1898.] 



NATURAL SCIENCES OF PHILADELPHIA. 



375 



pingo and the small Sierra de Patlachique — to the east as an outlier 

 in the Valley of Puebla, Monte de Rio Frio. These are all of vol- 

 canic origin. About the north foot of Cerro Tlamacas spread the 

 the fruitful plains of Otumba and Apam. The railroad to Vera 

 Cruz crosses here. 



The enclosure of the Valley of Mexico is completed to the west by 

 the Sierra de las Cruces, continued northward by the spurs called 

 Monte alto and Monte bajo, and ending finall}- in the Sierra de Tepot- 

 zatlan and Cerro de Sincoque, separated from the northern range of 

 hills by the railroad cut and drainage ditch, Tajo de Nochistongo. 

 The floor of the valley is generally level and uniform with six large 

 lakes filling the more depressed portions. Their size and elevation 

 in metres and square kilometers is given in the subjoined table : 



The relative elevation, minus or plus, has been referred to the 

 base of one of the corners of the National Palace on the Plaza Mayor, 

 as the zero level. During the diluvial period of geologic time the 

 lakes were very much more extended than now. The whole Valley 

 of Mexico was filled by a large inland sea with here and there a vol- 

 canic hill rising, as an island, or as a peninsula, out of its surface. 

 Texcoco was in the past quite saline. Fernando Cortez in a letter to 

 Charles V, dated 1500, says : " En el dicho llano (del Valle de Mex- 

 ico) hay dos lagunas, que casi lo ocupan todo. E la una de estas 

 lagunas es de aqua dulce, y la otra, que es mayor, es de aqua salada." 

 The earth of the plains surrounded Texcoco Lake is impregnated 

 with salt, and in many places the saline material forms a rich efflo- 

 rescence. The flora of this region of the valley has a marked char- 

 acter. Various species of Chenopodium, Atriplex, Salsola and Grati- 



