0E THE OUTER HEBRIDES . 861 



It has been shown that the whole of the Long Island from the Butt 

 of Lewis to Barra Head has been overflowed from the Minch by ice 

 that moved outwards from the Inner Islands and the mainland, and 

 the actual thickness of this ice-sheet it is now possible to measure 

 with some approach to exactness. We have seen that the only- 

 points in the Outer Hebrides which have escaped glaciation are 

 those that exceed a height of 1600 feet. Taking this, therefore, as 

 the thickness of the ice that overflowed the lowest ground of the 

 Long Island, and 3000 feet as the probable upper limits of the ice- 

 sbeet in "Western Boss and Sutherland, we readily arrive at the 

 depth of the ice-sheet that filled up the Minch. Immediately off 

 the east coast of Harris the thickness of the ice would be upwards of 

 2300 feet ; for the sea has a depth there of 121 fathoms. In the 

 centre of the North Minch the upper surface of the mer de glace 

 would be 2300 feet above what is now the sea-level, the actual 

 thickness of the ice being 2700 feet. Nearer the shores of the 

 mainland, in the Inner Sound, the depth attained by the ice would 

 be still greater, no less than 3700 feet. Measuring from the 

 Cliseam in North Harris to the mountains of Torridon, we have a 

 distance of 56 miles, so that the inclination of the surface of the 

 mer de glace was very little, the fall not being more than 1400 feet, 

 or about 1 in 211. But slight as that incline was, it was probably 

 twice as great as the slope of the mer de glace that filled up the 

 German Ocean. Of course no one can believe that such enormous 

 masses of ice could ever have been nourished by the snowfields of 

 the mainland alone. The thickness attained clearly points to ex- 

 cessive precipitation over the whole area covered by the mer de glace ; 

 and we may be allowed to suppose that the ice was probably of the 

 same peculiar laminated structure as that underneath which the 

 Antarctic lands lie buried. 



If the surface of the mer de glace continued to fall at the same 

 rate, we should find marks of glaciation in St. Kilda up to a height 

 of about 200 feet; whether this is actually the case I hope to 

 ascertain at an early opportunity. But whether it be so or not, it is 

 clear that the facts brought forward in this and the preceding paper 

 completely negative the notion of a great Atlantic glacier, which it 

 has been supposed flowed in upon Scotland. 



It may have occurred to some as a difficulty that the moraine 

 jprofonde, or till, of the ice-sheet contains so very few stones that do 

 not occur in situ in the Outer Hebrides. If the ice actually flowed 

 from the mainland and Skye across the Minch and over the Long 

 Island, why should not many recognizable Skye rocks occur in the 

 Hebridean till? The explanation appears to be as follows: — We 

 must remember that although the upper surface of the ice-sheet 

 flowed steadily towards the north-west, yet the bottom portion in 

 the Minch would move in quite a different direction. The lower 

 strata that impinged upon the foot-slopes of the Long Island, which 

 are now below water, would be deflected to right and left, and two 

 under-currents would set along the bottom of the Minch, one 

 flowing north-east and the other south-west. These currents being 



