IOWA ACADEMY OF SCIENCES. 
127 
The conditions under which the Arkansan series was 
deposited are of unusual interest. The deposition of such 
an enormous mass of sediment as is found making up 
the coal measures of the Arkansas valley must have re- 
quired some unusual conditions. Branner* has attempted 
to explain the circumstances as follows: 
If we inquire into the reason for the great thickness of coal measures 
sediment in the Arkansas Valley, I believe it to be found in the drainage 
of the continent during Carboniferous times. The rocks of this series in 
Arkansas contain occasional marine fossils, and these marine beds alter, 
nate with brackish or freshwater beds whose fossils are mostly ferns and 
Such like land or marsh plants. This part of the continent was, therefore, 
probably not much above tide level. The drainage from near the Catskill 
mountains in New York flowed south and west. The eastern limit of the 
basin was somewhere near the Archaean belt extending from New England 
to central Alabama. This Appalachian water-shed crossed the present 
channel of the Mississippi from central Alabama to the Ouachita uplift, or 
to a water-shed still farther south and now entirely obliterated and buried 
in northern Louisiana. In any case the drainage flowed westward through 
what is now the Arkansas valley, between the Ozark island on the north 
and the Arkansas island on the south. 
The chief objection to this idea is, that we now know 
that the northern Ozark isle and the Ouachita part of the 
uplift did not exist as mountainous uplifts in carboniferous 
times. North of the Missouri-Arkansas line the region was 
land, to be sure, after the lower Carboniferous marine beds 
were laid down. South of that line sedimentation con- 
tinued in deepening waters. The sediments were carried 
from the north or northeast and dumped off the shore, 
rapidly building the latter outward. 
There may have been a great land area in northern Lou- 
isiana, and probably was. If so, what is now the Arkansas 
river valley was a broad, deep estuary opening out to the 
west. And the sediments came in from both sides as w T ell 
as from the head towards the east. The conditions were 
then similar to those presented now by the Lower Mississippi 
plain, only the great embayment opened to the west 
instead of the south. 
The present Arkansas valley, however, has probably been 
formed entirely since Tertiary times, and by a system of 
♦Am. Jour. Sci.,(4),vol II., p. 236,1896. 
