110 The American Naturalist. [February, 
though less definitely by the cafion valleys of the interior por- 
tions of the North American continent. The period of high- 
level continued to the opening of the Kansan epoch of glacia- 
tion, when, through some cause not rightly understood, but 
probably largely the great weight of the accumulating ice- 
sheet, the land in the eastern half of North America was de- 
pressed, and has remained to the present time at a much lower 
altitude than before the Kansan epoch. The high-level period 
or epoch just mentioned, seems naturally set apart both from 
the preceding low-level Lafayette epoch and the succeeding 
low-level Glacial epochs. It has, therefore, been defined as a 
geologic time unit and designated the Ozarkian epoch, from 
the fact that its results are so well seen in the sharp cut cafion 
valleys of the Ozark mountain and plateau regions of southern 
Missouri and northern Arkansas. 
Although commonly considered as a unit, the Ozarkian 
high-level epoch was probably characterized by two main ele- 
vatory movements. The first began and ended at and very 
soon after the opening of the epoch, and was of great areal ex- 
tent but not very marked vertical movement. To it, however, 
may be traced nearly all the recognized effects of the Ozarkian 
high-level condition of eastern North America. The second 
decided disturbance of the earth’s equilibrium seems to have 
occurred very nearly at the end of the epoch. It was some- 
what local in nature and effected most the northeastern por- 
tion of America, (and probably all the lands surrounding the 
North Atlantic). I must acknowledge that Iam basing my 
claims for this elevatory movement on somewhat slender evi- 
dence. But the deep, narrow, now submerged valley channels 
in the border portions of the continental plateau off the mouths 
of such rivers as the Delaware and Hudson, certainly were not 
in process of excavation during the whole or even any large 
part of the Ozarkian epoch. They must be the result of a 
special elevation near the close of the epoch. And, further, the 
cafion valleys of the Mississippi basin sometimes have rock 
shelves buried under the present valley bottom. Often these 
rock shelves cannot be explained by any inequality of the 
strata excavated, but seem to indicate a renewed period of up- 
