146 AZOIC AGE. 



of the Silurian, in connection with the fact that Silurian and later 

 limestones are known to be mainly made from organic relics. 



2. The occurrence of graphite in the limestone and other strata, 

 — graphite being known to be a common result of the exposure 

 of mineral coal or charcoal to a high heat, and, in certain rocks 

 of Rhode Island and Massachusetts, having undoubtedly been made 

 from vegetable remains. 



3. The occurrence of anthracite in small pieces in the iron- 

 bearing rocks of Arendal, Norway, which rocks are probably Azoic 

 in age. 



Against this organic origin it may be urged that the limestones 

 and graphite may have been of chemical origin ; and in the earlier 

 age of the globe, when there existed a heat too great for animal 

 life, the occurrence of such chemical formations is not improbable : 

 yet the argument still leaves it an open question. 



Supposing the existence of life of some kind, it is more likely to 

 have been vegetable than animal. 



1. In the progressing refrigeration of the globe, a temperature 

 fit for vegetable life would have been reached before that which 

 animal life could sustain. If there is any exception to this, it is to 

 be found only among the lowest species of animal life ; and there 

 is as yet no evidence that such exceptions exist. 



2. The graphite and anthracite indicate vegetable life, if any 

 at all. 



3. There are among the Azoic rocks, slates, sandstones, quartz- 

 ites, and conglomerates which are not more altered than some 

 Silurian rocks containing fossils ; and, had Mollusks and Crinoids 

 existed, shells and Encrinites should be found in the beds. More- 

 over, the great beds of iron-ore rarely contain a trace of phos- 

 phates ; and this is some indication that there was little or no 

 animal life. 



4. The Silurian formation commences with the same genera and 

 partly with the same species of animal life in different parts of Eu- 

 rope and America, — indicating that the actual bottom of the series 

 of animal life had been reached alike in both countries. 



5. Again, in America a period of folding and crystallization 

 appears to have terminated the Azoic age, making a fitting close 

 to the era of the earth's primal inorganic history. The latter part 

 of this long time of revolution (whose centuries may have been 

 counted by scores) was the epoch of the unfossiliferous Huronian 

 beds, — since these terminate the Azoic, according to Logan. They 

 indicate, as far as studied, no existing animal races. 



6. It is possible that vegetable life may make strata of lime- 



