130 THE AMERICAN NATURALIST. {VOL XXXIIL 
Russia or of the plateaus of the Colorado Canyon. A striking 
example of these buried plains of denudation has recently been 
seen by the writer in that treasury of physiographic illustra- 
tions, the Crimea. As the steamer coasts the southwest shore 
of the peninsula, one sees a dark line running through the cliffs 
with almost ideal straightness for many miles; this line repre- 
sents an old land surface on the well-flexed Jurassic, in which 
the steeply dipping beds are truncated as nicely as if pared 
down by a huge knife. Upon them lie the light-tinted Eocene 
sediments, horizontal, and signifying a perfect case of uncon- 
formity. Now we believe that this smoothing of the older 
terrane in each case is capable of explanation, and as yet only 
two theories have been advanced to serve the purpose. The 
first, the theory of subaerial denudation, has by far the weight 
of evidence in its favor; the second, that of marine denudation 
(sea-benching on a large scale), is in the highest degree im- 
probable, for reasons that need not concern us here. Both 
theories alike demand a fairly constant relation of land and sea, 
not absolute quiescence from up and down movement of the 
land, but comparatively faint oscillations of the land during a 
long period. If we are going to explain these fossil land sur- 
faces at all, then we are forced to posit a more or less constant 
baselevel for each. We have, furthermore, in these same 
regions positive evidence that they have since been capable of 
a long-continued absence of important movement of the land 
with respect to their common baselevel, the level of the sea. As 
far as the wild Archzan tracts of Canada are known, it seems 
to be the fact that at least one million square miles of that 
country have remained above the sea during most of Palzeozoic 
time and all of Mesozoic and Tertiary time. Conversely, from 
the Cambrian to the Triassic, the Russian terrane was sunk 
below the sea. 
3. We must again join issue with Professor Tarr, when he 
contends that there are no extensive peneplains of modern date 
existing on the earth. The Russian Plain, from St. Petersburg 
eastward and northward, is essentially in this condition ; while, 
considering its vast extent, the whole of the European part of 
the empire has suffered but a minimum amount of warping from 
