162 OSCAR H. HERSHEY 
the intervening area is occupied by flat-topped, sandstone-capped hills of 
limited extent. 
Of the lower plain to the north, he says: 
This is a region of great denudation. .... Its streams are mature, the 
valleys- comparatively wide, and the topography in general presents the 
aspect of much greater age than that of the Boston Mountains. 
“Youth” and ‘‘maturity”’ as applied to streams and valleys 
are terms relating to types and not to age as measured in years. 
The topography and drainage of a land never greatly elevated, 
and possessed of a humid climate and soft formations may be 
ever so senile in type and yet no older in years occupied in its 
development than another region characterized by immaturity 
of its physiographic features. This is an axiom of the science 
of physiography. The less mature character of the drainage 
and erosion forms of the Boston Mountain region than of that on 
the north does not necessarily militate against a reference of the 
development of its summit plain to an earlier cycle of erosion. 
Some of the causes which have tended to bring about this 
result are as follows: 
Boston Mountain owes its prominence and preservation as a 
residual on the ‘“‘main Tertiary’ peneplain largely to the resist- 
ant properties of the Upper Carboniferous sandstone which enters 
so largely into its composition. On the south, the Arkansas 
valley was developed on a belt of soft shales and all traces of 
residuals were swept away. On the north, the elevation of the 
land was less, the streams soon cut down to baselevel and could 
devote a large part of the cycle of erosion to widening their 
valleys. The Boston Mountain was on the main divide between 
the White and Arkansas river systems, and only very small 
streams operated on it while the country on either hand was 
being ground away by powerful trunk streams. The latter 
developed meandering courses indicating maturity, but we could 
hardly expect to find similar winding courses in the small head- 
water streams of the main divide. 
The evidence of youth in the Boston Mountain valleys is 
somewhat deceptive, especially on the southern slope. Here I 
