THE NORTH AMERICAN LAKES. 329 



The Mexican lakes present a case which strongly cor- 

 roborates this doctrine. Of the two lakes which impart 

 health and convenience to the city of Mexico, the upper 

 one is fresh, the lowe r salt. This salt is not a muriate, but a 

 carbonate of soda, like that in the nitrian ponds of Egypt. 

 The two streams which feed the upper lake, have changed 

 both the mass and character of the water. The salt has 

 been washed out, and carried down to the lower lake. 

 There it stagnates, until it escapes, by evaporation, or 

 through the expensive aqueducts constructed by the go- 

 vernment. The rise of the water in the lower basin fre- 

 quently overflows an extensive surface of lowland, and 

 sometimes inundates the contiguous part of the city. When 

 it dries up and leaves bare the surface, an alkali is 

 often left, which the inhabitants gather and sell to ma- 

 nufacturers of soap. 



The Mediterranean has a communication with the At- 

 lantic, and its saftness is supported by the great supplies 

 it receives through the Herculean straits near Gibraltar, 



The same remark may be made concerning the Black 

 Sea, or Euxine. It seems to be now understood, particu- 

 larly since the publication of Mr. Ingigian's History of the 

 Thracian Bosphorus, that the level of the Marmora and 

 the Euxine is so nearly the same, that the current some* 

 times runs through the canal of Constantinople, as some 

 call it, northeastwardly to replenish the Euxine, and then 

 again, southwestwardly to evacuate it. When, therefore, 

 the supplies from the Danube, the Dnieper, the Dniester, 

 the Don, the Kuban, the Phasis, and other streams, fail 

 to raise the Euxine high enough to force into the Mar- 

 mora and the .Egean, there is still water, both heights 

 being the same. If the Black Sea heightens from new 

 accessions of water, the outward current runs. But 



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