330 COSMOS. 



even in the region of the tropics, that this limit attains its 

 greatest elevation above the level of" the sea. The phenome- 

 non ot" which we are treating is extremely complicated, de- 

 pending on the general relations of temperature and humidity, 

 and on the form of mountains. On submitting these relations 

 to the test of special analysis, as we may be permitted to do 

 from the number of determinations that have recently been 

 made,* we shall find that the controlling causes are the dif- 

 ferences in the temperature of different seasons of the year ; 

 the direction of the prevailing winds and their relations to the 

 land and sea ; the degree of dryness or humidity in the upper 

 strata of the air ; the absolute thickness of the accumulated 

 masses of fallen snow ; the relation of the snow-line to the to- 

 tal height of the mountain ; the relative position of the latter 

 in the chain to which it belongs, and the steepness of its de- 

 clivity ; the vicinity of other summits likewise perpetually 

 covered with snow ; the expansion, position, and elevation of 

 the plains from which the snow-mountain rises as an isolated 

 peak or as a portion of a chain ; whether this plain be part 

 of the sea-coast or of the interior of a continent ; whether it 

 be covered with wood or waving grass ; and whether, finally, it 

 consist of a dry and rocky soil, or of a wet and marshy bottom. 

 The snow-line which, under the equator in South Ameri- 

 ca, attains an elevation equal to that of the summit of Mont 

 Blanc in the Alps, and descends, according to recent measure- 

 ments, about 1023 feet lower toward the northern -tropic in 

 the elevated plateaux of Mexico (in 19^ north latitude), rises, 

 according to Pentland, in the southern tropical zone (14^ 30' 

 to 18"^ south latitude), being more than 2665 feet higher in 

 the maritime and western branch of the Cordilleras of Chili 

 than under the equator near Quito on Chimborazo, Cotopaxi, 

 and Antisana. Dr. Gillies even asserts that much further to 

 the south, on the declivity of the volcano of Peuquenes (lati- 

 tude 33*^), he found the snow-line at an elevation of between 

 14,520 and 15,030 feet. The evaporation of the snow in the 

 extremely dry air of the summer, and under a cloudless sky, 

 is so powerful, that the volcano of Aconcagua, northeast of 

 Valparaiso (latitude 32*^ 30'), which was found in the expe- 

 dition of the Beagle to be more than 1400 feet higher than 

 Chimborazo, was on one occasion seen free from snow.t In 



* See my table of the height of the line of perpetual snow, in both 

 hemispheres, from 71° 15' north lat. to 53° 54' south lat., in my Asii 

 Centrale, t. iii., p. 360. 



t Darwin, Journal of the Voyages of the Adventure and Beagle, p. 297 . 



