GEOLOGY. 269 



Africa, or Spain, might have been sufficient to rob the air of the moist- 

 ure \vhich it was wont to carry away and precipitate upon this great 

 inland basin. See how the Andes have made Atacama a desert, and 

 of Western Peru a rainless country, simply by the rising of a mountain 

 range between these regions and their vapor springs. 



The great inland basin of Asia in which are the Aral and the Cas- 

 pian Seas, is situated on the route which I make the thirsty winds 

 from Africa and America to take, and so scant of vapor are these winds 

 when they arrive in this basin, that they have no moisture to leave 

 behind. Just as much as they pour down, they take up again and 

 carry off. The level of the Caspian Sea is as permanent as that of the 

 whole ocean. We know that the volume of water returned by the 

 winds, the rains, and the dews, into the whole ocean, is exactly equal 

 to the volume which those seas give back to the atmosphere ; for, as far 

 as our knowledge extends, the level of each of these two seas is as per- 

 manent as that of the great ocean itself. It is estimated that three 

 times as much water as the Mediterranean receives from its rivers is 

 evaporated from it. This may be an over-estimate, but the fact is 

 made obvious, by the current which the Atlantic sends in through the 

 Straits of Gibraltar, that the evaporation from it is in excess of the 

 precipitation ; and that the difference, whether it be much or little, is 

 carried off to modify climate elsewhere. But supposing the Mediterra- 

 nean to be barred up across the Straits of Gibraltar ; the demand for 

 vapor from it would exceed the supplies of water to it, and it would 

 begin to dry up. As it sinks down, the area exposed for evaporation 

 would decrease, the supplies to the rivers would diminish, until finally 

 there would be established between the evaporation and precipitation 

 an equilibrium, as in the Dead and Caspian Seas ; but for aught we 

 know, the water level of the Mediterranean might, before this equi- 

 librium were attained, have reached a stage far behind that of the Dead 

 Sea level. The Lake Tadjura is now in the act of attaining such an 

 equilibrium. There are connected with it the remains of a channel by 

 which the water ran into the sea. Its surface is now 500 feet below 

 the sea level. If not in the Dead Sea, do we not, in the valley of this 

 lake, find out-cropping some reason for the question What have the 

 winds had to do with the phenomena before us ? The winds ,, in this 

 sense, are geological agents of great power. It is not impossible but 

 that they may afford us the means of comparing directly geological 

 events which had taken place in our hemisphere with geological events 

 in another. The tops of the Andes were once at the bottom of the sea. 

 Which is the oldest formation, that of the Dead Sea, or the Andes ? 

 If the former be the older, then the climate of the Dead Sea must have 

 been hygrometrically very different from what it now is. 



In regarding the winds as geological agents, we can no longer con- 

 eider them as the type of instability. We rather behold them in the 

 light of ancient and faithful chroniclers, which, upon being rightly con- 

 sulted, will reveal to us truths which nature has written upon their 

 wings in characters as legible and enduring as she ever engraved the 

 history of geological events upon the tablet of the rock. 



It is probable that the salt of the sea is washed into it by the rains 



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