352 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [JunC 9, 



Temperature of Height of 32° rr • uf f 



hottest month F. in hottest ° -, • 



4. 1 T .1 snow-une. 



at sea level. month. 



Pyrenees 74-5 12750 9300 



Caucasus 77 13500 10300 



Mont Blanc 72-5 12150 9000 



Bernese Alps 72*5 12150 8800 



Scandinavian Fjelde 61°43' N 59 8100 5500 



Mageroe, Norway, extreme north 45*5 4050 2160 



Himalaya, about 31° N., north side 83-75 15525 16620 



The same, „ south side 83-75 15525 12980 



Andes, near Quito 79-25 14175 15795 



do. 18° N 81-5 14850 14772 



do. near Valparaiso 68 10800 12780 



do. 37° 40' S 63-5 9450 7960 



Straits of Magellan 45-5 4050 3390 



It is evident by this table that the snow-line rises above the line 

 of 32° for the hottest month of the year where the snow-fall is small, 

 and sinks below it where the snow-fall is great. In the Caucasus, 

 the Alps, and the Pyrenees, the snow-line is about three-fourths of 

 the height of the line of 32° for the hottest month of the year ; in 

 the Fjelde of Norway, about two- thirds ; in the Peruvian and 

 Chihan Andes above, but in Patagonia and Tierra del Fuego below ; 

 above, on the north side of the Himalaya, but below on the south 

 side. These contrasts are all to be explained by the difference 

 in the amount of snow-fall, which is greater on the south than on 

 the north side of the Himalaya, greater in Patagonia and Tierra del 

 Fuego than in Chile and Peru, and probably greater, at least in 

 ■winter, in Norway than in Central, Southern, or Eastern Europe. 



The dependence of the height of the snow-line on summer tempe- 

 rature and on amount of snow-fall, to the exclusion of winter tem- 

 perature, may be best shown, perhaps, by two extreme cases. The 

 mean temperature of the Altai mountains (according to Mr. Hop- 

 kins's paper cited above) is below freezing ; yet, in consequence 

 of the comparatively warm summer, and the small snow-fall, 

 the height of the snow-line (Mrs. Somerville's ' Physical Geogra- 

 phy,' p. 61) is about 6000 feet. On the Straits of Magellan, on the 

 contrary, though the mean temperature is several degrees above 

 freezing, the height of the snow-line (see table) is little more than 

 half as much. 



It is well known that, other things being equal, the magnitude of 

 glaciers depends on that of the snow-fields in which they rise ; and 

 as of course any depression of the snow-line will enlarge the snow— 

 field, it foUows that the lower the snow-line the further will the 

 glaciers descend below- it. As a decrease of about 3° F. is due to 

 every 1000 feet of ascent in the hottest month, it follows that a fall 

 of temperature to that extent in the hottest month would lower the 

 snow-line by about 1000 feet ; and in many cases it is likely that the 

 glaciers in such a case would descend 1000 feet further below the 

 snow-line than at present, thus gaining a total increase of 2000 feet 



