258 The American Geologist. October, i897 
ont, as also in Illinois. Its surface everywhere slopes from 
the inner to the outer side of the curve or away from some 
I)oint in Wisconsin. It is the remaining portion of a great 
broad, but not high, dome-shaped "uplift'' which centred sf)me- 
where in Wisconsin, but which has been completely removed 
throughout the country east and north of the Trenton and Ni- 
agara escarpments, unless possibly its surface may be repre- 
sented by the summits of the Platte and Blue mounds in south- 
western Wisconsin. 
The concurrence of the surface of this great "uplift" with 
the Niagara limestone in Iowa, Illinois, and southwestern Wis- 
consin, and with the Trenton limestone in Minnesota, suggests 
that its characteristic features are due ti> structure and not 
to baseleveling conditions. But its failure to reflect the cor- 
rugations of the strata, and the thinning of the Niagara lime- 
stone at its eastern borders, which thinning is not similar to 
that of northern Iowa, there having originally been at least 200 
feet of this formation over the Erin mound in Illinois, seem 
to indicate that it is a portion of a widespread peneplain w^hich 
was uplifted differentially into a broad, low dome, and subse- 
quently much dissected by erosion. It has been demonstra- 
ted that in southeastern Minnesota and extreme northern Iowa 
this supposed peneplain corresponds to the late Cretaceous 
baseleveled plain, while there is some evidence of Cretaceous 
strata on its surface as far south as Delaware county. 1 con- 
sider it a legitimate inference from the evidence that the sur- 
face of the Niagara plateau in both Iowa and Illinois every- 
where represents the Cretaceous peneplain, so that a reference 
of the summit plane of the "mounds" in northwestern Illinois 
to this category is not mere hypothesis. 
Since beginning my investigation into the character of the 
topographical features of the Mississippi valley, I have had 
constantly in mind the probability of many of the dissected 
plains which I encountered having been due primarily to the 
structural inequalities of the rock strata. But, although oft- 
en corresponding to the surface of some specially resistant 
limestone, I have nearly ahvays been able to trace these plains 
across the beveled edges of soft strata. And I have concluded 
that, in all probability, practically all of the extensive so- 
called structural plains of eastern America, are parts of one 
