CORRELATION OF STRATA. XCV 
feet. The rocks contain considerable chert and the fossils are nearly always silicified. 
Characteristic fossils are Tetradiwm columnare, Columnaria halli, C. carterensis, Streptelasma 
profundum, Raphistomina lapicida, Stromatocerium rugosum, Triptoceras planodorsatum, 
Actinoceras bigsbyi, Orthoceras arcuoliratum, O.lesueuri, T. planoconvexwm, Orthis pectinella, 
Receptaculites occidentalis, and Camarocladia rugosa (Ulrich).* 
In Wisconsin and Illinois the group is represented by the ‘‘Upper Buff” and ‘‘ Upper 
Blue limestones.” The average thickness of the two beds here is probably less than 40 
feet. They are well exposed in the quarries at Rockton, Illinois, where the upper member 
carries fossils clearly indicating the Phylloporina and Fucoid beds of the Minnesota 
section. We collected here, namely, Orthis pectinella, O. testudinaria, Strophomena 
trentonensis, Agelacrinus marginatus, Bythotrypa laxata, Rafinesquina inquassa, Arthropora 
bifurcata, Prasopora conoidea and Bythopora alcicornis. In the Upper Buff, which we 
correlate with the Rhinidictya and Ctenodonta beds of Minnesota, we saw Streptelasma 
profundum, Pachydictya occidentalis. Phylloporina reticulata, Rhinidictya mutabilis, 
Cyrtodonta cingulata, Vanuxemia mota and Cyrtoceras corniculum. Numerous other 
Cephalopoda occur in this bed but they have not been identified in Minnesota. At Beloit 
and other localities in Wisconsin the Upper Buff contains some chert. 
The strata representing the Black River group in Minnesota are peculiar in consisting 
almost entirely of shales. They are also more fossiliferous than elsewhere, and the fauna 
taken as a whole is unusual in two respects. Namely, it includes a large number of 
Bryozoa which are wanting entirely in other regions, while the Cephalopoda, for which 
the group is noted, are here represented by relatively small species only. Another 
remarkable feature is the rapid reduction in volume between Cannon Falls and localities 
in Fillmore county. (See sections 6, 7 and 8). This is so marked that we are almost 
justified in assuming that the entire group failed a few miles east of Fountain. Unfortu- 
nately, this cannot be proved since the strata of the Trenton period have all been removed 
in that direction. While the Stones River group may still have continued without material 
interruption across the Mississippi valley, at the southern end of the state, the Black River 
group probably did not do so, and we are inclined to believe that the latter, as well as 
the succeeding Silurian and Devonian strata of southern Minnesota and northern Iowa, 
were deposited in a bay. 
* The last of these fossils is the large “fucoid” that is so extremely abundant and characteristic of the Fucoid bed 
in Minnesota. In Mercer county, Kentucky, it occurs at an exactly equivalent horizon, i. ¢., in a thin bed of shale near the 
top of the Black River limestone. The fossil seems to us to be a cast of a branching sponge similar to the Camarocladia 
dichotoma described by Ulrich and Everett from the sponge layer of the Stones River group at Dixon, [llinois, C. rugosa is a 
much larger species, its flexuous compressed branches varying from 5 toover 12 mm.in width. The bifurcations vary 
greatly, being sometimes very close, at other times far apart. Asa rule the specimens show little structure, appearing as 
mere stony flattened branches with more or less obscure transverse and oblique furrows. The most complete specimens are 
covered with anirregular network composed of coarse nodulose threads often exhibiting a longitudinal arrangement. 
Generally the network remains on one side of the branches only. When removed entirely the stems are seen to be composed 
of two elements: (1) a siphuncle-like, subcylindrical rod, with annulations and constrictions 3 to6 mm. apart, and (2) a 
series of oblique septa-like partitions—generally two to each annulation—clasping the annulated rod so as to leave about 
One-third of its circumference exposed to view. Not infrequently the rod changes suddenly from one side of the branch to 
the other. Inan unbranched fragment before us this occurs twice in the space of 40 mm. The best specimens were found 
in Goodhue county. 
