194 C. R. KEYES A DEPOSITION AL MEASURE OF UNCONFORMITY 



The dome did not rise as the " Ozark isle " until lono- after Carbon- 

 iferous sedimentation had ceased. 



THICKNESS OF THE CARBONIFEROUS 



The thickness of the Coal Measures of the Mississippi valley is greater 

 than anywhere else in the United States. In two east-and-west cross- 

 sections, one on the north side of the Ozark dome and the other through 

 the Arkansas valley, are contrasted the Carboniferous series which pre- 

 sent about the following measurements : 



Northern Soutliern 

 section. section. 



Oklahoman 1,500 1,500 



Missourian 2,000 1,500 



Des Moines 500 3,500 



Arkansan Wantin«:. 20,000 



Mississippian ! 1 ,000 1,500 



Conditions under which Arkansan Skriks was Deposited 



The deposition of such an enormous mass of sediments as is found 

 making up the Coal Measures of the Arkansas valley must have required 

 some unusual conditions. Branner * has attempted to explain the cir- 

 cumstance as follows : 



" If we inquire into the reason for the great thickness of Coal Measure 

 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 fresh water beds, whose fossils are mostly ferns and 

 such like land or marsh })1ants. This part of the continent was, there- 

 fore, 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 Arcbean belt, extending from 

 New England to central Alabama. This Appalachian watershed crossed 

 the present channel of the Mississip])i from central Alabama to the 

 Ouachita uplift or to a watershed still farther south and now entirely 

 obliterated and buried in northern Tvouisiana. 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 view is that we now know that the northern 

 Ozark isle and the Ouachita part of the u{)lift did not exist as the pres- 



* Am. Jour. Sci. (4), vol. ii, 1896, p. 236. 



