80 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DeC. 1/, 



latitude, an insular position will have a lower position of the snow- 

 line than a continental one. In comparing a place near the equator 

 with one in the higher latitudes, there will be much greater humidity 

 in the atmosphere and much less variation in the annual temperature 

 in the former case than in the latter, and both these causes tend to 

 produce a much lower position of the snow-line, with reference to the 

 line of 32°, in the former case than in the latter. These general 

 causes, independently of the action of mere partial causes, are suffi- 

 cient to explain the general results of observation above given. 



23. After the preceding considerations respecting the relative posi- 

 tions of the line of 32° F. and the snow-line, I proceed to examine 

 the distances, measured vertically, to which the principal known 

 glaciers descend below this latter line, which forms the limit of a 

 glacier's superficial increase. Such distance must depend on the 

 depth of the glacier, the rate of its motion, and the activity of the 

 destructive agencies to which it is exposed. It is of course widely 

 different for glaciers of different magnitudes, but for those which are 

 sufficiently large to be considered of the first order it varies much 

 less than might perhaps be expected, as appears from the following 

 table, p. 81. The glaciers specified are all of the first order in mag- 

 nitude, except, perhaps, that of the Maladetta. 



It will be observed that the descent of the Aar glacier below the 

 snow-line is considerably less than that of any other equally large 

 glacier enumerated in the opposite table. This I conceive to be due 

 to the very small inclination of the bed of that glacier towards its 

 lower extremity. If we reject this example as anomalous, the mean 

 descent of the remaining nine great glaciers below the snow-line is 

 about 4500 feet. 



There are also three glaciers in the Himalayas, from the extremi- 

 ties of which the Pindur, the Gori, and the Ganges issue. The 

 heights of these sources have been incidentally given by observers *, 

 as being respectively 11,946, 11,543, and 13,500 feet. Taking 

 Capt. Strachey's estimate of the height of the snow-line as 16,000 

 feet in that region, we have 4054, 4457, and 2500 feet for the 

 descents of these glaciers below the snow-line. The two former are 

 in very near accordance with the mean of the cases above given ; the 

 last more nearly accords with the smaller descents of the glacier of 

 the Aar, and is probably due to a similar cause. 



The glaciers which proceed from more limited spaces for the accu- 

 mulation of snow and ice descend below the snow-line to distances 

 much shorter than those above given. 



(2.) Height of the Snow-line, and Descent of Glaciers below it in 

 the hypothetical cases o/" § I. 



The degree of cold requisite to produce glaciers on a mountain 

 might obviously be caused by the elevation of the mountain. I shall, 



* See Captain Strachey's Paper " On the Snow-hne in the Himalaya," Journ. 

 Asiatic Soc. Bengal, April 1849; and Edinb. New Phil. Journ. vol. xlvii. 1849; 

 and *' On the Physical Geography of the Provinces of Kumaon and Garhwal," 

 Journ. R. Geograph. Soc. 1851. 



