Provincial Carboniferous Terranes. 307 



pian period. Over the entire area now known as the Upper 

 Mississippi valley a prolonged period of subaerial erosion 

 took the place of sedimentation. West of the line now occu- 

 pied by Mississippi river the land border extended southward 

 beyond the latitude of the present Missouri-Arkansas boundary, 

 as lately clearly shown by Marbut.* The full significance of 

 this position of the old shore-line at the close of the Lower 

 Carboniferous is considered elsewhere.f Taken in all its vari- 

 ous aspects, the data at hand indicate that the length of time 

 occupied in the formation of the Mississippian was but little 

 more than that during which the Des Moines series was laid 

 down. Compared with the Des Moines, the Mississippian 

 should probably be placed at about one and one-half. 



The enormous development of Coal-measures in central 

 Arkansas has been an anomaly in the stratigraphy of the Car- 

 boniferous. The thickness, estimated by Brannerj: to be over 

 20,000 feet, is much greater than that of the entire Carbonif- 

 erous elsewhere on the American continent. The conditions 

 permitting this vast accumulation were certainly very local and 

 unusual. They are briefly stated§ as follows : South of the 

 latitude of the Boston mountains in northern Arkansas, Car- 

 boniferous sedimentation continued on without interruption 

 from the Mississippian; while in the north erosion took place. 

 The sediments from the northern land area, where erosion was 

 going on vigorously, were carried southward 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 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 embayment opened to the west instead of to the south. 



The present Arkansas valley, however, has probably been 

 formed through erosion largely, if not entirely, since Tertiary 

 times and by a system of drainage in no way dependent upon 

 the Carboniferous drainage. When the great uplift of Mis- 

 souri and Arkansas rose, the northern part, embracing the 

 so-called Ozark isle, and the southern portion, comprising the 

 Ouachita mountains, were made up of resistant limestones, and 

 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 uplifted area, was able to main- 



* Missouri G-eol. Sm\. vol. x, 1896, p. 83. 



+ Bull. Geol. Soc. America, vol. xii, 1901, p. 173. 



% This Journal (4), vol. ii, 1896, p. 235. 



£ Bull. Geol. Soc. America, vol. xii. 1901, p. 195. 



