CORRELATION OF STRATA. . i 



gibbota, Cyclotpirn biaulcalii, 

 RrrrplacuIHe otceni and Mnrrhi.,ni<t 1,,-llicinctn. 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 fossil* 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 Fuaispirn inJJata (Meek & Worthen, sp.) were obtained here. 



Marlurra bftl. We adopt this name from Dr. Sardeson's section. It is an easily 

 recognised 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 Stewart vi He 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 

 (astropoda. of which Mucluren cratta, Maclurinn mneala, M. munifoltrtuis, and iMphosjiirn 

 ni : /ii.<tinii 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, at a 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. 



THK HUDSON KIVEK 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 1hc present it must be sufficient to 



