330 On the Geological Arrangement of Ancient Strata, 



still farther out into the deep sea, the cod and haddock would prevail, 

 till at last, in the schistose strata formed in the still depths of the ocean, 

 we would in vain look for any traces of animal existence. 



We can thus explain, too, why strata of considerable thickness, hav- 

 ing the same mineral character throughout, apparently formed under 

 similar circumstances, and within one epoch, should yet differ materially 

 Ml the specific characters of the inhabitants of its upper and lower beds. 

 The difference of a few hundred feet in the depth of water ; and, conse- 

 quently, in the temperature, and other circumstances, being sufiicient to 

 influence the instincts of the animals by which the beds are tenanted. 

 Agassiz found no species of fish common to two formations, although 

 the same genera are distributed among several ; this is also the case with 

 molluscous testacea. In general, the species are confined to distinct 

 beds, although several genera have a wide range over different strata. 

 Thus, trilobites, goniatites, and pennatulse, are found only in the oldest 

 er deepest fossiliferous strata; productse and spiriferse range through 

 the middle strata ; terebratulse have a still wider range, up to the chalk, 

 and the familj' of ammonites seem also to have been much dispersed, 

 although they center most numerously in the oolitic series. 



Although the same arrangement of fossil animals, in regard to super- 

 position, appears to be very uniformly the same in every part of the 

 earth hitherto explored, yet the mineral characters of the equivalent 

 fossiliferous strata are not always similar ; so that the presence of par- 

 ticular fossils is no test of the mineral characters of rocks. This charac- 

 ter of sedimentary strata is dependent upon the type of the older rocks, 

 out of which they have been formed, and hence may vary in different 

 localities. In the same way, strata may be mineralogically alike, and yet 

 differ essentially in their fossils. Thus, we have various denominations 

 of sandstone, according to the depths at which the deposits were made, 

 which, although mineralogically identical, are tenanted by different 

 genera of fossils. The same occurs in calcareous and aluminous de- 

 posits. 



Neither can the opinion any longer be tenable that fossils are the key 

 to the relative ages of strata, unless in those cases where there is an 

 actual super-position of one series of strata on another, and even in those 

 cases there may have been a contemporaneous formation of strata by a 

 gradual and nearly equable extension of the upper and lower beds from 

 the shores out into the deep sea. In cases where there is no super-posi- 

 tion, it is sufficiently evident that the schistose depositions, called pri- 

 mary, may have been going on accumulating in the depths of the ocean, 

 at the same period at which the lias and oolites were forming in the shal- 

 lower seas. 



By the same process of reasoning, it will appear evident, that it was 

 possible for the whole range of fossil animals to have been contempo- 

 raries in the same primeval ocean ; and yet, that not even a single species 

 should have obtruded upon the appropriate domains of another. 



