544 proceedings op the geological sociEiY. [June 25, 



glacier-ice. The ice with which the mountain -valleys of Harris and 

 the south were filled had no share whatever in the glaciation of the 

 northern part of the island, extending from the hase of the moun- 

 tains to the Butt, a distance of not less than 35 or 40 miles. Where, 

 then, did the ice come from which overflowed this by far the largest 

 portion of the island ? There is only one place whence it could have 

 come — the mainland. Standing on the watershed of the island and 

 looking away towards the south-east (the direction from which the 

 ice moved), the observer sees on the edge of the horizon the moun- 

 tains of "Wester Ross fringing the eastern borders of the Minch. It 

 was amongst the wild glens of that distant region that the glaciers 

 which overflowed Lewis were nourished. Loch Broom, Loch Greinord, 

 Loch Ewe, Loch Gairloch, and Loch Torridon have each, as their 

 severely glaciated mountain-slopes attest, brimmed with ice that 

 flowed outwards to the Minch. So likewise the Outer and Inner 

 Sounds of Baasay formed the channels of gigantic glaciers. The 

 Island of Baasay, as my brother has shown, is glaciated from end to 

 end. So is the Island of Bona ; and the land-ice which did the 

 work must have attained a prodigious thickness. The depth of the 

 Inner Sound, between Baasay and the mainland, is not less than 

 120 fathoms, while off the coast of Bona it is as much as 138 

 fathoms. Now the height to which one may trace a moutonnee 

 surface in Baasay is not less than 1300 or 1400 feet, which gives a 

 depth of ice in the Inner Sound of upwards of 2000 feet ; I believe, 

 however, it was more than even that. The extreme height reached by 

 the ice along this north-west part of Scotland has not yet been deter- 

 mined. The upper portions of the mountains, formed of Cambrian 

 sandstones, have suffered much under the action of the weather ; 

 and while the tougher gneiss below still shows a finely mammillated 

 surface, the overlying horizontal beds of sandstone, riven and shat- 

 tered by the frost, have failed in great measure to preserve any dis- 

 tinct traces of glaciation. A detailed examination of that most 

 interesting region would, I have little doubt, enable us to carry 

 the surface of the old mer de glace up to a height of at least 3000 

 feet above the present sea-level : certainly on the mainland (as, 

 for example, on the mountains overlooking Loch Torridon) roches 

 moutonnees reach a considerably greater elevation than even the 

 highest point in the Island of Baasay. 



But let us take only the thickness I have given for the ice in the 

 Sound of Baasay, as approximately that of the mer de glace that 

 flowed into the Minch. The average depth of the Minch not being 

 more than between 50 and 60 fathoms, it is evident that no part of 

 our ancient oner de glace could have floated, but the whole mass 

 must have pressed on over the bottom of the sea, just as if that had 

 been a land surface. That the ice did not flow northwards up 

 the Minch shows that' an equally massive ice-sheet was at the 

 same time streaming out from the deep fiord-valleys of Sutherland. 

 The only course it could possibly take was precisely that which we 

 know it did take — namely, across the broad low grounds of Lewis. 

 Moreover the height to which Lewis has been glaciated leads irre- 



