OSCILLATORY CHARACTER OF CONTINENTAL SEAS 367 



they owe their cosmopolitanism to other modes of migration than trans- 

 continental currents. 



Widely distributed species, whether pelagic or bottom dwellers, are 

 of the highest service in determining contemporaneity of geologic events, 

 but it is only the pelagic and semi-pelagic types that are dependable 

 for exact correlation between distinct provinces and in proving the exist- 

 ence of unobstructed current highways. The last fact, considered in 

 connection with the known rarity of pelagic faunas and species in the 

 interior continental basins, is of vital importance on the question at 

 issue. The other species, on the contrary, may occur in totally distinct 

 continental basins; and for these it is only when a large percentage of 

 a bottom-dwelling fauna is recognized from place to place that we are. 

 justified in assuming continuity of shoreline between them. The Low- 

 ville, for instance, indicates such a continuous shore and a corresponding 

 uniformity of faunal distribution in the rudely triangular area (Ohioan 

 province) between the mouth of the Mississippi on the south, Minnesota 

 on the northwest, and western Quebec on the northeast. If there are 

 deposits of this age to the west of this area, then they were laid down in 

 a distinct basin, for the Lowville fauna is not found in them. In this 

 and similar instances a few of the species are common to two or more 

 otherwise distinct faunas, but these are the descendants of ^^cosmopolitan" 

 types found in the Pacific and Arctic realms as well as in the Lowville 

 proper which invaded the continent from the south. 



Improbability of transcontinental marine currents shown by the geo- 

 graphic limitation of Mohawkian sediments and faunas. — A close analysis 

 of the "late Black Eiver and early Trenton fauna," whose distribution in 

 North America, as mapped by Schuchert and Ulrich^'' [middle Ordovicic 

 (lowest Trenton)"], suggests the greatest submergence of the continent 

 known, proves a synthesis of several faunas differing not only in age but 

 also in kind and to a considerable extent in their respective derivations. 

 It includes (see accompanying map, figure 7), first, the Decorah shale 

 fauna, a highly characteristic and therefore easily recognized association 

 of species. This fauna is best known from its development in towa and 

 Minnesota. It has been identified as far south as middle Tennessee, but, 

 excepting one doubtful occurrence in northeast Tennessee, seems to be 

 entirely absent in the Appalachian Valley. It is well developed on the 

 cast side of Ozarkia, but has not been seen on the other sides of this 

 uplift. Still, it probal)ly exists under cover of later deposits in an old 



^ BuU. Geological Society of America, vol. 20, pi. 58, 1910. 



