454 E. O. ULRICH CORRELATION OF THE STRAND-LINE 



cm waters and faunas would be excluded and replaced by north Atlantic 

 waters and faunas. Such reversals happened many times during the 

 geologic history of the North American Continent. There were times, 

 too, when the northern continents were connected by strips of low land 

 or shallow water that favored intermigration of sublittoral faunas. These 

 and other warpings of the sea-bottom resulted also in changes in the 

 course of marine currents and in corresponding changes in the distribu- 

 tion of floating organisms. 



All of these changing conditions, save the last, tend to invalidate the 

 principle of correlation by comparison of the general aspect of faunas. 

 Unless short-lived species are positively identified in both, mere similarity 

 in aspect, however great, can not of itself establish the contemporaneity 

 of two fossil faunas. 



INFLUENCE OF BOTTOM AND DEPTH ON MARINE FAUNAS 



Marine faunas are greatly influenced by the character of the bottom 

 on which they live. Some species prefer and may be confined to sandy 

 bottoms, others live only on muddy bottoms; but such variations in en- 

 vironment are of smaller consequence to the paleontologist than is com- 

 monly supposed. He works chiefly with the fossilized, hard parts of 

 animals, and these, obviousty, are carried by waves and currents far and 

 widely beyond the areas in which they lived. The more or less frag- 

 mentary remains of faunas, whose natural habitat is in one case a sandy 

 bottom, in another a calcareous mud bottom, or in a third a clayey mud, 

 may thus be mechanically associated in the same layer. This happens 

 commonly in shale formations which contain small lenses of sandstone or 

 limestone, or both. A true indication of the limits imposed by bottom 

 conditions on the existence and distribution of particular faunas is pre- 

 served chiefly in formations that consist of or include widely extended 

 and uniformly developed lithologic units. The fossils preserved in such 

 beds doubtless represent many originally distinct colonies, but all of them 

 lived on essentially the same kind of bottom. Occasionally, too, original 

 colonies, which must have been rapidly or suddenly buried, are found. 

 Original associations also are found in the crevices of algal and coral 

 ■ reefs and between the boulders of a rocky shore. 



Correlations requiring the assumption of lateral change in faunas 

 caused by differences in character of bottom are, so far as my experience 

 is concerned, commonly found to be in error. As a rule, the observed 

 differences in the faunas resulted from altogether different causes. The 

 use of the idea in correlation seems to be warranted only as a last resort. 



Bathymetric differences also cause great changes in marine faunas; 



