328 REPORT OF NATIONAL MUSEUM, 1892. 



VII. CRITERIA OF PAST AQUEOUS CONDITIONS. 



Among the more conspicuous facts in geology are some of those which 

 relate to the maimer of origin as well as to the original and present 

 conditio!; of the sedimentary formations. These subjects have been 

 discussed at some length in Essay [I, and among those discussions are 

 some references to the character of the water in which each formation 

 was deposited. Studies of the sedimentary formations, especially those 

 made from a biological standpoint, have demonstrated that the bodies 

 of water in which they were deposited were of the various kinds that 

 are now known; that is. sonic were marine, some fresh, and some brack- 

 ish. 



It is by no means for biological reasons alone that it is desirable to 

 obtain a knowledge of the character of formerly existing bodies of water 

 and of the character of their respective aquatic faunas. On the con- 

 trary, such knowledge conveys important information concerning vari- 

 ous subjects in general geology, notably concerning formerly existing 

 physical conditions and many of the physical changes which took place 

 from epoch to epoch of geological time. For example, the presence of a 

 marine formation within any given geographical area shows that when 

 it was deposited that area was beneath ocean level, and consequently 

 that marine waters prevailed there. The presence of a nonmarine 

 formation shows that land areas surrounded the body of water in which 

 it was deposited and separated it more or less completely from marine 

 waters. Therefore, an extensive nonmarine formation necessarily im- 

 plies that a large land area, which may have been of continental ex- 

 tent, surrounded the body of water in which it was deposited. 



In the present advanced state of geological knowledge the distin- 

 guishing characteristics of marine formations are well understood, 

 because they have been exhaustively studied by geologists, and found 

 to agree iu general character with the faunas and deposits of existing 

 seas, and because opportunities for the study of these formations are 

 abundant in almost all parts of the world. For various reasons the 

 distinguishing characteristics of nonmarine formations are generally 

 not so thoroughly understood as are those of marine origin, among 

 which reasons are their comparative rarity and usually their compara- 

 tively small geographical extent. Usually, also, they are not so abun- 

 dantly fossiliferous as are marine formations, and, therefore, the prin- 

 cipal means for their characterization are often not available. More- 

 over, these formations require some modification of the usual methods 

 of investigation because they are themselves of different kinds, as is 

 shown by inherent evidence possessed by each, and because they have 

 in common certain characteristics which distinguish them from those 

 of marine origin. 



