326 STUDIES FOR STUDENTS 



the earth's surface are laid down a great variety of deposits, varying 

 from the oozes of the oceanic depths to the muds of protected bays 

 or of epicontinental seas, and the sands and gravels which front the 

 beaches facing the more or less open seas. It is only in regard to the 

 detrital shallow-water deposits that serious doubt as to their origin 

 is liable to arise, and it is only these, therefore, which need to be con- 

 sidered in the present connection. 



Where salt or brackish water fossils occur in abundance, a pos- 

 sible continental origin is eliminated, and there will be expected also 

 species which do not live within the zone of the littoral, thus proving 

 the absence of a littoral origin. Where fossils are absent, the more 

 uniform and widespread character of the deposit, the color, and other 

 features, summed up by Walther,' may settle definitely the marine 

 origin of an ancient deposit. The marine deposits are, however, as 

 thoroughly characterized by the absence of most of those features 

 which mark continental and littoral deposits, as by their own dis- 

 tinguishing features. Chief among these may be mentioned the 

 absolute absence of mud-cracks, rain-prints, and the foot-prints of 

 terrestrial animals. 



To the littoral division belong, strictly speaking, only those deposits 

 which are laid down between the limits of high and low tide. The 

 term is frequently, however, rather broadly used as relating to the 

 neighborhood of the shore. Thus one may encounter expressions 

 in regard to dune sands of the littoral belt or conglomerate deposits 

 as indicative of the littoral; yet dunes are entirely beyond the limits 

 of the tides, and gravels may be laid down at some distance from the 

 actual beach. The littoral zone, with its deposits, is regarded by 

 Walther as related most closely to the land; but this is a view upon 

 which a difference of opinion may be justly held, and the majority of 

 geologists would doubtless decide that its affinities were rather with 

 the sea. 



For present purposes it will be necessary to define the littoral zone 

 more exactly, and to sharply restrict its limits. It may, consequently, 

 be considered as the zone embraced between the average of the 

 highest flood and the average of the lowest ebb tides of the month. 

 This means that, on the average, the highest portions of the littoral 



I Einleitung in die Geologic, III. Theil, "Lithogenesis der Gegenwart." 



