CONCLUDING OBSERVATIONS. 321 



tion, then, has not been of use solely or chiefly in pre- 

 paring the soil for vegetation. Its principal use has 

 been to procure the proper arrangement and condition 

 of the earth's crust. You have seen how largely, in 

 some cases, denudation has been carried on. This is 

 chiefly for the purpose of removing the material to local- 

 ities where it is required. It is by this reconstruction, 

 so extensively prosecuted, that the proper diversification 

 of the surface of the earth has been effected. 



436. Mechanical, Chemical, and Vital Agencies. In all 

 these disintegrations and reconstructions you see a great 

 interplay of mechanical, chemical, and vital forces. I 

 will cite here a single example. Carbonate of lime, in 

 the forms of limestone, chalk, and marble, enters largely 

 into the structure of the crust of the earth ; but water is 

 continually dissolving some of this as it finds it in the 

 rocks, and so the ocean is kept supplied with it. This is 

 a chemical operation, the carbonic acid in the water en- 

 abling it to dissolve considerable of the carbonate of 

 lime. Then, following it to the ocean, shell-animals there 

 gather it to make their shells, or coral animals to make 

 their skeletons. This is a vital operation, for in the bod- 

 ies of these animals, by a vital power, the carbonate of 

 lime is separated from the water in which it is dissolved, 

 and is deposited in a solid form. Then, by a mechanical 

 operation, these shells and skeletons become massed into 

 solid rock. 



437. The Atmosphere. One of the most signal exam- 

 ples of the interplay above referred to is seen in the re- 

 lations which the atmosphere bears to the earth that it 

 envelops like a robe. Before the Carboniferous age it 

 was a very different atmosphere from what it is now. 

 It was then highly charged with carbonic acid gas, which 

 is carbon, or charcoal, united with oxygen. Now this 

 excess of carbon in the air was brought down in the 

 Carboniferous age, and lodged in the bowels of the earth, 

 as coal, in immense stores, for the future use of man. 



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