GEOLOGICAL IMPORTANCE OE SEDIMENTATION 317 



sedimentation already recorded in the geological literature of the past 

 fifty years, rather than to new observations; and it is thought that a 

 certain value will be attained by bringing together this widely scat- 

 tered body of information. The references are frequently given to 

 the most accessible source, rather than necessarily to the original 

 statement of facts, frequently difficult of access and sometimes in 

 foreign languages; but the effort has been, not to found arguments 

 upon commonly quoted generalizations, but to go back to their 

 sources, often largely lost sight of, and to examine from this starting- 

 point the quantitative value of those generalizations. The ultimate 

 conclusions will be found to be sometimes in accordance with, some- 

 times at variance with, views which have frequently been expressed in 

 geological literature. The outline of the work is as follows: 



The littoral zone is strictly limited to those portions of the shore 

 lying between the average highest flood-tide and the average lowest 

 ebb-tide of the month. Above this all deposits belong to the land 

 surface, even though they may be in the form of low-lying deltas 

 which perhaps once a year are flooded by the sea. Below the littoral 

 zone all deposits belong to the general class of marine sediments, 

 which may in turn be subdivided into estuarine, shallow open sea, 

 and true deep sea. 



It is pointed out that the sediments of the continental, littoral, and 

 marine zones are accumulated under distinctive conditions which 

 should frequently allow them, upon sufficient study, to be sharply 

 separated. It is, moreover, concluded that, in comparison with con- 

 tinental and marine deposits, those of the littoral zone should form 

 but a small fraction of the stratigraphic series, and that therefore, in 

 the case of certain formations bearing marks of shallow-water origin 

 and occasional exposure to the air, their frequent reference to an ori- 

 gin between tidal limits over the mud-flats of a shallow sea is inher- 

 ently improbable, and should only be accepted where indubitably 

 proven. 



On the other hand, it is argued on inductive and deductive lines 

 that an appreciable portion of the mechanical sediments incorporated 

 into the geological record should have been made as subaerial delta de- 

 posits and therefore continental in nature, laid down in close association 

 with the sea, and more or less interbedded with marine formations. 



