278 REPORT OP NATIONAL MUSEUM, 1802. 



principal value of these Judications consists in their availability as 

 accessories to biological evidence. These physical indications of the 

 identity of formations are further discussed in Essay VI. They are 

 mentioned here only for the purpose of comparing them with biological 

 indications. 



Formations are biologically characterized only by the fossil remains 

 of animals and plants which lived while they were in process of depo- 

 sition, and the more intimate the natural relation of any of those 

 animals and plauts to the physical conditions which produced a forma- 

 tion, the more characteristic of it are their remains. This implies that 

 while no kind of fossil remains is to be rejected in practical studies of 

 structural geology, there is much difference in the value of the different 

 kinds for this purpose. These differences in value will be specially 

 discussed in following essays. 



In the preceding essay it was shown that there was an intimate 

 relation between the geographical boundaries of each aqueous area 

 within which the sedimentation took place that resulted in the produc- 

 tion of a formation and those of the habitat of the aquatic fauna the 

 remains of which are now found to characterize it. Also that each 

 aquatic fauna began its existence as such with the beginning of the 

 deposition of the formation and ended its faunal existence with the 

 completion of that deposition, although, as a rule there was some ge- 

 netic connection with both the preceding and succeeding faunas. 

 Again it was shown that the conditions which attended the establish- 

 ment of those boundaries and controlled the deposition of the sediments 

 also constituted each area a congenial habitat for its aquatic fauna and 

 that consequently the whole life history of each of those faunas was 

 intimately connected with the production of the formation in which its 

 remains are found, while contemporary land faunas and floras bore no 

 such direct relation to it. Furthermore, reasons were given why it 

 may be accepted as a fact that as a necessary consequence of the con- 

 ditions of their habitat, every species of every aquatic fauna which 

 possessed fossilizable parts, was originally fully represented in the for- 

 mation to which the fauna pertained, while all other kinds of fossil re- 

 mains have always very imperfectly represented the faunas and floras 

 to which they belonged. Besides this, their presence in any formation 

 was always the result of accident. 



Although these are sufficient reasons why remains of aquatic faunas 

 are always of greater value than any other in the identification of for- 

 mations, that fact does not imply that other kinds are not of the high- 

 est value for other purposes nor that they are valueless for this purpose. 

 For example, although land faunas and floras bore no direct relation 

 to the production of a formation, it is evident that the effects of the 

 physical changes which respectively inaugurated and closed its depo- 

 sition would in each case have been of such a character, and that they 

 would have been so extended upon the land, as to cause important 



