46 MEXICO, CENTRAL AMERICA, WEST INDIES. 



From the descriptions handed down by the Spanish conquerors, and the 

 comparative observations made at different epochs, it is evident that the extent 

 and vohxme of these Mexican lakes have continued to diminish during the last 

 three hundred and fifty years. The capital was formerly represented as a 

 " lacustrine city " surrounded by flooded plains, whereas at present it stands on 

 dry land, the lakes no longer occupying even a third of the " valley." They 

 have also become shallower, and the bed of the Texcoco basin is steadily silting 

 up with the sands of the plains moving forward under the action of the winds. 

 Its level would even be raised and its contents discharged on the city but for the 

 excessive evaporation, by which the volume of water is gradually diminishing. 

 In 1804, at the time of Humboldt's visit, its depth varied from 10 to 16 feet, but 

 in 1885 it had fallen to 5 feet 6 inches in the deepest parts, with an average 

 of scarcely more than 2 feet. In 1881 it was even much shallower, little over 

 12 inches in many places, and in exceptionally dry years Texcoco, San Cristobal, 

 Xaltocan, and Zumpango have been exhausted. In fact this brackish depression 

 would have long ago been emptied but for the flow from Chalco and Xochimilco. 



It is generally supposed that the local climate has really become drier since 

 the time of the conquest. The disappearance of the forests from the slopes and 

 plains would appear to have increased the evaporation by giving greater play to 

 the winds, without a corresponding increase, perhaps even a decrease, in the 

 rainfall. At present the contents of the lacustrine basins in the valley of Mexico 

 are insignificant compared with their volume in a former geological epoch. The 

 bed of the old lake, that is, the so-called "valley," consists of quaternary débris, 

 sands, clays, pumice, scoriae, organic remains, superimposed in successive layers 

 so thick that they have not yet been pierced by the shafts of an artesian well sunk 

 to a depth of 1,270 feet. In some places the calcareous strata of lacustrine origin 

 have jdelded spring water at a comparatively slight distance from the surface ; but 

 elsewhere nothing has been met except the quaternary deposits.* 



The chemical composition of the Texcoco waters is itself an indication of their 

 gradual concentration in a continually narrowing basin. Xochimilco and Chalco 

 are both fresh- water reservoirs, their contents being constantly renewed ; on the 

 opposite side of the valley the other small depressions are also flooded with fresh- 

 water. But the central lake is always brackit^h even after the heavy rains, when 

 it covers a considerable surface. 



At a remote geological epoch, when the whole valley of Mexico was filled with 

 fresh water, the overflow was discharged through a breach in the mountains 

 northwards to the Tula or Montezuma, a headstream of the Panuco river. But 



* Superficial area and relative altitude of tte lakes iu the Valley of Mexico (1865) : — 



