350 SNOW-LINE—TEMPERATURE. 
and destitute of vegetation ; which arises from the 
rapid evaporation in high plains, and the circum- 
stance that few of the mountains enter the region 
of perpetual snow, which under*the equator com- 
mences at the height of 15,748 feet, and in the 45th 
degree of latitude at that of 8366 feet. In Mexico, 
in the 19th and 20th degrees, perpetual frost com- 
mences, according to Humboldt’s measurements, at 
15,092 feet of elevation ; so that of the six colossal 
summits, which are placed in the same line in the 
19th parallel of latitude, only four, namely, the 
Peak of Orizaba, Popocatepetl, Iztaccihuatl, and 
Nevado de Tolucca, are clothed with perennial 
snow; while the Cofre de Perote and the Volean 
de Colmia remain uncovered during the greater part 
of the year. None of the other mountains rise into 
so lofty a region. 
In general, in the equinoctial part of New Spain, 
the soil, climate, and vegetation, present a similar 
character to those of the temperate zone. Although 
the table-lands are singularly cold in winter, the 
temperature is much higher in summer than in the 
Andes of Peru, because the great mass of the cordil- 
lera of Mexico, and the vast extent of its plains, pro- 
duce a reverberation of the sun’s rays never observed 
in elevated countries of greater inequality. 
To the north of 20° the rains, which fall only in 
June, July, August, and September, very seldom 
extend to the interior. The mountains, being com- 
posed of porous amygdaloid and fissured porphyries, 
present few springs ; the filtrated water losing itself 
in the crevices opened by ancient volcanic erup- 
tions, and issuing at the bottom of the cordilleras. 
The aridity of the central plain, on which there 
