Baseleveiing and Organic Evolution. — Woodworth. 221 
ation in organisms than are the rapid changes which attend 
submergence. 
Uplift and dissection of the peneplain. 
With the revival of erosion through the elevation of the 
land and the consequent entrenchment of the rivers in the 
surface of the uplifted peneplain, the subtle organic coating 
will feel again the changed environment. Valleys and divides, 
cradles, barriers, and a choice of location, will again be offered 
to animals and plants as a change from the monotonous con- 
ditions of the completed peneplain. Following the altered 
environment will come the modified organism, varied by rea- 
son of migration or isolation and the multitude of causes 
which geographic vicissitudes originate in the organic realm. 
In this change from a lowland, interrupted only by monad- 
nocks, to a dissected upland, the lowland species will find the 
scales turned against them in favor of the survivors of the 
old highland life still clinging to the elevated monadnocks or 
some adjacent region of continued uplift. 
PART III. THE JURA-CRETACEOUS PENEPLAIN. 
From these general considerations we now turn to a period 
of baseleveiing which is particularly well marked in North 
America in the existing topography and also, as I hope to 
show, in the organic history of the Mesozoic, the era during 
which the Jura-Cretaceous peneplain was elaborated. This 
peneplain forms the most conspicuous topographic feature in 
eastern North America, and traces of a plain of denudation of 
this date are not wanting on the Pacific coast* and over the 
interior of the continent. While it is yet too early to state 
that this peneplain was so widespread as to affect the atti- 
tude of the continent as a whole, the evidence points to this 
conclusion and so to the effects in the organic world which 
would follow from such ;i condition of geography in the mid 
die Mesozoic. 
The peneplain is well developed in the middle Atlantic 
statesf and southern New England, as it is also in the south- 
*Mr. J. S. Diller's generalized section across the Sierra Nevada range 
exhibits thf <>u 1 1 ii fan uplifted, tilted and Faulted peneplain. See 
fig. 1. Bulletin 33, U. 8. Geol. Survey, p. Ki. 
fProf. W. M. Davis, Rivers and Valleys of Pennsylvania, Nat. Geo- 
graphic Magazine, vol. i, 1889. 
