TJie Peneplain. — Tarr. 367 
terfere. This rate of denudation among these neighbors 
would at first be rapid; for, in the inception of the work, the 
elevation was great, the slope steep, and the rocks were ex- 
posed to strong winds and powerful frost action, while they 
were not protected by trees. The rate of downwearing, as 
we see upon similar peaks in the higher mountains of the 
present, would have been much more rapid than at anv later 
stage, decreasing in rapidity as they were lowered; 
though still being worn down rapidly until the zone of the; 
timber line was reached. Then conditions of an entirely new 
kind would have been introduced, and, from that line down- 
ward, the rate of denudation of the peaks would greatlv de- 
crease, partly because of the lessened slope, but chiefly be- 
cause of the protection of the forest, which holds the disinte- 
grated pieces in place, and helps make a protection of residual 
soil. As the forest covering became greater, and the slope 
less, the soil covering would become deeper, and the rocks 
more and more protected, until denudation had become ex- 
ceedingly slow. These two peaks, starting at the same level, 
having the same kind of rock throughout, and exposed to the 
same conditions, would reach this stage of development 
(namely their crests at approximately the timber line), at about 
the same time; and, as their crests sank lower below this 
level, the peaks would still stand at about the same elevation. 
In an extensive mountainous region there may have been a 
number of such cases. 
But there would not be many such peaks of the same 
hight or so similar that they would be reduced at nearly the 
same rate. Some would be of easily denuded rock, and, in 
time, these would be very much lowered, while the harder 
ones stood well above the base level. There would at first 
be very marked ruggedness. partly the result of difference in 
original elevation, and partly the result of the effects of sub- 
aerial denudation upon the much elevated and differentiated 
surfaces. One peak, perhaps of slightly less durable rock 
than a neighbor, would be lowered at a very much more rapid 
rate than its neighbor. But there would come a time when 
this difference in rapidity would be very much diminished, 
even if the rock of the two peaks were quite different. This 
time would come when the zone of trees was reached; and 
