IOWA ACADEMY OF SCIENCES 
119 
DEPOSITIONAL EQUIVALENT OF HIATUS AT BASE 
OF OUR COAL MEASURES; AND THE ARKAN- 
SAN SERIES, A NEW TERRANE OF THE 
CARBONIFEROUS IN THE WESTERN 
INTERIOR BASIN. 
BY CHARLES R. KEYES. 
For a long time it has been known that in Iowa and the 
neighboring states to the south a break in sedimentation 
exists at the base of the coal measures. It has been noted 
in various places in the reports of the Iowa geological sur- 
vey and reference has been made to it in various other 
publications. Of its real significance no hint has ever 
been given. 
Recently the correlation of the various formations 
making up the coal measures has been in progress, and 
some exceedingly interesting results have been attained. 
It has been possible to compare the sections in the northern 
part part of the Western Interior coal field with those of 
the southern part. The basal horizon of Iowa and Mis- 
souri coal measures has been found to belong some 20,000 
feet above the Lower Carboniferous or Mississippian. Our 
Lower Coal Measures are high up in the middle Carbonif- 
erous, instead of being near the stratigraphic bottom. 
West of the Mississippi river the unconformity at the 
base of the coal measures is known to extend in a north 
and south direction from about the north boundary of 
Arkansas to the southern limit of Minnesota. 
From the Mississippi river the rocks have a general dip 
westward. Over a considerable belt of country west of the 
great river the juncture of the coal measures with the 
underlying formations is visible. The width of this belt is 
from 100 to 200 miles. How much farther westward it 
