toWA ACADEMY OF SCIENCES. 
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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. 
Arkansan 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 Arkausan 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 
