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 Archsan 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, 

 rapidl}^ 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 

 w^est. And the sediments came in from both sides as well 

 as from the head towards the east. The conditions were 

 then similar to those presented now by the Lower Mississippi 

 plain, only the great embay ment 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, i8g6. 



