REVISION OF STRATIGRAPHIC CLASSIFICATION 295 



SHIFTING OF FAUNAS 



General discussion. — The idea of "shifting of faunas" — that is, tan- 

 gential transgression of faunas in migration — is of course true theoret- 

 ically and in fact. The passage between two points requires some, how- 

 ever infinitesimal, lapse of time. But, speaking geologically, the propo- 

 sition is, in my opinion, purely theoretical. Its practical application in 

 the correlation of geological formations seems impossible and can be 

 assumed only in entire disregard of the general coarseness of geological 

 time units. Admitting, for the sake of argument, Walcott's very con- 

 servative estimate of Paleozoic time as representing something like 

 17,500,000 years, and that this great sum is divisible into less than 100 

 succesisve time units of sufficient importance to be inserted in a general 

 time scale, we find by simple division that the average length of time 

 represented by each formation is not less than 175,000 years. Walcott's 

 estimate being based on an aggregate thickness of 21,000 feet of Paleozoic 

 sediment in the Cordilleran sea, the average time value of each foot of 

 deposit is accordingly about 833 years. For limestone deposits, of course 

 this value is much greater. Now, while a few beds or formations 50 feet 

 or less in thickness may be recognized by correlation over wide areas, the 

 average interval within which satisfactory correlation is possible is not 

 less than 100 feet. Multiplying this by 833, the average time value of 

 practical correlation units would be about 833,000 years. Considering 

 that the living shell Littorina littorea required less than 50 years to 

 migrate along the Atlantic coast from Halifax to Cape May, a distance of 

 over 700 miles,^ it might at the same rate encircle the globe no less than 

 46 times in the time-equivalent of a single Paleozoic correlation unit. 



If Paleozoic invertebrates traveled only one-fiftieth or one-hundredth 

 as fast as this living shell, then we may reasonably assert essential con- 

 temporaneity for stratigraphic correlations extending across tlie continent. 



As evidence opposed to the slow progress of faunal migrations, I would 

 cite the great geographic distribution of the Richmond faunules, which 

 will be discussed in the argument for low relief of Paleozoic lands. One 

 of these Richmond faunules has been recognized at numerous localities in 

 the broad area limited on the east by Illinois, on the west by Nevada, on 



tlons, and to a large extent on the same or similar facts, the classification adopted by 

 Schuchert, while agreeing in essential respects, differs in several important details from 

 my own. It was chiefly to give me an opportunity to discuss these differences that the 

 publication of the present work was delayed a year. 



"Case of Littorina littorea; first observed at Halifax, Nova Scotia, 1852 ; at Bathurst, 

 Bay of Chaleur, 1855 ; coast of Maine, 1868 ; Portland, Maine, 1870 ; Salem, Massachu- 

 setts, 1872 ; Barnstake, Cape Cod, Massachusetts, 1875 ; Woods Hole, rare, 1875 ; com- 

 mon, 1876 ; New Haven, Connecticut, 1880. (E. S. Morse : The gradual dispersion of 

 certain mollusks in New England. Bull. Essex Institute, vol. xii, 1880, pp. 3-8.) 



