490 CHARLES R. KEYES 



all compensating, or all cumulative in their effects. Between 

 the two extremes the sum of the antagonistic tendencies may 

 have very variable values. The present valley of the Arkansas 

 River as it crosses the Ozark highlands is a noteworthy illustra- 

 tion in which the combined effects of perfectly independent 

 processes are curiously cumulative in character. It is on this 

 account chiefl}^ that the real facts concerning the development 

 of the great uplift have been so largely obscured. 



Summing up : The different geological conditions when the 

 Arkansas River initiated its course across the Ozark region, (i) 

 an undeformed lowland flat in which the strata had been folded 

 to a marked degree before being beveled and the country 

 reduced to the state of a peneplain, (2) a remarkable, yet nar- 

 row, belt, bordered on either side by resistant rocks, of soft 

 shales of prodigious thickness which, when a new epoch of 

 uprising was inaugurated, enabled the stream to easily keep its 

 channel down to the general base level of the country surround- 

 ing the uplift, and (3) a broad structural trough, which, how- 

 ever, was only one of many synclines nearby and parallel to it — 

 were highly cumulative in effect in imparting to the uplift the 

 present aspect of twin elevations. By this singular combination 

 of geological conditions the Arkansas River instead of being 

 forced to turn aside by the great topographic dome which, out 

 of the Cretaceous peneplain, arose athwart its path, was able to 

 saw in two the arching strata. 



Topographically, the Ozark highlands form two distinct ele- 

 vated, regions. Structurally as well as topographically the 

 Arkansas valley is a trough. But structurally the Ozark high- 

 lands, as a whole, form an immense dome bowed from the Red 

 River to the Missouri. 



Charles R. Keyes. 



