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IOWA ACADEMY OF SCIENCES' 
of the Mississippi valley, and even of other portions of 
North America. 
Periods Northern Section 
Southern Section xThiekngss 
Figure 4. --Shows the relative thickness of the members of the Mississippi valley coal measures 
north and south. 
The thickness of the coal measures of the Arkansas 
valley as estimated by Branner* is nearly 24,000 feet. If 
present correlations be correct the highest of these beds in 
Arkansas are not above the horizon of the Bethany 
limestone of Kansas. For the deposition of such an enor- 
mous sequence there must have existed exceptional con- 
ditions. The great development of the coal measures in 
Arkansas is not widespread, but is confined to a compara- 
tively limited area. 
The noteworthy feature in the lithology of the Arkansas 
coal measures is their make-up of shales and sandstones, 
with an almost total absence of marked limestones. While 
this characteristic is remarkable through such an extensive 
succession, it points clearly to attendant physical condi- 
tions that are unmistakable, and that are now known to 
be in perfect harmony with the historical record of other 
parts of the region. 
The Lower Carboniferous formations are well understood 
in Arkansas. It is now known that the Boone cherts are 
Am. Jour. Sei., (4), Vol. II, p. 235, 1896. 
