THE SIERRA DE AHUALCO. 
Popocatepetl and Ixtaccihuatl are the highest peaks of a moun- 
tain range or Sierra about 60 miles in length, and 18 in breadth, 
which extends in a general north and south direction from Jon- 
acatepec in the State of Morelos to Otumba in the State of 
Hidalgo. This Sierra is called the Sierra Nevada or snowy Sierra 
of Ahualco. Its extent in latitude is from 18° 55’ to 19° 47’ N. Lat. 
The Sierra is, therefore, well within the tropics. Its longitude is 
from 98° 53’ 15” to 98° 55’ W. from Greenwich. It is 35 to 50 miles 
east of the City of Mexico. Other important peaks of the ridge, all 
lying to the north, are Tlaloc, Telapon and Papayo. Rising out of the 
plains of Apam to the north, the Sierra gradually increases in height 
until it culminates in the cone of Popocatepetl at its southern 
extremity. Its average height above the plateau on which it stands 
is 5,000 feet and above the level of the sea 13,000 feet. From the 
culminating point at the south the Sierra descends sharply to the 
plateau, which itself then falls off rapidly—in many places abruptly— 
several thousand feet. 
The Sierra constitutes a barrier which Sonaraiee the valley of 
Puebla on the east, from the valley of Mexico on the west; the 
shortest line of communication between the two valleys is a saddle- 
shaped pass between Popocatepetl and Ixtaccihuatl, whose lowest 
point has a height of 12,118 feet. 
The Sierra also determines the flow of the waters into these 
valleys. Thus the waters descending the western slopes of the ridge 
“flow into Lakes Texcoco and Chalco, of the valley of Mexico, and 
- those of the eastern ‘slopes into the plains of Apam and the river 
Atoyac, which skirts the southern end of the ridge, and empties into 
the Pacific Ocean.. 
While the ridge constitutes a topographic unit, viewed as a 
volcanic series it is made up, in the opinion of Felix and Lenk,* 
of two elements; the first, Popocatepetl, is situated on the line 
of a great fissure running east and west, with which are connected 
the volcanoes Ajusco, Toluca, Patzcuaro and others; the second, 
Ixtaccihuatl and the peaks of Telapon and Tlamacas, stands 
*Beitrage zur Geologie und Paleontologie der Republik Mexico, I Theil, p. 6. 
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