be explained by deeper entombment 

 under superincumbent strata, or nearer approach to regions of ele- 

 vated temperature, of which indications are given by the great 

 granitic masses of the Central Highlands. But how are we to explain 

 this alternation of metamorphism in a vertical direction? It shows, 

 I think, how important the mineral constituents of the bed and their 

 mode of association are as factors in the general result. In the 

 Torridon Sandstone well-rolled grains of moderate size occur in a 

 matrix, scanty indeed, but commonly just enough to separate them. 

 In the quartzite clear and perhaps rather less-rounded quartz grains 

 are in apposition. The latter have become agglutinated ; and the 

 same result may be seen to have happened in the former, where 

 two " clean " grains are in contact ; but the interposition of a little 

 " dirt," as we may sometimes see in the quartzite, is an obstacle to 

 union f. Eelspar grains seem to unite less readily. The amount 

 of mineral change in the matrix seems to depend on both its consti- 

 tution, if homogeneous, and mode of association, if not. If, for ex- 

 ample, it contains the constituents of viridite or epidote in a right 

 state of division, these minerals are readily formed ; if not, change 

 is slow. 



The muddy beds overlying the quartzite appear to have resisted 

 change, other than the formation of crystallized calcite (and perhaps 

 dolomite). The series above these, however, has been of a more 

 favourable composition ; and in it we see bed differing from bed in 

 the amount of metamorphism. But into these interesting questions 

 I must abstain from entering. I trust some day, when I have added 

 yet more to the large quantity of material which I have been for 

 some time past collecting, to invite the attention of the Society to 



* Since the above was written, Prof. Henard has seen the slide, and was of 

 opinion that this mineral was more probably staurolite. Some of it certainly 

 suggests that the mineral is orthorhombic ; but unfortunately no crystal that I 

 can discover in the slide gives quite conclusive evidence. 



t Of this the preservation of Annelid-tubes in the quartzite is a marked in- 

 stance. These certainly occur occasionally, so that we need not here refer to 

 the so-called fucoidal markings. 



