1859.] MURCHISON NORTH HIGHLANDS. 225 



The lower or fundamental gneiss, near the coast of Scourie and 

 Loch Laxford, occupies low hills, which have been much worn down 

 by glacial action and drift. These, as they range eastwards up to 

 the summit of the mountain called Ben Stack, constitute the base or 

 support of the overlying white quartz-rocks of Arkle and Fionaven. 



Nowhere in the North-west Highlands are grander geological 

 features seen than where these quartz-rocks, weathering white hi 

 the escarpments of Arkle and Fionaven, repose upon the massive old 

 grey gneiss, with its numerous intrusions of red granite. Passing 

 to the cast from either of theso mountains, the observer meets first 

 ■with a limestone covering the lower quartz-rock, and next with 

 superimposed masses of quartzose, micaceous, flag-like strata. I 

 specially call attention to a section from Ben Stack, at the lower 

 end of Loch Stack, to the head of Loch More, because no traveller 

 will have any difficulty in examining it, since the high road from 

 Scourie to Lairg runs parallel to it. (Sec fig. 6, p. 226.) 



The old or massive gneiss, trending from S.S.E. to N.N.W., is well 

 exposed on the sides of the high road on the south bank of Loch 

 Stach, where, dipping 35° west of south, it has been much cut into, 

 and where huge granitic intrusions are finely displayed. He wbo 

 climbs to the summits of Ben Stack will see that this old gneiss is 

 covered by a thin pebbly conglomerate, which dips at a low angle to 

 the S.E. and there forms the base of the lowest of the Silurian 

 quartz-rocks. The latter rock, though much denuded, and not 

 nearly so well exposed as in Arkle and Fionaven, occupies consider- 

 able dimensions in the hills north of the upper end of Loch Stack 

 ( Ben Strome), whence it slopes down to the western end of Loch 

 Siore with an easterly dip. A little to the cast of a shooting-lodge 

 on Loch More, the limestone succeeds, as expressed in this section, 

 fig. 6. Here, as at the head of (Jlencoul, and all the tract extend- 

 ing to the S.W. between Loch More and Assynt, and again to the 

 N.E. between Loch More and Loch Eriboll, the limestone is dimi- 

 nished to a thin course. In this respect the limestone bears the 

 same irregular proportions to the quartzose rock in which it is inter- 

 calated as the Lower Silurian limestones of Wales do to their asso- 

 ciated slaty and siliceous rocks. In Wales, as in Sutherland, the 

 calcareous matter is every here and there of considerable dimensions, 

 and at other places it dwindles away to mere threads, and is often 



t-iii irely lost. 



In examining the limestone upon the steep banks <>t' a mountain- 

 torrent which flows into the south side of Loch Ifore, Prof. Ramsay 

 and myself observed, it is true, an intercalated igneous rook ; but, as 

 we also saw thai the cahareous band was followed conformably, as 

 in Assynt, bj an overlying quartzose rock, we remained of opinion 

 that here, as elsewhere, the occurrence of a felspathic or porphyritic 

 rock, whether above or below the limestone, did aol interfere with 

 the general order of succession*. 



* Prof, rTiool states that this igneous rook acquires Larger dimensions higher 

 up in the mountain, and l do not doubt the fact, though neither Pro£ B 



