PHYSIOGRAPHY OF THE BOSTON MOUNTAINS 701 



the time the extensive denudation was oroinsf on over the lat- 

 ter. So low must it have stood that the strata now composing 

 their summits suffered but little erosion, while the same beds 

 extending northward suffered much because of their greater 

 height. If this be true, the actual amount of degradation 

 suffered by the Boston Mountain region is indeterminable ; 

 but as there was more or less of it, and the region stood at a low 

 level, it would be considered a peneplain. The elevation, which 

 must have occurred in late Tertiary or in post-Tertiary time, 

 was greatest along the present east-west axis of the plateau, 

 gradually decreasing to the northward, and changing the region 

 from a low, monotonous plain to a plateau approximating 2,500 

 feet i \ height, greatly modifying the former drainage and insti- 

 tuting that of the present. 



Aside from the difference in topography between the region 

 under discussion and the one to the north, the writer cannot at 

 present claim very great support for the idea herein presented. 

 There are, however, some other facts that seem to lend the 

 hypothesis support, (i) The region being on the border of the 

 Ozark uplift, it is probable that during the greater part of its 

 history it lay at a low level and consequently suffered compara- 

 tively little from erosion. (2) The outliers of the Boston 

 Mountains to the north are as a rule lower than the main 

 plateau, though capped by the same rocks, thus indicating an 

 axis of elevation to the south of the outliers. (3) The eastward 

 course of White River and its tributaries may be due to their 

 having been diverted from what would seem a more natural 

 southern course, at the time of the uplift. 



A. H. Purdue. 



University of Arkansas, 

 Fayetteville, Ark. 



