RELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION. 333 



of a receding- and encroaching' shore line, and other results of the ele- 

 vation and depression of the land surface upon which they were formed, 

 generally were of a destructive character. It is of course only portions 

 of any of these ancient fluviatile deposits that have been discovered, 

 but portions of some of the later ones have fallen under unusually pre 

 servative conditions. 



Those narrow bodies of water, usually called lagoons, which are sepa- 

 rated from the open ocean only by sand reefs, often partake of the charac- 

 ter of estuaries as regards both their aquatic life and their varying pro- 

 portion of soluble salts. Their deposits also so often resemble those of 

 estuaries that upon physical grounds alone it probably will always be 

 difficult and generally impracticable to distinguish from each other the 

 ancient deposits of these two kinds which may exist among geological 

 formations. 



The extent of an estuarine deposit of course depends largely upon 

 the size of the inflowing river, the largest sometimes rivaling in ex- 

 tent the deposits of lakes and inland seas. In some respects their 

 physical characteristics resemble those of fluviatile deposits. That is, 

 like the latter, they contain accumulations of silt and shingle, and they 

 generally are wanting in that regularity of stratification which charac- 

 terizes the deposits of broad bodies of water. This irregularity is usu- 

 ally apparent even upon the outer border of an estuary, where it shows 

 the effects of the litoral wash of the great body of water between which 

 and the inflowing river it holds an intermediate place. Kstuaries exist 

 upon the borders of both lacustrine and marine waters, but the physi- 

 cal character of their deposits is essentially the same in both cases. 

 It therefore is impracticable upon physical grounds alone to distin- 

 guish an estuary deposit made upon the border of marine waters from 

 one made upon a lake border. 



The physical characteristics of those sedimentary deposits which 

 are made in lakes and inland seas are similar in all essential respects 

 to those made in marine waters, except that, as a rule, calcareous mate 

 rial is more prevalent among marine deposits than any other. The 

 materials of which they are composed, like those of marine deposits, are 

 more or less evenly bedded, and they constitute characteristic members 

 of that great class of sedimentary deposits to which the term stratified 

 rocks is applied. Because of this uniformity of general characteristics 

 it is always difficult, and generally impossible, to demonstrate by means 

 of physical data alone whether a given formation was deposited in 

 marine waters or in those of a lake or an inland sea. Still, a geologist 

 who has much experience in the application of all available evidence 

 may often approximate a correct judgment in such cases by means of 

 physical data, but the almost certain presence in such strata of bio 

 logical data leaves him without excuse for relying only upon the 

 physical. 



