ATMOSPHERIC CAUSFS OF CHANGE. 271 



CHAPTER IX. 



^ Atmospheric Causes of Change. 



" The seas have changed their beds the eternal hills 

 Have stooped with age the solid continents 

 Have left their banks and man's imperial works 

 The toil, pride, strength of kingdoms, which had flung 

 Their haughty honors in the face of heaven, 

 As if immortal have been swept away." 



Henry Ware. 



WE have before described the atmosphere as an elastic fluid 

 encompassing the earth, and capable of absorbing large quanti- 

 ties of moisture, and have illustrated the formation of clouds and 

 the origin of winds. We can easily perceive that so important 

 an ngent may be capable of effecting the most marked changes, 

 and we do not now allude to those effects produced by a secon- 

 dary agency. Thus, the moisture deposited from the air in the 

 form of rain, or snow, upon loftjir mountains, causes deluges of 

 water to descend artd overflow the plains below, or becomes the 

 source of those mighty rivers which roll through thousands of 

 miles, bearing their immense deposits down to the ocean's bed. 

 Changes like these we have already considered, and do not there- 

 fore include them in our present description. The atmosphere 

 acts as an agent in destroying rocks and changing the face of a 

 country, mechanically and chemically. The formation of im- 

 mense dunes, or downs, which are heaps of blown sand, is an 

 illustration of the former action, and the disintegration and de- 

 struction of rocks by the absorption of oxygen, and carbonic acid, 

 is an example of the latter ; both of these actions we will now 

 briefly consider. 



The fine sands of the African desert for a long series of ages, 

 have been blown by the westerly winds over all the lands capa* 

 ble of tillage on the western banks of the Nile, involving in the 



