BOSTON MOUNTAIN PHYSIOGRAPHY 161 
appreciate the true significance of the evidence, and the figure 
which he gives on p. 487 is virtually in accord with my present 
conception of the subject. It is believed (for the same reason 
that a dissected peneplain is recognized in the summits of the 
Appalachian ridges) that the summits of the Ouachita moun- 
tains are remnants of an old plain of denudation (whether in age 
Cretaceous or early Tertiary matters not), and that this plain 
would descend rapidly to the axis of the Arkansas valley had it 
not been totally destroyed over that broad structural and topo- 
graphic depression; and that then it would rise northward 
because of a monoclinal folding inaugurated in early Quater- 
nary time, and perhaps continuing today. The question is, 
Would this projected plane coincide with the summit of the 
Boston Mountain? 
Let us eliminate later deformation and straighten out the 
supposed lower or “main Tertiary’ plane of erosion baselevel. 
Our theory supposes that the earlier or ‘‘ Cretaceous” baselevel 
plane would first become distinctly differentiated from the later 
in southern Missouri, and the difference between them would 
have increased to about five hundred feet on the northern edge 
of Boston Mountain. Continued to the first ridge of the Oua- 
chita system, the difference between the two planes might be 
expected to be one thousand feet, but through the concavity 
common to the border of all symmetrical dome-like uplifts it 
would be natural to expect an increase to at least the present 
difference in level of the planes at the summit and base of 
Sugar Loaf Mountain, about 1200 feet, I believe. 
But Professor Purdue has raised a question of the validity of 
this interpretation by his erosion studies, the pertinence of which 
cannot be ignored. He says, in reference to the Boston Moun- 
tain region: 
The drainage of the region is that intermediate between youth and 
maturity. The streams are vigorous and have completely dissected the 
plateau bythe formation of gorges from 500 to 1000 feet deep, thus producing 
a very rugged topography over the whole region. Between these gorges the 
slopes often meet, forming more or less rounded hills; but more frequently 
