102 Wisconsin Academy of Sciences, Arts, and Letters, 
In the section at Prairie du Chien the Oneota is at least 150 feet 
thick, and the first of the fossiliferous cherts comes in about 65 feet 
above the bottom. The formation therefore comprises at least 90 
feet of more or less cherty beds at the top that are wanting at 
Stillwater. At Dresbach, Minn., which lies about midway between 
Prairie du Chien and Stillwater, the Oneota is approximately 100 
feet thick. The fossiliferous cherty zone referred to in this and the 
preceding paragraph occupies the upper 30 feet. The thinning of 
the Oneota to the northward, apparently by erosion of the upper 
beds, is suggested in similar manner by other sections in the valley 
north of Prairie du Chien. 
A sandstone, which may or may not correlate with the lower of 
the two beds of sandstone at New Kichmond, Wis., is locally present 
at the base of the Shakopee. A four to five-foot bed of sandstone 
occupies this position at Prairie du Chien. Five miles to the north 
of this city the same bed has thickened to about 10 feet. As re¬ 
marked in a preceding paragraph a bed of sandstone may occur at 
the Shakopee-Oneota contact, but it is very unsafe to rely on its 
presence. 
It has been shown that good physical evidence of the break be¬ 
tween the Shakopee and the Oneota dolomites—in other words, be¬ 
tween the representatives respectively of the Canadian and 
Ozarkian systems—is generally procurable in the numerous and 
often excellent exposures of the “Lower Magnesian limestone’’ in 
the Upper Mississippi Valley. But we must go elsewhere for the 
data required to appreciate the true significance of the break. By 
study of the sections in Wisconsin and Minnesota alone it would 
be impossible to reach the absolutely established conclusion that 
the Shakopee represents only the concluding stage of a great series 
of limestones and dolomites having an aggregate thickness of over 
5,000 feet. In northern Arkansas the Powell limestone carries the 
same peculiar gastropod fauna that is found to the north in the 
Shakopee dolomite. In Arkansas and Missouri the Powell is under¬ 
lain by three Canadian formations (Cotter, Jefferson City, and 
Eoubidoux) each with its own characteristic fauna of which not a 
trace has been observed north of Illinois. On the other hand, all 
three of these faunal zones are clearly recognized in the Canadian 
limestones in the Appalachian Valley, particularly in northeastern 
Tennessee and western Virginia and also in central Pennsylvania. 
In Virginia the Eoubidoux fauna occurs at the base of over 2,000 
feet of dolomitic limestone; and the younger Canadian faunas 
