CHAPTER III. 



ORGANIC AGENCIES. 



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ORGANIC agencies are less powerful than aqueous in 

 modifying the surface of the earth ; yet, even in this 

 respect, they are of no mean importance, since enormous 

 beds of limestone are formed by this means. But their 

 true importance is perceived when we remember that 

 organisms are the most delicate indicators of physical 

 conditions, and therefore of the changes through which 

 the earth has passed. Organic remains or fossils are, as 

 it were, the characters in which the history of the earth 

 is written. 



The subject may be best treated under four heads, each 

 having a special application in explaining some important 

 point in the history of the earth, viz. : 1. Vegetable accumu- 

 lations, to throw light on the formation of coal and lignite. 



2. Iron accumulations, to throw light on the great beds of 

 iron-ore, found in the strata of earlier geological times. 



3. Lime accumulations, to explain the formation of lime- 

 stones. 4. Geographical distribution of species, to throw 

 light on the geographical diversity of species in earlier 

 epochs, and on the laws of succession of organic forms 

 in the history of the earth i. e., the laws of evolution. 



The phenomena under all these heads can be observed 

 by each one for himself ; but it must be remembered that 

 nearly all geological causes are very slow in their opera- 

 tion, and, therefore, the phenomena are not forced upon 

 our attention, but must be looked for by intelligent, ever- 

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