BOSTON MOUNTAIN PHYSTOGRAPHYV 163 
recognize valleys of several cycles trenched beneath each other 
and coalescing so as to be apparently the product of a single 
cycle of erosion. (I am using the term ‘‘cycle”’ as referring to 
the time between uplifts with rejuvenations of the drainage.) 
Eliminate all those portions of the valleys which are below the 
supposed ‘‘main Tertiary’? baselevei plane, say five hundred 
feet below the general summit level of the mountain, and much 
of the youthfulness will disappear; the valleys remaining will be 
comparatively broad and flat-bottomed. 
Another indication of immaturity recognized is the flat sum- 
mits. This feature is maintained by the rather resistant sandstone 
strata capping the hills and its importance is exaggerated by its 
abnormal character. 
If the summit plain of the Boston Mountain was not 
developed anterior to that of the lower county on the north, it 
must have been elevated by faulting. The northern face of the 
Boston Mountain is ‘‘an irregular, but bold escarpment from five 
hundred to one thousand feet high.”’ This is entirely too sinuous 
to be a degraded fault scarp. Great promontories project out 
into the plain country on the north. The phenomena are char- 
acteristically those of differential erosion. If a fault with a throw 
of five hundred or one thousand feet existed, it must have dis- 
located the Upper Carboniferous sandstone in such a conspicu- 
ous manner as would long since have attracted attention. In 
short, I cannot see any escape from the conclusion that the Boston 
Mountain was a residual on the baselevel represented by the 
plain to the north—the Ozark plateau. 
The criteria on which I would base the recognition of a dis- 
sected peneplain on the nearly horizontal rocks of the central 
Mississippi region are: (1) many of the summits must be trun- 
cated; (2) these flat summits must fall into a single plane, only 
very slightly tilted; (3) the plane must pass across diverse 
formations without deformation; (4) the plane must to some 
extent bevel the edges of the slightly inclined strata; and (5) 
the dissected plain must be extensive enough to make it improb- 
able that it was developed by marine erosion. Such other 
