RELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION. 327 



of actual contemporaneity of origin, that they can not be used as con- 

 current chronological evidence. 



The latter statement applies chiefly to the remains of aquatic faunas, 

 but the case is little if any changed by the association with them of 

 remains of land faunas and floras unless such faunas and floras should 

 be represented in the fresh-water as well as the marine formation. 

 Such association and identity are to be regarded as the only direct evi- 

 dence of contemporaneity of a fresh- water and marine deposit. All 

 other evidence is indirect and of more or less uncertain value. 



Such a dual commingling of remains of a land fauna and flora with 

 those of aquatic faunas implies that the two bodies of water in which 

 the commingling took place were separated by a land area, the whole 

 breadth of which was occupied by the fauna and flora represented by 

 the remains. It also implies that those remains reached their intomb- 

 ment in the sediments of both bodies of water in the manner described 

 in Essay i. It is a fact, however, as already pointed out, that re- 

 mains of land animals and plants are very rarely found in marine de- 

 posits, even in case there is reason to believe they lived abundantly in 

 the vicinity of the waters in which those deposits were made. This 

 circumstance greatly lessens the chances of discovering direct proof of 

 contemporaneity of fresh water and marine formations. 



The indirect evidence of contemporaneity of fresh water and marine 

 formations which may occur upon one and the same continent is in part 

 that which is afforded by the position of each in their order of succes- 

 sion in a series of formations of known geological age, and in part that 

 which pertains to the general subject of correlation. I have already 

 shown that the best of the evidence which pertains to that subject, 

 especially when applied to so small a portion of the geological scale as 

 is represented by even the greatest of the fresh water formations or 

 series of deposits, is of very uncertain value. I may now add that such 

 evidence is still less valuable when it rests upon the remains of fresh- 

 water faunas alone, because of their remarkably slow evolution, both 

 progressive and differential, during the whole of that portion of geo- 

 logical time in which they are known to have existed. It should also 

 be stated that whatever of accuracy may have been attained in assign- 

 ing the fresh water formations of Europe to their respective taxonomic 

 positions in the geological scale it does not necessarily follow that fresh 

 water formations upon other continents bearing closely similar faunal 

 and floral fossil remains can be assigned upon such evidence alone to 

 exactly the same taxonomic positions. Therefore, in attempting to cor- 

 relate interior fresh water formations with border region marine forma- 

 tions, such as those which occur in North America, for example, the 

 geologist must, as a rule, to which no exceptions are yet known, rely 

 upon general indications and cumulative evidence. 



