2o8 The American Geologist. April, i899 
reason, as it seems to me. that antecedent rivers have been so 
frequently mentioned in the last thirty years, and peneplains 
in the last ten. 
It was in Powell's "Exploration of the Colorado River" 
(1875) that the "peneplain idea," along with a number of other 
important facts and principles, first came to my notice. The 
idea is not stated categorically, but when describing the even 
surface of deformed rocks beneath the horizontal Carbonifer- 
ous strata in the Colorado can\-on, Powell said that "aerial 
forces carried away 10,000 feet of rocks by a process slow yet 
unrelenting, until the sea again rolled over the land," and the 
evenly denuded surface is referred to as "the record of a long 
time when the region was dry land" (p. 2 12). In a later work, 
the same author writes: — "Mountains cannot long remain as 
mountains; they are ephemeral topographic forms. Geologic- 
ally speaking, all existing mountains are recent; the ancient 
mountains are gone" (Geology of the Uinta Mountains, 1876,^- 
196). Again, "in a very low degree of declivity approaching 
horizontalit), the power of transporting material is also very 
small. The degradation of the last few inches of a broad 
area of land above the level of the sea would require a longer 
time than all the thousands of feet which might have been 
above it, so far as this degradation depends on mechanical 
process — that is. driving or flotation; but here the disintegra- 
tion by solution and the transportation of material by the 
agency of fluidity come in to assist the slow processes of 
mechanical degradation, and finally perform the chief part 
of the task" (Ibid., 196). Button referred to Powell's having 
given precision to the idea of baselevel, an idea probably 
known previous!}- in a general way to many geologists. '"All 
regions" — Button says — "are tending to baselevels of erosion, 
and if the time be long enough each region will, in its turn, 
approach nearer and nearer, and at last sensibly reach it" (U. 
S. G. S. Monogr. II, 76).* In Great Britain, where the litera- 
*I had expected to find some similar sentences in Gilbert's "Geology 
of the Henry ]\Iountains."" but discover instead the following state- 
ment: — ''It is evident that if steep slopes are worn more rapidly than 
gentle, the tendency is to abolish all differences of slope and produce 
uniformity. The law of uniformity of slope thus opposes diversity of 
topography, and if not complemented by other laws, would reduce all 
drainage basins to plains. But in reality it is never free to work out 
its full results: for it demands a unifoimity of conditions which nowhere 
exists" (p. 115). 
