326 REPORT OF NATIONAL MUSEUM, 1892. 



justly considered. In .short, the idea of absoluteness in such cases is as 

 much out of place as is the assertion or recognition of personal authority. 



Although the remarks in the last paragraph refer directly to North 

 American geology and geologists, they are equally applicable to other 

 parts of the world when reference is made to the scale as represented by 

 the European rocks. 



Notwithstanding the great excellence of the scale now in general use 

 and the fact that so little change has been made in it since it was first 

 devised by the early geologists, the future progress of geological science 

 will demand modifications the necessity for which will be especially 

 urgent when the true character of correlation for all the principal parts 

 of the earth has been ascertained. Hitherto correlation has been inves- 

 tigated with the single purpose of adjusting the series of formations 

 which occur in each of the various parts of the world to the scale now 

 in use, but although its general applicability to that purpose is not to be 

 questioned the ultimate result of the study of correlation will be to 

 modify this scale and adjust it to the systematic geology of the whole 

 earth. That is, the scheme of stratigraphic classification which has 

 been the main factor in adjusting the elements of systematic geology, 

 must in turn be itself adjusted to the great system which it will have 

 been the principal agent in producing. 



There is another subject which properly pertains to correlative 

 geology, but which does not come under the head of identification of 

 formations and only in part under that of correlation as the term has 

 been defined and the subject discussed on preceding pages. It relates 

 to the great obscurity or absence of evidence of chronological relation 

 between the marine and freshwater deposits which may occur upon one 

 and the same continent, and also to the equally great uncertainty as 

 to the correlation with one another of the nonmarine deposits of widely 

 separated parts of the earth. 



When the geologist is seeking to systematically classify the forma- 

 tions of a continent or region which consist of both marine and fresh 

 water deposits, among the physical facts with which he is confronted is 

 that in no case can a formation of one of these kinds be .continuous 

 with one of the other kind because they were necessarily deposited in 

 separate bodies of water. Therefore there can in no case be any direct 

 physical proof of contemporaneity of a fresh-water with a marine forma 

 tion, and there can be no physical indication of chronological relation 

 between them except in case of observable superposition. These re- 

 marks are made with special reference to intracontinental fresh-water 

 deposits on the one hand and border-region marine deposits on the 

 other. 



He is also confronted with the biological fact stated on preceding- 

 pages that the fossil faunas pertaining to fresh-water formations are 

 so different from those pertaining to mari e formations, and the two 

 kinds are so incongruous in their respective characters, even in case 



