THE PHYSIOGRAPHY OF MEXICO 85 



The northern, western, and eastern boundaries have already 

 been described in connection with other provinces. The southern 

 boundary line extends in a general way from Cape Corrientes 

 along the courses of the Rio Armeria, the Rio Tepalcatepec, the 

 Rio Balsas, the Rio Mexicala, and the Rio Atoyac, all of which 

 skirt the southern edge of the recent volcanic flows, to the vicinity 

 of Esperanza, and then around the volcano of Orizaba and the 

 Cofre de Perote to Jalapa. The change in topography when one 

 crosses the boundaries of this province are sharp and definite, and 

 if not always comprehended in a single view, at least become 

 apparent within a few miles. 



Topography. — Viewed from any commanding eminence, this 

 province presents a wildly rugged topography — "a variegated sea 

 of rolling, timber-covered, constructional, volcanic hills, mesas, 

 peaks, and monticules, alternating with basin valleys and plains." 1 

 The great altitude of the major portion of the province (8,000 feet 

 and upward) is not due alone to the higher altitude of the plateau 

 itself, although it is 5,000 feet above El Paso, but to the vast 

 quantities of volcanic material which have been ejected from numer- 

 ous vents and piled upon it. 



This volcanic material consists not only of lava, but also of tuffs 

 and pumice, in some places in situ and in others as old lacustrine 

 deposits. The material from which these lacustrine deposits were 

 made was derived by erosion from the surrounding volcanic heights 

 and carried into the lakes, raising their waters until they over- 

 flowed into neighboring streams, and finally filling the lakes com- 

 pletely. As many of these Pliocene and Pleistocene lake beds were 

 of considerable areal extent, their deposits have effaced much of the 

 pre-existing volcanic topography. 2 



It is probable that the plain upon which the higher peaks rest 

 was formed simultaneously with them. According to Ordonez the 

 more fluid portions of the magma were ejected from the vents by 

 violent explosions and raised into such prominent andesitic cones as 

 Popocatapetl and Ixtaccihuatl, while the denser basalts flowed from 

 connecting fissures to form the "badland" plains surrounding them. 3 



1 Hill, Eng. and Mm. Jour., LXXIX, 410. 



2 Ordonez, Guides des excursions, 10th Inter. Geol. Cong. Mex., iqo6, No. 8 and plate. 



3 Ordonez, ibid., No. 1, p. 4. 



