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 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 niust 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 Kaskaskia 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 u'^rthern 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 



ilMissouri Geol. Sur. , Vol. VIII, p. 351, 1895. 

 §IowaGeol. Sur., Vol. I, p. 118, 1893. 



