Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/462

 Temperature of hottest month at sea level.

Height of 32° F. in hottest month.

Height of snow-line.

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 Chilian 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 temperature and on amount of snow-fall, to the exclusion of winter temperature, may be best shown, perhaps, by two extreme cases. The mean temperature of the Altai mountains (according to Mr. Hopkins'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 Geography,' 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