IOWA ACADEMY OF SCIENCES 
121 
The topographic contrasts are certainly nearly as marked 
in the old as they are to-day over the same area. 
The phenomenon under special consideration has been 
generally regarded as local in its nature; the same, as many 
unconformities recurring at many places in the coal meas- 
ures. That it signifies an important sequence of events 
has never been sufficiently emphasized. That the horizon 
is really a great hiatus has never been fully considered. 
That the interval represents a period in the history of the 
region of much longer duration than it took to form all of 
the coal measures above it is a phase of the subject never 
before suggested. 
It has lately been shown jj that the present Ozark uplift 
is of comparatively recent date; that is, Tertiary. In con- 
sidering the region as it was in Carboniferous times, the 
dome must be neglected, and the area regarded as forming 
a lowland plain, the same as the rest of the region was 
known to be. This is farther indicated by the fact that on 
the highest parts of the dome remnants of the coal meas- 
ures are still found on the beveled edges of the older strata- 
The oscillation of the Carboniferous shore-lines in the 
upper Mississippi valley has already been described in 
detail§. This evidence goes to show that immediately 
after the Ivaskaskia beds were laid down, land existed north 
of the present Arkansas-Missouri boundary. This was a 
region of profound and prolonged denudation. South of 
the line sedimentation continued. The land waste from 
this northern district was carried into the southern water 
area. 
The northern area, after the close of the early Carbonif- 
erous period, being an area of denudation suggests an area 
to which the waste must have been carried and deposited. 
There is also suggested a depositional measurement of the 
erosional period. 
In correlating the Iowa and Missouri formations of the 
coal measures with those of the Arkansas valley a tabular 
UMissouri Geol. Sur. , Vol. VIII, p. 351, 1895. 
§Iowa Geol. Sur., Vo!. I, p. 118, 1893. 
