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liEVOHT — 1884. 



stone of Durness. llcnce, that al though it and some associatod beds arc 

 Ordovician, yet this is no proof of the ago of the Torridon sandstone, 

 quartzite, and eastern gneiss, which njay bo Archaean (Heddle). 



The last view at one time seemed to offer a promising way out of tlie 

 difificulties, but the recent examinations of tlie more critical sections by 

 more than one skilled stratigraphist seem to show that the quartzite 

 must be admitted to bo of Ordovician age, and fully confirm the views of 

 Murchison and his helper. , Mr. Peach and Dr. A. Geikie ; so that if the 

 eastern gneiss do overlie it in true .sncoession, its Palceozoie age is settled. 



Thus the great question at issu(* is. What is the relation of tlic 

 eastern gneiss to the admittedly Palajozoic group ? Is the conform- 

 able upward succession a real one, or only an apparent one, due to 

 faulting with overtlirust on a grand scale 't The question is one of 

 unusual diliiculty, where to have erred is only human. 



Perhaps the simplest way of explaining the difficulties wiL be by 

 describing one of the sections generally regarded as among the most 

 important, that on a line passing roughly along or parallel with Loch 

 Maree as far as the valley, which is followed by the railway from Ding- 

 wall to Loch Carron : — 



In many places by the shore of Loch !Maree the great masses of the 

 Torridon sandstone are seen to rest upon the Hebridean series. This cou- 

 sists in the lower part (well exposed along the eastern shore of the 

 more southern part of the lake) of coarse granitoid gneisses of a pinkish 

 colour, often traversed by veins of pegmatite. The characteristic struc- 

 tures of a granite are not revealed by the microscoj^e. Among the felspars, 

 orthoclase, albite, or oligoclase and microcline, can be identified. A 

 greenish mineral, present in variable quantities, is sometimes an altered 

 biotite, sometimes hornblende. Sphene, garnet, and white mica are 

 occasionally present. Foliation, as a rule, is rather faintly marked. 

 Massiveness, constancy of mineral constitution through considerable thick- 

 nesses, and slowness of change, are the dominant characteristics. A.s thi 

 series is traced upwards indications of bedding, evidenced by change in 

 mineral character, become more marked ; the gneiss is more distinctly 

 foliated, and contains well marked beds of hornblende-schist, mica-scbist, 

 and (though rarely) of crystalline limestone ; the general strike is, roughly 

 from NW. to SE. Of the succession Dr. A. Geikie (as above quoted) 

 says, ' In traversing the western seaboard, from Cape Wrath to Loch 

 Torridon, I have ascertained that these ancient rocks are disposed in 

 several broad anticlinal and synclinal folds. . , . The upper division 

 cannot be sharply defined, but is on the whole marked by the relative 

 thinness of its beds, with a mucli larger development of schists, and a 

 great dimiimtion of the quantity of pegmatite — characters particulurly 

 well seen at Gairloch.' 



In Glen ]jng.^an, or Logan, near the head of Loch Maree, the 

 Ordovician limestone, here dolomitie, dips down towards the bed of tho 

 glen, at an angle of some 30°, and is then cut ofi" by a mass ot 

 granitoid rock of variable breadth ; on the opposite side of this rises thi 

 flscarpment of the eastern gneiss, forming the steep craggy left bank m 

 the valley. Its dip and strike, though not iduutical with that of tin 

 limestone, is not vei'v divergent, (ind its lithological characters arc so 

 ditlerent from those of the Hebridean series as to be insisted upon 1'}' 

 Murchison as one of its best distiiiclions. This dith'rence happenstobe 

 especially eonspicu()n« at this lilace, where the stratilied character is so 



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