402 



NA TURE 



\_August 26, 1880 



presented by the gneiss. A very slight examination , landshire is comparatively small. As shown in Fig. i it 

 shows that every dome and boss of rock is ice-worn. The 1 rises somev/hat steeply from the west, its chief area and 

 smoothed polished and striated surface left by the ice of drainage lying towards the east. I have visited those 

 the glacial period is everywhere to be recognised. Each tracts of the Highlands where the rocks approach nearest 

 hummock of gneiss is a more or less perfect yociic to the type of the ancient gneiss, and where the conditions 

 moutonncc. Perched blocks are strewn over the ground have been most favourable for intense glaciation. No 

 by thousands. In short, there can hardly be anywlicrc more promising locality for a comparison of this kind 

 else in Britain a more thoroughly typical piece of , could be found than the deep defiles of Glen Shiel and 



glaciation. 



An obvious answer to the question of the origin of the 

 peculiar configuration of this gneiss country is to refer it 

 to the action of the last ice-sheet which covered Britain. 



Kintail. The rocks have there been extremely meta- 

 morphosed and have been exposed to the action of ice 

 descending from some of the highest uplands in the West 

 of Scotland. Yet we look in vain among them for any 





That the gneiss was powerfully ground down by that ice '■ semblance of the bare bossy surface of the old gneiss. 



A further difficulty arises when 



" ' we reflect that in the general 



-^ .=^- — ~ ^ erosion of the country the gneiss, 



~f^' / ,- being covered by later formations, 



would be the last to be attacked, 

 ^- v-=^7r7~~ I and in so far as it was so covered, 



_;._^_- - — _ _ must have been exposed to the 



erosive action of the ice for a 

 shorter time than the overlying 

 rocks. We might therefore have 

 presumed that instead of being 

 more, it would have been less 

 trenchantly worn down than these. 

 Its great toughness and durability, 

 which have enabled it to retain the 

 ice-impress so faithfully, must have 

 given it considerable powers of 

 resistance to the grinding action 

 of the glacier. 



Every fresh excursion into these 

 northern wilds has increased my 

 difficulty in accounting for the 

 peculiar contours of the gneiss 

 ground by reference merely to the 

 work of the Glacial period. A 

 recent visit, however, seems at last 

 to have thrown some light on the 

 matter. I had long been familiar 

 with the fact that the platform of 

 gneiss on which the red sandstones 

 and conglomerates were laid down 

 abounded in inequalities even at 

 the time of the deposit of these 

 strata. Its uneven surface rose 

 heie and there into high ridges 

 and cones, of which Stack is a 

 diminished representative, and 

 sank into depressions now occu- 

 pied by thick masses of sandstone. 

 1 ut I have lately observed that 

 not only do these larger features 

 pass under the sandstone, but that 

 the minor domes and bosses of 

 neiss do so likewise. On both 

 sides of Loch Torridon, for exam- 

 ple, the hummocky outlines of the 

 gneiss can be seen emerging from 

 under the overlying sandstones 



en Shicldag. Locli Turr.Jo 



Fig. 3. — View of outlier of Cambi 



and sandstone among gneiss hil's 



is sufficiently manifest. But if the peculiar bossy surface I (Fig. 2). On the west side of Loch Assynt similar 

 is to be thus explained we are confronted by the difficulty junctions are visible. But some of the most impressive 

 that the ice must have acted far more effectively on the sections occur in the neighbourhood of Gairloch. Little 

 gneiss than on any other rock in the region. Yet there more than a mile to the north of the church the road to 

 is nothing in the configuration of the ground to make the Poolewe descends into a short valley surrounded with 

 erosion greater on the gneiss than on the red sandstone gneiss hills. From the top of the descent the eye is at 

 or quartzites and schists. The same side of a sea-locli once arrested by a flat-topped hill standing in the middle 

 may be seen to present slopes both of gneiss and sand- of the valley at its upper end, and suggesting some kind 

 stone ; the gneiss is always worn into smooth domes, | of fortification ; so different from the surrounding hum- 

 ridges, and hollows ; but the sandstone retains its parallel I mocky declivities of gneiss is its level grassy top, flanked 

 bands of rocky terrace. The difference is evidently not 1 by wall-like cliffs rising upon a glacis-slope of debris and 

 due to any greater glacial abrasion of the gneiss. The herbage (Fig. 3). Closer examination shows that the little 

 area of high ground above the gneiss platform in Suther- eminence is capped with a coarse reddish breccia made 



