1856.] NICOL — QUARTZITES, ETC. OF N.W. SCOTLAND. 37 



of geological phsenomena in this portion of Scotland which I would 

 wish to introduce. The oldest formation is undoubtedly the gneiss 

 seen underlying the red sandstone, from Cape Wrath to the Gairloch 

 and Skye. Its chief mineralogical peculiarity is the prevalence of 

 hornblende and the comparative rarity of mica, though the more 

 ordinary kinds are fur from being unknown. Though granite-veins 

 often intersect this rock, there is no mass of granite which can be 

 assigned to this first period. Indeed the fragments in the red sand- 

 stone, and the composition of its grits, would lead us to conclude 

 that granite was not an abundant rock in this region, more especially 

 the regular compound of quartz, felspar, and mica, which appears to 

 have been chiefly formed at a more recent period*. This older 

 gneiss, in the region indicated, has a general direction to the N.W., 

 apparent not only in the strike of the rocks, but also in many of the 

 lakes and valleys. In the mountain ranges this direction is far less 

 marked, having, it appears, been obliterated by subsequent denuding 

 action. 



Over the gneiss thus elevated, a large deposit of red sandstone has 

 been thrown down. Where the bottom beds are a coarse angular 

 breccia, intercalated amidst the broken ends of the gneiss, as on the 

 Gairloch, the deposition of this rock must have immediately suc- 

 ceeded some violent convulsion, either local or general. The red 

 sandstone on the west forms a narrow band along the shore, and never 

 extended far into the interior ; still less over the whole country, as 

 has often been imagined. This is proved by its thinning out on the 

 east, below the quartzite ; whilst the conglomerates on its margin in 

 Caithness and Easter Ross also appear to indicate proximity to the 

 shore on that side of the island. The thickness of this deposit must 

 be very great, — from 2000 to 3000 feet being still exposed in the 

 mountains of Applecross, Gairloch, Loch Broom, and Assynt, whilst 

 the dip of the strata implies that its original dimensions were far 

 greater. 



At the close of this period the country must have been still more 

 depressed towards the east, allowing the quartzites to extend much 

 farther into the interior. At present the quartzite-fragments micon- 

 nected with the red sandstone stretch from five to ten miles beyond 

 its boundary, but may probably have once reached much farther over 

 the gneiss. A change in the mineral character of the deposits also 

 took place, the red sediments ceasing and others more favourable to 

 organic life coming in their place, as shown in the fossiliferous lime- 

 stone above. The quartzite including the limestone has no great 

 thickness, probably not exceeding from 300 to 500, or at most 800 

 feet. To this, however, must be added the quartzite of Ben More 

 Assynt, if truly overlying, and also the upper gneiss, so far as this 

 has been metamorphosed in place. 



* Micaceous granites are also rare in the Old Red Conglomerates in many parts 

 of Scotland, and the granite-boulders in the Silurian conglomerates on the Ayr- 

 shire coast rarely contain this mineral ; thus showing that they have not been 

 derived from the granite-mountains in the vicinity, but from an older formation. — 

 See Murchison on the Silurian Rocks of Scotland : Journ. Geol. Soc. vol. vii. 

 p. 153. 



