1897.] ASHLEY — GEOLOGY OF ARKANSAS. 219 



paper, but the structure of the region shows that they are from 

 rocks occupying the same geologic horizon. 



The general structural features of the Ouachita uplift are so much 

 like those of certain parts of the Appalachian structure that one nat- 

 urally assumes at the outset that we have here an Appalachian type, 

 and indeed a part of the original Appalachian folding. It cannot 

 be stated positively that this is not the case, but it is my own opin- 

 ion that the Ouachita uplift is the structural equivalent, not of the 

 Appalachians, but of the Nashville Silurian basin and of the Cin- 

 cinnati arch. To be sure the Ouachita anticline is closely pressed, 

 while the Nashville and Cincinnati folds are very gentle. This 

 difference is, however, a matter of structural detail only. The 

 equivalent of the Appalachian chain and the source from which the 

 Lower Coal Measures sediments of south Arkansas appear to have 

 been derived (in part) was a continuation across the present 

 Mississippi drainage of the pre-Cambrian rocks of Alabama and 

 Georgia, etc. This region, that is the gap through which the 

 Mississippi river now flows, has been lowered, partly by erosion, 

 but chiefly by downward orographic movements and it is now 

 buried beneath the Cretaceous and Tertiary sediments of south 

 Arkansas and of north Louisiana and Texas. 



This hypothesis seems to be borne out by the following facts : 



L The fossils found on the south side of the Ouachita anticline 

 are coal plants like those from the Cumberland plateau of east 

 Tennessee, eastern Kentucky, and West Virginia. 



IL The sediments thicken and become coarser to the south as 

 one leaves the Ouachita uplift (Ashley) just as in the upper Ohio val- 

 ley they thicken and become coarser east of the Cincinnati arch. 



in. In central Texas, northwest of Austin, is an Archean and 

 Cambrian area which appears to be the southwestern end of the old 

 Appalachian chain, and north of it are Carboniferous rocks. 



IV. If the Cincinnati, Nashville and Ouachita arches are assumed 

 to be the same general fold, this line is found to continue into 

 Indian Territory and Oklahoma, and the arch as a whole is parallel 

 with the Appalachian system save across the break made by the 

 present Mississippi river depression where the same conditions 

 would be expected. 



V. The peneplain south of the Ouachita mountains and on which 

 the Cretaceous beds were deposited slopes toward the Mississippi. 



VI. The Arkansas river drainage formerly (in Carboniferous and 



