TJie Pe?ieplai/i. — Tarr. 365 
plain it by one of three conditions, either post-peneplain de- 
nudation, or ancient irregularities upon this peneplain surface, 
or diflferential elevation of the peneplain since its formation. 
In the present condition of New England and New Jersey. 
I am unable to see any evidence that the region was ever re- 
duced further than the condition of full maturity of topogra- 
phy, — that is, a region of hills and valleys of considerable vari- 
ety, and, away from the sea shore, of rounded but considerably 
elevated mountains. That this mature mountain region has 
been subjected to later elevation, which has rejuvenated the 
rivers, seems certain. According to this, the present New 
England topography is mainly one of reduced mountains, 
lowered to" the stage of full maturity, then elevated and made 
more rugged. By this explanation it is held that the region j 
was never reduced to the peneplain stage, but has always been,/ 
as it still is, a mountainous section, though once less mountain- 
ous than now, because of the recent uplift. 
So far as I can see, the facts in the fielTi are in fuller har- 
'mony with this explanation than with that of the peneplain./ 
The present marked irregularity of surface is explained with- 
out other assumption than that certain places were formerly, 
as now, either high or low, as they would naturally be in a re- 
gion of mature mountains. It does away with the necessity 
of assuming long periods of time during which the land re- 
mained at approximately one level. To reduce a mountainous I 
region to the stage of maturity is an easy task compared with \ 
the reduction of a mature mountain region to a peneplain. It 
would be impossible to state what the ratio of time is. but it 
is certain that to lower a mature mountain to a peneplain must 
take many times as long as to reduce a mountainous area to 
that of maturity. Moreover, much more variation in elevation 
is possible under the explanation here proposed than under 
that of the peneplain. While the mountains were being low- 
ered to the stage of maturity, there might be very much fluc- 
tuation of level without marked interference with the contin- 
uation of the process of production of mature forms. Besides 
this, while there are no existing peneplains, there are at pres- 
ent many regions of reduced mountains approaching the stage 
of maturity — witness the very regions under consideration. 
