CORRELATION OF STRATA IOV11 



or nrthi* pectinella bed. This bed is scarcely recognizable in Fillmoro county, 

 but at St. Paul and in Goodbue county it is a well marked horizon. It is full of one of the 

 so-called fucoids, the Oamarocladia rugosa. a fossil which we regard as the cast of a branch- 

 ing sponge. (See foot note, p. xcv. ) It is very characteristic of the bed in Minnesota 

 ami iM-curs in the same group in Kentucky. Other characteristic fossils of the bed are Orthi* 



iflln and x>ri>hmena*eptata. In Minnesota the bed is generally terminated above by a 

 roughly bedded, rusty, semi-crystalline layer, one to three feet in thickness. The rest of 

 the bed. with the occasional exception of one or two thin limestone layers, consists en tin- 1 y 

 of blue shales similar to those of the preceding beds, excepting that it is largely made 

 up nf comminuted fragments of organic remains. 



The Furoid bed may in a measure be considered as a passage from the Black River 

 group to the Trenton group. On both paleontological and lithological grounds, however, 

 we are satisfied that it is really a part of the former. The rather limited fauna is more 

 clearly related to the Black River than to the Trenton and it was not till its close that any 

 marked lithological change took place. In Minnesota, it is true, the strata following are 







at first still shaly. but instead of the preceding blue and green colors, we now have a 

 yellowish or gray tinge, while the prevailing fossils, excepting several Branchiopoda. are 

 nearly all distinct In Wisconsin and Illinois the two groups are just as easily separated, 

 while in Tennessee and Kentucky, no one could fail in separating the Orthis bed from the 

 Carter's Creek limestone. Paleontological ly there is always a decided break between the 

 two groups. This is. perhaps, least in eastern Canada where the Black River group is 

 also lithologically much like the Trenton limestone. 



In the eastern states and Canada the Black River group is remarkable for the 

 abundance and great size of the Cephalopoda. In other regions, however, this class of 

 fossils is not so strongly represented, although the group everywhere presents some of the 

 leading species less of them in Minnesota than anywhere else. But in Wisconsin and 

 Illinois the "Upper Buff limestone" again contains more Cephalopoda than anything 

 else, although most of the species occur also in the underlying " Lower Blue limestone". 

 Still, this seems to be the case with the Cephalopoda not only in Wisconsin but in Canada, 

 K. -mucky and Tennessee as well. The summary tables immediately following the list of 

 fossils show that of the 296 species found in the Black River group of Minnesota. 189 are 

 restricted to the group, 72 occur also in the Stones River group, and 58 pass into the 

 following groups. 



Trenton group. (Oatena limestone, and tholes. NtuhviUe group.) 



When the Lower Silurian faunas of Canada, the eastern states and of Kentucky and 

 Tennessee are compared with those which characterize the various divisions of the Lower 

 Silurian in the northwest, it seems strange that it has not been recognized heretofore that 

 the Galena limestone, instead of being a local upper member of the Trenton or the 

 equivalent of the Utica slate, is really equal to the whole of the Trenton group in New 



