546 keport— 1884. 



stone of Durness. Hence, that although it and some associated beds are 

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

 quartzite, and eastern gneiss, which may be Archasan (Heddle). 



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

 difficulties, bat the recent examinations of the more critical sections by 

 more than one skilled stratigraphist seem to show that the quartzite 

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

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

 eastern gneiss do overlie it in true succession, its Palaeozoic age is settled. 



Thus the great question at issue, is, What is the relation of the 

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

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

 faulting with ovei-thrust on a grand scale ? The question is one of 

 unusual difficulty, where to have erred is only human. 



Perhaps the simplest way of explaining the difficulties will be by 

 describing one of the sections generally regarded as among the most 

 important, that on a line passing roughiy 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 con- 

 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 microscope. 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. As the 

 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-schist, 

 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 denned, but is on the whole mai-ked by the relative 

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

 great diminution of the quantity of pegmatite — characters particularly 

 well seen at Gairloch.' 



In Glen Lag^an, or Logan, near the head of Loch Maree, the 

 Ordovician limestone, here dolomitic, dips down towards the bed of the 

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

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

 escarpment of the eastern gneiss, forming the steep craggy left bank of 

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

 limestone, is not very divergent, and its lithological characters are so 

 different from those of the Hebridean series as to be insisted upon by 

 Murchison as one of its best distinctions. This difference happens to be 

 especially conspicuous at this place, where the stratified character is so 



