308 REPORT OF NATIONAL MUSEUM, 1892. 



The physical incongruity between marine and nonmarine formations 

 is manifest in 1 1 it* tact that the presence of one or more of the latter 

 in a series of the former kind of formations always implies that there 

 have been such disturbances of physical conditions as to cause at least 

 a local break ia the continuity of marine sedimentation as well as in 

 that of faunal succession. Such breaks also imply a greater or less 

 interruption of the chronological record, the extent and character of 

 which can be determined, if at all, only by indirect means. It is, how- 

 ever, the want of reciprocal relation between marine faunas on the one 

 hand and nonmarine and land faunas and laud floras on the other that 

 more concerns the question of the relative value of the different kinds 

 of fossil remains in characterizing the time divisions of the geological 

 scale than docs a similar want with reference to other kinds. 



The biological contrast between marine faunas and land floras and 

 between their respective conditions of existence is so complete that it 

 is unreasonable to assume that the evolutional changes which have 

 taken place in each during geological time were chronologically concur- 

 rent. Therefore, whatever of intrinsic value in the characterization 

 of the divisions of the scale the fossil remains of the one series may 

 possess, it is quite independent of that of the other. 



It is true that the biological contrast between marine faunas and 

 land faunas is not so complete as it is between marine faunas and land 

 floras, but as regards interdependence and common conditions of ex- 

 istence the want of reciprocal relation between marine faunas and a 

 largo proportion of the members of all the land faunas is well nigh 

 complete. Rsal or apparent exceptions to such completeness are ob- 

 servablein the exclusively marine habitat, or marine resort for subsist- 

 ence, of certain mammals, reptiles, ami birds, but these are cases of 

 adaptation to conditions which are abnormal of exceptional for the 

 respective classes to which they belong. 



Notwithstanding these exceptions if is evident that during geological 

 time there has been no necessary concurrence of rate or degree of pro- 

 gressive evolution between murine and land faunas, and therefore that 

 the chronological value of the one series of faunas has in no ease a 

 necessary relation to that of the other, or no other than a common sec- 

 ular relation,* which is at best obscure. 



The relation of the marine to the nonmarine aquatic faunas is less 

 incongruous than it is in the case of the land faunas already noticed, 

 the respective members of nonmarine faunas having much in common 

 with corresponding members of marine faunas as regards zoological 

 affinity. Such a relation, however, does not make nonmarine fossil 

 faunas of concurrent chronological value with the marine, because of 

 the conspicuous fact that the rate of both progressive and differential 

 evolution has been remarkably slow in the case of nonmarine. especially 



v The difficulty or impossibility of correlating mariue deposits with those of non- 

 marine origin is discussed al the close of Essay vi. 



