SNOW-LINE TEMPERATURE. 303 



which comprehend a fourth part of its surface ; that 

 of Patzcuaro in Valladolid ; and, finally, the lakes 

 of Mexitlan and Parras in New-Biscay. 



The interior of New-Spain, and especially a great 

 part of the elevated table-land of Anahuac, is arid 

 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 Volcan de 

 Colmia remain uncovered during the greater part of 

 the year. None of the other mountains 4se 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 Cor- 

 dillera of Mexico, and the vast extent of its plains, 

 produce a reverberation of the sun's rays never ob- 

 served in elevated countries of greater inequality. 



To the north of 20° the rains, which fall only in 

 June, July, August, and September, very seldom ex- 

 tend to the interior. The mountains, being composed 

 of porous amygdaloid and fissured porphyries, pre- 

 sent 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 



