Ixii PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



been observed. The principle on which I have proceeded is easily 

 explained. The snow-line is that line on the side of a mountain 

 which forms the highest limit to which the boundary of the snow 

 ascends during the year. It bears an important relation to all gla- 

 ciers, being that limit below which the glacier receives no permanent 

 superficial increase. Below this limit the destructive begin to pre- 

 vail over the productive agencies. The distance to which the glacier 

 descends below it depends on local circumstances, but we find that 

 nearly all glaciers, large enough to be considered of the first order, 

 descend to levels lower than the snow-line by an amount varying 

 from about 4000 to 5000 feet. In smaller glaciers the descent is 

 proportionally less. Again, the snow-line bears certain relations ta 

 the line which I have defined as the line of 32° F., or that along 

 which the mean annual temperature is equal to 32° F. At certain 

 places in sufficiently high latitudes this line will lie at the level of 

 the sea. In lower latitudes it will lie at higher levels, and in still 

 higher latitudes the mean annual temperature will be less than 32°. 

 It is found that at the equator the snow-line lies about 1000 feet 

 below the line of 32°, while in the higher north latitudes it lies above 

 the latter line, the vertical distance between them being very variable. 

 A continental climate, in which the atmosphere contains compara- 

 tively little moisture, and the variation from summer to winter tem- 

 perature is very great, is favourable to a relatively high position of 

 the snow-line ; while an insular climate, in which the quantity of 

 moisture is comparatively large, and the annual variation of tempe- 

 rature comparatively small, superinduces a relatively low position of 

 this line. Thus in the north-eastern part of Asia, the snow-line is 

 probably from 4000 to 6000 feet above the line of 32°, while in Ice- 

 land its height above the latter line does not exceed a few hundred 

 feet. A knowledge of these facts enables us to estimate approximately 

 the vertical distance between these lines in any proposed hjrpothetical 

 case. To estimate the absolute height of the snow-line above the 

 sea-level, we have only then to calculate the height of the line of 32° 

 at the place proposed. For this purpose we must estimate the mean 

 annual temperature there by means of the isothermal line passing 

 through the place, and then calculate the vertical height to which 

 we must ascend to reach the point at which the mean annual tempe- 

 rature is equal to 32° ; and to effect this we must know the height 

 which corresponds to a decrease of temperature of 1°. Humboldt 

 and others have shown that this height may be taken as varying from 

 about 320 to 350 feet in ascending steep mountains, or vertically in a 

 balloon ; but Humboldt has also shown, from his own observations, 

 that, in an ascent presenting a succession of high and extensive 

 table lands, the increase of height for each degree may amount to 

 450 or 500 feet. This is an important distinction. 



In this manner, then, the height of the snow-line above the sea-level 

 can be estimated at any proposed place, with any hypothetical distri- 

 bution of land and sea. If a mountain rise a few hundred feet at least 

 above the snow-line, and the configuration of its summit be favourable,, 

 glaciers will be formed upon it, of which the magnitude will depend 



