538 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 25, 



behind them, but are distinctly ice-worn in the same direction as 

 the whole of the northern moory districts — that is, from south-east 

 to north-west. Nay some of the higher mountains themselves seem 

 to be merely gigantic roches moutonnees. Such, at all events, appears 

 to be the case withSuainabhal, whpsebare rocky head and sides strongly 

 suggest that the whole mountain from base to summit has been 

 smothered in ice that moved outwards to the Atlantic ; but a 

 particular account of the mountain tracts of Lewis and Harris is 

 reserved for a future paper. I shall only remark at present that all 

 the principal valleys in the western districts of the Lewisian mountain 

 region trend north and north-west, only a few streamlets finding 

 their way down the southern slopes' of the hills into Loch Resort, 

 while no valley of any consequence opens out upon the low grounds 

 that stretch away to the north-east. In the southern or, rather, 

 south-eastern mountain districts, the valleys radiate to various points 

 of the compass, and one of considerable size, occupied by Loch 

 Langabhat, expands into the low grounds. Notwithstanding this, 

 however, the direction of the glaciation over these low grounds is in 

 no wise affected. The roches moutonnees still sweep persistently along 

 the base of the mountains from south-east to north-west. 



It thus becomes evident that the ice which passed from sea to sea 

 across the whole breadth of the low grounds of Lewis did not come 

 from the southern mountain tracts. And it is no less evident that 

 this ice was of sufficient thickness to keep on its course towards 

 north-west undisturbed by the pressure of the glacier masses, which 

 must at the same time have filled to overflowing the deep glens and 

 valleys of the mountainous region referred to. 



Some further account of the glaciation falls to be given when I 

 come to consider the origin of the freshwater lakes. 



V. Bottom Till of the low grounds. 



Throughout all the low grounds of Lewis we find a deposit of till 

 lying in the valleys, nestling in hollows between roches moutonnees 

 and stretching more or less continuously for long distances over 

 gently undulating flats. The materials of which this deposit con- 

 sists appear to be derived almost exclusively from the degradation 

 of gneissic rocks. The paste, or matrix, is generally a dark or light 

 reddish brown hard clay, frequently rendered coarse and gritty by 

 the quantity of gneissic matter diffused through it. Indeed to such 

 an extent is this the case that in some places the matrix partakes 

 more of the nature of an earthy grit than of clay. It exhibits an 

 abundance of blunted angular and subangular fragments of gneiss, 

 varying in size from mere grit up to boulders several feet in 

 diameter ; but boulders of a large size do not occur very frequently. 

 The largest one I saw measured only 5| feet across, and was esti- 

 mated to contain about 70 or 80 cubic feet. It would be difficult 

 to give an average size for the stones. They seemed not to differ in 

 dimensions from the boulders in the till of the Scottish Lowlands — 

 stones measuring from 2 or 4 to 6 or 8 inches across being per- 



