8 , MK. T. F. JAMIESON SUPPLEMENTARY 



. ensue during a Glacial period when that fall took the shape of 

 snow. Por, with regard to snow, there is an important circumstance 

 to be kept in view, the full result of which is apt to be lost sight of. 

 Kain runs off as it falls, and there is no storage from year to year ; 

 with snow the case is very different. Take two adjoining districts 

 where the snowfall is as two to one. Say that in the course of a 

 year twenty measures of snow fall on A and only ten on B, and that 

 the loss by melting and evaporation amounts to eight measures in 

 each of them. The result is that on B two measures of snow 

 remain for conversion into ice, and on A twelve ; so that although 

 the fall is only as 2 to 1 the growth of ice will be as 6 to 1. Let 

 this process go on, not for one year but for a thousand or many 

 thousand years, and it is easy to see how the ice will grow upon 

 the one and gain upon the other. Put A for the West Highlands 

 and B for the East, and this will help to show how it came to pass 

 that there was such an excess of ice on the former. It was the 

 cumulative effect of this storage of the surplus continuing through- 

 out so many ages that gave rise to such strange results, not only 

 in Scotland, but also in Ireland, Scandinavia, and North America. 

 The mass of ice lying over Argyllshire and the West Highlands 

 being immensely thick must of necessity have lingered on for a 

 very long time after the thinner ice to the north-east had melted 

 away, and as this region of thick ice lay right across the mouth 

 of the glens where the Lochaber lakes were situated, it explains 

 in a consistent and reasonable manner that very remarkable circum- 

 stance which has appeared so inexplicable to Prestwich, Milne- 

 Home, and many other observers. 



II. Level oe the Si^ow-line. 



Prof. Prestwich, however, finds a serious objection to this view 

 of the matter on account of " the excessive inequality in the level 

 of the snow-line in closely adjacent districts which it would 

 necessitate." ^ 



But the level of the snow-line is a different question altogether, 

 and is regulated chiefly by the summer heat. An excessive quantity 

 of snow would, I imagine, discharge itself below the level of the 

 line as a glacier. Where there was much snow there would be a 

 big glacier ; where there was less there would be a smaller one, 

 and where there was little snow there would be none at all ; yet 

 the general level of the snow-line might be comparatively regular 

 all along. Permanent snow when it accumulates thickly passes 

 into the form of ice, and this glacier-ice does not conform to the 

 level of the snow-line, but may descend far below it. In West 

 Greenland, where the glaciers come down into Baffin's Bay, the 

 level of the snow-line is more than 2,000 or even 2,500 feet above 

 the sea ; but we must not exaggerate its regularity, for Mr. Hol- 

 land " tells us that patches of snow occur here and there far below 



1 Op. cit. p. 674. 

 ^ Quart. Journ, Geol. Soc. vol. xxxiii, (1877) p. 153. 



