108 The American Naturalist. [February,. 
on organisms, of the changes in physical geography which 
arise in the process of baseleveling.” One of his most im- 
portant conclusions is that the great change at the close of the 
Mesozoic era, from a characteristically reptilian to an equally 
characteristic mammalian land fauna, was due primarily to 
the great change in the physical features of the continent 
which is known to have closed the Cretaceous and introduced 
the Eocene period. The low-lying denudation plain of the 
later Mesozoic era was rapidly changed into the more elevated 
hilly land surface of the early Tertiary era. Toward the clos- 
ing stages of Tertiary time, a peneplain had again been 
formed over a large portion of the continent, but it was not so 
perfect nor so extended as the previous one, and, hence, its. 
effect upon organic life was less marked. 
The main Tertiary or Tennesseean period of base-leveling 
in the eastern portion of North America was terminated by a 
slight seaward tilting of the continental plateau. A portion 
of the former land area was depressed beneath the sea, and om 
it was laid down a marine formation of red and orange grav- 
elly loam—the Lafayette formation. The land on the alterna- 
tive side of the axis of general deformation was slightly up- 
lifted, thereby stimulating the streams to greater activity, 
which carried the products of rock disintegration into the 
neighboring sea to form the Lafayette formation. The soil and 
subsoil of the land area of that time were prevailingly of a 
red color, for had it been otherwise the Lafayette formation 
would not be so generally of that tint. Red soils are charac- 
teristic of lands whose climate is comparatively mild. The 
soils of the central and upper Mississippi basin, from whence 
chiefly was derived the red Lafayette loam of the Mississippi 
embayment region, are not known to have been of a decided 
reddish tint in the Quaternary era, except locally during the 
mild Aftonian interglacial epoch. On the contrary, the col- 
ors which almost exclusively dominate the deposits and buried 
soils of the Quaternary era are blue, yellow, brown and black. 
The fineness of tho materials introduced by glaciation, the 
broad swampy flats, and the usually severe winter climate 
were the combined causes of the dark color of the Quaternary 
