ART. 21 OEDOVICIAN TEILOBITES ULRICH 61 



the Bryozoa,^'' a considerable number of specific identities. In 

 view of these otherwise inexplicable facts I see no way to escape the 

 conviction that these Baltic and American faunas originated in and 

 at opportune times migrated from the same oceanic basin ; and that 

 basin must have been in either the Arctic or the North Atlantic Sea. 



Another point to be brought out is the genetic connection between 

 these Decorah and New York-Ontario Trenton faunas, on the one 

 hand, and the already discussed and mostly older Chambersburg, 

 Little Oak, Ottosee, Holston, and Lenoir faunas of the southern 

 Appalachian region and of the Chazyan in the Champlain Valley, 

 on the other. Whatever the modifications and special peculiarities 

 that pertain to and enable us to recognize and distinguish each of 

 these faunas from the others there still remain many genetic threads 

 that are common to them all and indicative of a more or less strongly 

 manifested common source. 



But the depositional data pertaining to this hypothetical North 

 Atlantic faunal province are as yet too insufficiently known to be 

 presented as anything better than more or less vague clues to an 

 interesting chapter in the history of Ordovician oscillations and 

 marine faunal migrations. Accordingly, my confidence in the fore- 

 going facts and suggested inferences goes no further than the strong 

 belief that a distinct marine faunal province existed in the North 

 Atlantic region during a considerable part of Paleozoic time. Also 

 that the Champlain Chazyan, the Little Oak of Alabama, the 

 Chambersburg of Pennsylvania, Maryland, and Virginia, the Lenoir 

 and parts of the Blount group of Tennessee, Alabama, and Vir- 

 ginia, the eastern and Upper Mississippi Valley Decorah and Tren- 

 ton formations, the deposits in the Baltic province, and the typical 

 JStinchar of southwestern Scotland all participated in its history. 



POST-CHAZYAN— PROBABLY EARLY SILURIAN— FORMATIONS 



IN EUROPE 



Generalized comiments on the Ordovidan-Silurian houndary. — A 

 problem in stratigraphic correlation on which opinions differ very 

 greatly concerns the proper classification of such European forma- 

 tions as the Drummuck in the Girvan District in southwestern Scot- 

 land,^" the Keisley limestone in northwestern England, the Leptaena 

 limestone in Sweden, and the Lyckholm and Borkholm formations in 

 Estonia. Reed identifies some of the Drummock trilobites with 

 Keisley species and some of the same and other species of the Keisley 

 with characteristic members of the Leptaena limestone fauna; and 

 there is general agreement among British and Scandinavian geolo- 



29Bassler, R. S., Early Paleozoic Bryozoa of the Baltic Provinces: U. S. Nat. Mus. Bull. 

 77, 1911. 



'" See also notes on the "Craighead " limestone, p. 84. 



