DKPOSITION OF ARKANSAS SERIKS 105 



ent mountainous uplifts in Carboniferous times. North of the Missouri- 

 Arkansas line the region was lantl, to be sure, after the Lower Carbonif- 

 erous marine beds had been laid down. South of that line sedimentation 

 continued 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 Louisiana, and 

 })robably 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 well as from the head toward the east. The condi- 

 tions were then similar to those presented now by the lower Mississij)pi 

 plain. Only the great embayment opened to the west instead of to the 

 south. 



The present Arkansas valley, however, has probably been formed 

 through erosion entirely since Tertiary times and by a system of drain- 

 age in no way dependent upon the Carboniferous drainage. When the 

 great uplift of Missouri and Arkansas rose, the northern part embracing 

 the so-called Ozark isle and the southern part comprising the Ouachita 

 mountains, were made up of resistant limestones, yielded less quickly 

 to erosion than the central soft shales, and the Arkansas river, which 

 happened, in the old peneplain, to traverse the central part of the up- 

 lifted area Avas able to cut its way down as fast as the region rose, and 

 was thus able to maintain its old course. The present uplift, which is 

 due to one general movement, is now apparentl}^ divided into two ele- 

 vated regions separated b}^ a broad valley. 



Recapitulation 



Summing up, it may be said that the facts set forth in the foregoing 

 account indicate that : 



(1) The Coal Measures of the Western Interior basin find a greater 

 development than in any other locality known in America, no less than 

 three great series being recognizable, each of which is as important as 

 the combined series as usually developed in Pennsylvania. 



(2) The Lower Coal Measures of the upper Mississippi valley, though 

 resting in great part directly on the Lower Carboniferous, are in reality 

 high up in the Carboniferous — in the upper half. 



(3) The hiatus at the base of the Coal Measures in the upper Missis- 

 sippi valley has in the south a depositional equivalent, the importance 

 of which is greater than the whole of the Coal Measures to the north. 



