CORRELATION OF STRATA. X¢@vil 
Fucoid or Orthis pectinella bed. This bed is scarcely recognizable in Villmore county, 
but at St. Paul and in Goodhue county it is a well marked horizon. It is fullof one of the 
so-called fucoids, the Camarocladia 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 
and occurs in the same group in Kentucky. Other characteristic fossils of the bed are Orthis 
pectinella and Strophomena septata. In Minnesota thebed 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 entirely 
of blue shales similar to those of the preceding beds, excepting that it is largely made 
up of comminuted fragments of organic remains. 
The Fucoid 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. Paleontologically 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, 
Kentucky 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. (Galena limestone and shales. Nashville 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 
