SxNOW-LINE — TEMPERATURE. 299 



height of 15,748 feet, and in the 46th degree of latitude CHAP. XXIV 



at that of 8366 feet. In Mexico, in the 19th and 20th ^, T" <■ 



Elevation of 



degrees, perpetual frost commences, accordmg to Hum- perpetual 



boldt's measurements, at 16,002 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 tlie Volcan de 



Colima 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 Temperatiire 

 ., ,. ' , ^ . ^ . ., \ ' ot the table 



soil, climate, and vegetation, present a similar character land. 



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 cordillera of Mexico, and 

 the vast extent of its plains, produce 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, K^i^^*- 

 July, August, and September, very seldom extend to the 

 interior. The mountains, being composed of porous 

 amygdaloid and fissured porphyries, present few springs ; 

 the filtrated water losing itself in the crevices opened by 

 ancient volcanic eruptions, and issuing at the bottom of 

 the Cordilleras. 



The aridity of the central plain, on which there is a Dcstmction 

 gi'eat deficiency of wood, is prejudicial to the working 

 of the mines ; and this natural evil has been augmented 

 since the arrival of Europeans, who have not only 

 destroyed the trees without planting others, but have 

 drained a large extent of ground, and thus increased the 

 saline efflorescences which cover the surface and are 

 hostile to cultivation. This dryness, however, is con- 

 fined to the more elevated plains ; and the declivities of 

 the cordillera being exposed to humid winds and fogs, 

 their vegetation is uncommonly vigorous. 



