282 REPORT OF NATIONAL MUSEUM, 1892. 



dicators of the chronological divisions of the geological scale and of the 

 correlation of its divisions in different parts of the world than are those 

 of nonmarine faunas is apparent to everyone who is familiar With even 

 the general facts of biological geology, but it does not follow, and it is 

 not true, that the latter are intrinsically less valuable than are the for- 

 mer in held studies of practical geology. For this practical work both 

 marine and nonmarine fossils are treated by the empirical method 

 already explained, and both are found to characterize the respective 

 formations in the same manner. 



Certain conditions, however, give each an advantage over the other 

 under different circumstances. For example, the geographical range 

 of the nonmarine invertebrate fossil faunas, especially those of fresh 

 water, having been sharply denned by shore lines, the species which 

 constituted them are to that extent more characteristic of the forma- 

 tions in which they occur than is the case with marine faunas. Cer- 

 tain species of the latter faunas, as already shown, usually ranged 

 beyond the limits of the area which was occupied by each fauna as a 

 whole. 



Xonmarine formations as a rale occur singly in a series of marine 

 formations, in which case the vertical as well as the geographical range 

 of their invertebrate species is sharply defined. It is true that in the 

 interior portion of North America there is a continuous series of fresh 

 water formations and that certain of the species range from one into 

 another. These, however, are notable exceptions to the rule referred 

 to, and they at most only make such nonmarine faunas equal to the 

 average marine fauna as regards exceptional vertical range of species. 

 Again, nonmarine formations usually have the advantage of the pres- 

 ence of remains of plants and of land vertebrates and invertebrates, 

 which in marine formations are usually so extremely rare as to be 

 unavailable. 



On the other hand, marine faunas embrace such a wide diversity of 

 forms as compared with the nonmarine, and their progressive and dif- 

 ferential evolution from epoch to epoch has been so much greater that 

 they offer as faunas much more abundant means for the characteriza- 

 tion and identification of formations. It is clear, however, that the 

 opinion which some geologists have expressed or implied that the fos- 

 sil contents of nonmarine formations are of little value in practical 

 geological investigation is not well founded. The following are the 

 principal conclusions reached in the foregoing discussions : 



Formations being the only true units of local or regional strati- 

 graphic classification, their correct identification is the first, and an in- 

 dispensable, step in the practical field work of structural geology. 



Although formations as such have only a physical existence, their 

 biological characteristics are always the best, and often the only, means 

 of their identification, and therefore the exhaustive study of fossils is 



