No, 386.] THE PENEPLAIN. 135 
cutting downward with energy, the eastern part of Massachu- 
setts has been lowered to a degree where, in Professor Tarr’s 
words, ‘there may well have been an approach toward the con- 
dition of a local peneplain.”’ That is, the relatively insignificant 
amount of work necessary to hew out the Deerfield valley must 
have taken as much time as the wasting to an almost-plain of a 
coastal belt from twenty to thirty miles broad at least. From 
his own point of view, Professor Tarr would explain this strong 
contrast in the rate of cutting by the fact that the“ coastal 
region is the locus of river systems with short courses to the 
sea, and, consequently, great head and erosive power. But how 
is it that the plateau of western Massachusetts is not similarly 
worn down by the rivers which have a quick course to the sea 
via the long-opened-up Triassic lowland on the west? (This 
implication, in the last quotation, that subaerial erosion may pro- 
duce a peneplain near the coast on the scale permitted by the 
range of elevations in Massachusetts, seems to be a virtual aban- 
donment of his whole position on the part of Professor Tarr.) 
The theory of differentiat uplift is antecedently probable, and 
the facts of present geographic form are exactly those that 
would be expected to result from the elevation of New England 
at the close of a former complete cycle of denudation. A tilt- 
ing toward the east and south would give the revived rivers of 
the northern and western parts of the peneplained region power 
to etch out of the terrane narrow valleys on the hard rocks and 
broader valleys on the less resistant ones. The broadening of 
the valleys would progress as the time allowed, as the area of 
soft rocks permitted, and as the river was strong. Where the 
second and third conditions are satisfied, as in the case of 
the Connecticut, we have a broad open (Triassic) lowland of the 
new cycle; when the second is not so well fulfilled, the same 
river gives us a much narrower lowland, that on the “ Calciferous 
mica-schists’’ of southern and central Vermont. 
One of the most strongly emphasized objections of Professor 
Tarr to the theory of peneplanation is based on the occurrence 
of monadnocks, or residual hills, on the New England upland. - 
He asks: “Are the monadnock rocks essentially harder than 
the other hilltops of the neighborhood ? ” and answers the ques- 
