THE PHYSIOGRAPHY OF MEXICO 87 



without. The names used herewith are applied to the peaks with 

 craters. Their accompanying craterless cones are known respec- 

 tively as Nevado, Ixtaccihuatl, and Cofre de Perote. From a 

 distance, Nevado appears to have a crater, but this is in reality but 

 a furrow, produced jointly by the sinking of the lava crust and later 

 erosion. The few geologists that have studied the phenomena 

 have apparently avoided giving an explanation of the genesis of the 

 craterless peak, and the single reference available does no more than 

 cite the analogy between these and a type described by Stiibel in 

 Ecuador. 1 



Numerous basins, formed in constructional depressions in the 

 volcanic surface, are scattered over the province. These abound in 

 the environs of the city of Mexico. The city itself is situated in the 

 so-called Valle de Mexico, which is an inclosed basin 50 miles long 

 by 35 miles wide. This basin contains six shallow lakes and a con- 

 siderable area of marshes. This marsh area is all the more unique 

 and interesting when it is considered that it lies at an altitude a 

 mile and a half above sea-level. The drainage of this basin, which 

 is of the first order of necessity for sanitary living during the rainy 

 season, was an engineering problem which defied successful solution 

 for three centuries. Finally, in 1900, there was consummated a 

 successful plan involving a seven-mile tunnel under the Xalpam 

 Mountain, and an aqueduct over the Guadalupe River, by which 

 the sewage and storm water of the city of Mexico is drained from the 

 basin and emptied into a stream which carries it to the Gulf of 

 Mexico. 2 Many of the basins are filled with water, forming lakes 

 of considerable size, namely, Chapala, Quitzeo, and Patzcuaro, all 

 in the state of Michoacan. 



Among other interesting topographic features are the cinder 

 cones and their crater lakes of the Valle de Santiago, and the pit 

 craters of the state of Pueblo. The former are low cones of ash and 

 breccia, eleven in number, the craters of which vary in diameter 

 from 1,500 feet to one mile. The bottoms of some of these craters 

 contain shallow lakes of clear, fresh water (xalalpascos) . This 



1 P. Waitz, Guides des excursions, 10th Inter. Geol. Cong. Alex., igo6, No. 13, 

 pp. 10-21. 



2 S. F. Emmons, Science, N.S., XVII, 309. 



