IOWA ACADEMY OF SCIENCES. 123 



terrane is easily identified, passing upward, south of the 

 Boston ridge, into the coal measures. 



The basal horizon of the lowest coal measures of Missouri, 

 or Des Moines series, is believed to extend southward and 

 to the south of the Arkansas river to coincide approxi- 

 mately with the Grady coal horizon or the base of the 

 Cavaniol. 



With the base of the Des Moines series of Missouri thus 

 located in Arkansas, and the top of the lower Carbonif- 

 erous well defined it leaves in the south an immense thick- 

 ness of nearly 19,000 feet of sediments that are in the north 

 wholly unrepresented by deposits. The 19,000 feet of sed- 

 iments were laid down during the period represented by 

 the stratigraphic break at the base of the northern coal 

 measures. 



The magnitude of the hiatus at the base of the coal 

 measures of Iowa, Missouri, and Kansas is readily appreci- 

 ated when we find a place where sedimentation uninter- 

 rupted attained a vertical measurement of 19,000 feet. The 

 period of which there is no measurable record in one part 

 of the region finds in an adjoining district sediments of 

 greater significance than all the coal measures above the 

 break. 



Here, then, is a case in which, on the one side of an old 

 shore-line, is the land that suffered profound denudation, 

 and on the other the water area in which sedimentation 

 was carried to a prodigious extent. In point of time the 

 one is the exact equivalent of the other. 



Arkansas Series. 



If the recent correlations of the different sections of 

 the coal measures -n the Western Interior basin can be 

 regarded as even approximate, there exists in the south, 

 below the basal horizon of the Des Moines series, another 

 great series which is now called the Arkansan series. 



Heretofore the coal measures of Arkansas have been 

 regarded as anomalous. They present an enormous devel- 

 opment as compared with the coal measures of other parts 



