ZOOLOGICAL STRATIFICATION. 



all that is necessary for the solution of the proposed question. 

 The outlines of this deposit are, in fact, identical with the outlines 

 of land and water during the Silurian period. The land con- 

 sisted, of course, of those strata which, being more ancient than 

 the Silurian, were pushed above the level of the waters by the 

 convulsion which preceded the period of tranquillity during which 

 the Silurian strata were deposited. 



229. If, therefore, the extent and limits of the Silurian deposit 

 could be certainly and definitely ascertained, the problem would 

 be solved ; but independently of the knowledge, more or less 

 imperfect, which direct observation has supplied as to the extent 

 and limits of this deposit, some uncertainty attends the identi- 

 fication of the various strata to which the name Silurian is 

 properly given, by their mere mineralogical characters. It has 

 been already explained, that strata of simultaneous deposition 

 vary in their mineralogical characters from place to place, those 

 which have been deposited in one part of the world consisting of 

 mineral constituents wholly different from those which have been 

 deposited at the same epoch elsewhere. 



230. The doubt which this raises, however, has been removed 

 by the light thrown upon geology by fossil zoology and botany. 

 If the mineralogical characters of contemporaneous strata be 

 doubtful, there can be no uncertainty as to the zoological 

 character of their fossil contents. 



Although it be true that many of the genera of animals and 

 plants, deposited in strata of different dates are common, this is 

 not the case with the species which, with very rare exceptions, 

 are peculiar to each period. If, therefore, the observer, guided 

 merely by the mineralogical character of the strata under examina- 

 tion, be uncertain as to its date, his doubts will disappear upon 

 a due examination of its organic deposits. !Xo other than the 

 strata of one particular epoch can contain the same combination 

 of species of animals and plants. 



231. But the aid afforded by the organic deposits is not limited 

 merely to the determination of the date of the strata. They also 

 supply the means of determining, with much greater precision 

 than can be obtained from the mere mineralogical constituents, 

 the outlines of land and water. It is known that certain genera 

 of animals can live only in the tranquil bottoms of deep seas ; 

 there they live and there they die, and there their remains are 

 buried in the strata deposited by such waters. Where such 

 remains, therefore, are found in the strata of the crust of the 

 earth, such strata must have been at the epoch of its deposition 

 at the bottom of a deep sea or ocean. 



232. The bodies of certain animals when dead and not dis- 



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