EAELY PALEOZOIC BEYOZOA OF THE BALTIC PKOVINCES. 27 



BLACK RIVER GROUP. 



The extensive development of this group in the Minnesota-Iowa 

 area was pointed out about 15 years ago by Ulrich, and is indicated 

 by the same writer in his recent work upon the revision of the Paleo- 

 zoic systems of North America. During Black River time, oscilla- 

 tions of the continental sea were frequent and are manifested in 

 numerous unconformities. At no single locality are all of the divi- 

 sions noted in the composite section developed, but most of these are 

 present at and in the vicinity of St. Paul. To the south in Minnesota 

 and in northern Iowa the overlaps are more pronounced and the 

 hiatuses more frequent. Farther south, in eastern Missouri, the 

 Black River deposits of Minnesota and Iowa are overlapped by the 

 uppermost division assigned to the group, the Kimmswick formation, 

 a crystalline limestone containing specimens of EchinospTierites which 

 can not be distinguished from the common Baltic species. The 

 faunas of the several Black River formations are quite large and 

 characteristic. They have been described by Ulrich, Clarke, Schu- 

 chert, Winchell, and Scofield, in the third volume of the ''Geological 

 and Natural History Survey of Minnesota," where the Black River 

 group is referred to as the "Trenton shales," or as the "Black River 

 shales." Complete lists of these various faunas are given in the 

 introduction to part 2 of the volume. Here the Black River shales 

 are divided into four beds, named in ascending order, according to 

 the characteristic fossils, the Rhinidictya bed, Ctenodonta bed, 

 Phylloporina bed, and Fucoid bed. In this introduction, the Sticto- 

 porella bed, forming the lowest division of the "Trenton shales" of 

 previous reports, was assigned to the Stones River group, but more 

 extended study on the part of Ulrich has convinced him that this 

 bed should form the introductory strata of the Black River shales, 

 or, as they are now known, the Decorah shales. Trenton shales, 

 Black River shales, and Decorah shales, as the terms are employed 

 for this area, are, therefore, synonymous. The underl5dng Upper 

 Buff and Lower Blue limestones of the region contain the fauna 

 characteristic of the Lowville formation of New York, and, following 

 the more recent stratigraphic work, they are regarded as the lowest 

 division of the Black River group. To this division the name 

 Platteville limestone has been applied by Bain. 



THE BRYOZOAN FAUNAS OF AMERICAN BLACK RIVER AND 

 EARLIEST TRENTON DEPOSITS. 



The earliest Black River deposits of the upper Mississippi Valley 

 are of blue to gray fine-grained dolomitic and argillaceous limestone 

 apparently little suited to bryozoan life, although fossils of other 

 classes are often quite numerous. The few bryozoans that do occur 



92602°— Bull. 77—11 4 



