The Peneplain. — Davis. 217 
dred feet below the general upland level. These valleys are 
so young that the Missouri itself has not yet developed an 
even slope; witness its several leaps at Great Falls. Innumer- 
able wet-weather side-streams are cutting sharp ravines in the 
larger valle}- sides. It does not seem possible to avoid con- 
cluding that the upland plain is to-day in process of destruc- 
tion by an agency that could not have been in operation while 
the finishing touches were given to its production. It was 
upon this peneplain in 1883 that the necessitv of believing in 
penultimate denudation was first strongly impressed upon me. 
Dr. Waldemar Lindgren, now of the U. S. Geological Sur- 
vey, who was with me in the field, may recall how the con- 
viction grew upon our minds; if I am not mistaken, he ac- 
cepted it before I did. A brief account of the region is pub- 
lished in volume XV of the Tenth U. S. Census Reports. 
The extended plains of Central Russia, as lately described 
b}' Philippson (Zeitschr. Ges. f. Erdk. Berlin, xxxiii, 1898. 
37-68, 77-1 10), have a gently undulating surface at a height of 
200 or 300 meters, broadly continuous, but here and there 
dissected by relatively narrow, steep-sided, young valleys. The 
upland surface is not a structural plain, for it bevels across 
formatioiTS of very different ages: it is therefore a plain of 
erosion. In the south, there is a partial covering of loess, a 
thin veneer often absent and leaving the rock surface visible 
over large areas. In the north, the drift cover is heavier and 
more continuous; but the plateau surface is st4ll the continua- 
tion of the same plain of erosion as in the south. There is 
no record of marine action on the great plain, hence its ero- 
sion is ascribed to the lateral swinging of the lower courses of 
large rivers, but the origin of the rivers is unknown: it can 
only be said that when the erosion was going on, the Russian 
"Scholle" must have stood 200 meters lower than to-dav. The 
narrow valleys have been cut since the uplift of the plain and 
are older than the glacial period (see especially p. 38-42, 54. 
55, 62, of the above-cited essay). This is the largest pene- 
plain of which I have found any account. 
A 6. TJie asserted discordance of peneplain surface and 
rock stmcture is open to question. It is said to be question- 
able "whether there is after all such a lack of sympathy be- 
tween topography and rock a., v.'cture" as has been rcpre- 
