CLOSED LACUSTEINE BASINS. 45 



of the neighbouring Rio Bravo del Norte. Other marshy tracts, like the lagoons 

 of Santa Maria and dos Patos, have a similar origin, and the bed of the Bolson de 

 Mapimi is also occupied by a closed reservoir, the Tlahualila lagoon. 



Farther south the Rio de Nazas, which is a somewhat cojnous stream in the 

 upper valleys of the Sierra Candela, is arrested in the Laguna del Muerto, while 

 the Rio d'Aguanaval does not always reach the Laguna de Parras. In various 

 parts of these desert spaces occur numerous ojos or " eyes," that is, springs, some 

 thermal, some cold, but nearly all richly charged with chemical substances. 

 Several have gradually raised circular margins of siliceous or calcareous deposits 

 round their orifice, and in some places these accumulations are high enough to 

 form veritable hillocks. Froebel saw a streamlet flowing from a knoll about thirty 

 feet high, which had been built up in this way by the water itself. 



In the State of San Luis, where the plateau is already divided by the mountain 

 ranges into numerous small basins, there are no extensive lagoons like those of the 

 northern provinces ; but this district contains over one hundred small hikes or 

 rather ponds, nearly all of which have become saline. The plains are largely 

 covered with various kinds of efflorescences, some composed of saltpetre, others 

 consisting for the most part of carbonate of soda. They still retain their old Aztec 

 name of tequesquite in Mexico, where the smelters use them in treating the various 

 silver and argentiferous lead ores. 



Closed lacustrine basins are also found in the valleys of the border range south 

 of the plateau. Such is the Patzcuaro or "Greater Lake," in the State of Mexico, 

 an island-studded depression encircled on all sides by mountains, and containing a 

 slightly brackish, but still potable water. Such is also the Cuitzeo, a deep reser- 

 voir which is filled by the river Morelia, whose extremely salt water sterilises all 

 the surrounding lands during the inundations. 



But of all these flooded depressions the most remarkable are those from which 

 the Mexican plateau takes its name of Anahuac, that is, Anal-huatl, "Amid the 

 Waters," a term afterwards extended to all the upland plains of this region. 

 These lakes, or rather shallow ponds, are disposed in a chain running north and 

 south for a distance of about 46 miles ; but their superficial area varies from year 

 to year and from season to season, so that they present different contour lines on 

 maps constructed at different periods. 



The southern lakes Xochimilco and Chalco really form only a single sheet of 

 water divided into two basins by a narrow dyke. Thanks to the copious streams 

 descending from the neighbouring hills this depression has maintained its old 

 outlines with little change. A canal, rvmning northwards to the city of Mexico, 

 discharges the overflow into Lake Texcoco, which occupies the bed of a periodically 

 flooded basin from five to seven feet below the level of the capital. The northern 

 Lakes San Cristobal, Xaltocan, and Zumpango stand like Xochimilco and Chalco 

 above that level. Hence during the inundations, when the rivulets converge 

 from the plain of Pachuca, descending from basin to basin towards the south, the 

 city would be threatened with total destruction were the embankments to burst 

 which have been constructed below each reservoir. 



