CORRELATION OF STRATA. Cl 
Rafinesquina deltoidea, Plectambonites gibbosa, Cyclospira bisulcata, Plewrocystites squamosa, 
Receptaculites oweni and Murchisonia bellicincta, all of which may be considered as 
characteristic of the lower half of the Fusispira bed. 
To the northward at Berne, also in Dodge county, the Mantorville layers are more 
argillaceous and much less firm, while the fossils generally retain their shells. About six 
miles north of Kenyon (Goodhue county) the same layers are exposed in a bluff near the 
headwaters of a tributary of the Cannon river. Here, however, they are so thin and soft 
that they are quite unfit for building purposes. Immediately beneath them this bluff 
presents also a good exposure of the Nematopora horizon. About three miles south of 
Cannon Falls a good section of the greater part of the bed is exposed on a hill-side and in 
large cuts along the road to Hader P.O. Here we have, resting on the Nematopora 
layers, nearly 50 feet of shaly and sometimes apparently arenaceous strata in which after 
a careful search not a single good fossil was observed. Above them are about 20 feet of 
thin bedded fossiliferous limestones, which doubtless are equivalent to the layers quarried 
at Hader. ‘The latter are at or near the top of the bed and contain a considerable fauna. 
Some fine specimens of Fusispira inflata (Meek & Worthen, sp.) were obtained here. 
Maclurea bed. We adopt this name from Dr. Sardeson’s section. It is an easily 
recognized bed of buff magnesian limestone, averaging about 50 feet in thickness in 
Olmsted and Fillmore counties. This entire bed is exposed in Prosser’s ravine near 
Wykoff (see section 4) and the lower layers are quarried at Stewartville and other 
localities in the state. The bed resists decomposition very well and as a rule forms bold 
bluffs. The fossils occur chiefly in the lower half, and consist almost exclusively of large 
Gastropoda, of which Maclurea crassa, Maclurina cuneata, M. manitobensis, and Lophospira 
augustina are sometimes abundant and always characteristic. At the top of the bed several 
hard though porous layers are usually present forming a durable cap when they have not 
been weathered into rough prominences. Above these, or taking their place, we have 
noticed at several points in Fillmore county, notably, ata small quarry about two and a half 
miles north of Spring Valley, from five to ten feet of unevenly laminated bluish-gray, 
crinoidal limestone, presenting unmistakable evidence of disturbance at the close of the 
period. This layer corresponds with current formed limestones occurring quite generally 
at the top of the Trenton in Kentucky and Tennessee, and will be further considered 
in our general remarks on the Lower Silurian. 
THE HUDSON RIVER OR CINCINNATI PERIOD. 
Under this term we include all the rocks lying between the top of the Trenton and 
the base of the Upper Silurian. Space is wanting, nor are we fully prepared to give all 
our reasons for preferring the term Cincinnati for the period instead of Hudson River 
group or period, Hudson terrane, or that oldest name of them all, the ‘‘ Gray Sandstones 
and Shales of Salmon River ” as described and named by Conrad in 1837, in his first report 
on the geology of the third district of New York. For the present it must be sufficient to 
