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 



