RELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION. 359 



of habitat is not of itself sufficient to enable one to decide whether it 

 was of marine, brackish, or fresh water origin, and other evidence 

 must be sought. 



The evidence upon which criteria of the character of formerly exist- 

 ing bodies of water are based is usually more or less direct, but it is 

 sometimes cumulative and concurrent in its character. Still, when 

 properly applied, the latter kind of evidence is usually nearly or quite 

 as valuable as if it were absolute and direct. 



The criteria of past aqueous conditions which are discussed in this 

 essay are of course only such as may be derived from sedimentary 

 formations and their contents. It can not be said that there are any 

 fully trustworthy physical criteria because a nonmarine formation 

 rarely presents any condition of stratification, or any lithological 

 character, which is not observable in some marine formations. Still, 

 there are many more or less valuable indications which may be ob- 

 served and to some degree relied upon in the absence of fossil remains. 



For example, although considerable accumulations of calcareous 

 strata are sometimes found among the generally arenaceous strata of 

 fresh water formations they have never been found to contain any im- 

 portant accumulations of regularly bedded limestones. Furthermore, 

 estuarine deposits are often still more of a detrital character than are 

 fresh-water formations and also they more rarely contain calcareous 

 layers. Therefore if one should encounter a series of regularly bedded 

 limestones, either magnesian or fully calcareous, he will rarely, if ever, 

 be at fault in regarding them as of marine origin even without biologi- 

 cal evidence. 



In a large proportion of the nonmarine formations the stratification 

 is less regular than is usually the case with marine formations. Still, 

 this by no means is a certain criterion, and in some cases nonmarine 

 formations are found to rest so conformably upon the marine and to 

 be so conformably overlain by them as to give little indication of the 

 great difference in the condition of their origin. 



The foregoing examples show how indefinite is the character of physi- 

 cal evidence as to the past aqueous conditions under which the various 

 sedimentary formations have been produced, but they serve to empha- 

 size a statement of the fact that almost entire reliance must be placed 

 upon the evidence furnished by fossil remains. 



With reference to general indications of difference between marine 

 and nonmarine formations which are furnished by their fossil remains 

 we observe that a conspicuous difference lies in the comparative abun- 

 dance and variety of forms of life which the fossil faunas of the for- 

 mations respectively represent. Marine waters have always teemed 

 with life in a wonderful variety of forms, and their fossil remains are 

 proportionally abundant. The variety is less in brackish waters and 

 least of all in lacustrine waters. It is true that ichthyic life is abun- 

 dant in some fresh waters, but never so generally abundant or so 



