138 THE AMERICAN NATURALIST. 
The most valuable part of the paper is that in which Pro- 
fessor Tarr emphasizes the effect of the tree-zone in retarding 
the progress of denudation and the consequent tendency of the 
higher summits: of a mountain range. Pike’s Peak is a good 
example in point, the rounded slopes below that zone contrast- 
ing finely with the serrate spurs of rapidly degrading naked 
rock far up. Professor Tarr would find in the halting of one 
peak at the snow-line until overtaken in the downward journey 
by its neighbors an explanation of the even tops of “ mature ” 
mountains, in particular those of New England. But the very 
rough accordance of levels expected in this ideal scheme, and 
represented in parts of the Alps and Carpathians, is not in the 
slightest degree comparable with the New England conditions, 
nor with the “ very even-topped Kittatinny Mountains,” referred 
to in the article. Thus it is impossible to conceive that the 
New England facet can be accounted for as formed at a van- 
ished tree-zone. Having reached the tree-zone, the forces of 
denudation then tend to reduce the mountains still further, but 
now according to the law of soft and hard. Now, since the 
effective tree-zone may be roughly put at an elevation of be- 
tween 4000 and 5000 feet in New England, we are as badly off 
as ever in arriving at a conception of how a nearly plane surface 
can be produced on the rocks of differing hardness composing 
the mountains, unless that surface is the result of the beveling 
down of all the terranes nearly to baselevel. Finally, there _ 
are well-established peneplains, such as that on the old rocks of 
Missouri, where the tree-zone theory cannot avail, because the 
_ district was undoubtedly never raised, at any time during the 
geographical cycle represented, to a height that would bring 
about the differentiation of a tree-zone. It is an old plateau of 
horizontal or but gently folded rocks. , 
HARVARD UNIVERSITY, December 10, 1898. 
