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. 

 Moreover, remains of what are believed to be Ehizopods, and 

 named Eozoon, have been detected in the Azoic rocks of Ottawa, 

 Canada. Limestone beds have been formed out of the microscopic 

 shells of Rhizopods ; for chalk is largely due to their growth and 

 accumulation. These Ehizopods, although animal, are extremely 

 low in the scale, — little above the spores of sea-weeds : so that, if 

 existing then, they simply foreshadowed the future animal kingdom. 

 It is also possible that vegetable life may make strata of limestone. 

 There are coral-secreting plants as well as animals ; and such plants 

 are called Corallines. There are also microscopic infusorial plants ; 

 and those •secreting silica — the siliceous infusoria (Diatoms) — are 

 known in later times to have made extensive beds of rock. 



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 the occur- 

 rence of such chemical formations in the earlier age of the globe 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 ; or, if animal also, only of the 

 very lowest kind. 



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. 



4. The Silurian formation commences with the same genera and 

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

 Europe and America, — indicating by its variety of kinds, and its 





