528 JOSEPH LE CONTE 
Tertiary; and the post-Tertiary erosion was supposed to be 
wholly Glacial. According to my view, on the contrary, the 
post-Tertiary erosion was by far the greater and was pre-Glacial 
in time, z. ¢., Ozarkian. 
In the eastern portion of the continent, therefore, the Ozar- 
kian grades into the Tertiary, but is well marked off from the 
present by the depression and sedimentation of the Champlain. 
2. The mid-continental or plateau region.—In the plateau 
region, on the contrary, the Ozarkian work is: sharply marked 
off from the Tertiary, but grades insensibly into that of the 
present. 
It is well known that the plateau region has been rising ever 
since the end of the Cretaceous—that from being the lowest 
part of the interior continental basin, it has become the highest 
part of the continental arch. The amount of elevation during 
this time has been at least 20,000 feet, about 12,000 of which 
has been carried away by erosion, leaving still 8000 feet of gen- 
eral elevation. Of this enormous general erosion, the largest 
part—shown by the receded and still receding cliffs — prob- 
ably belongs to the Miocene. The canyon-cutting is certainly 
post-Miocene. The outer canyon (of the Grand Canyon) ten to 
fifteen miles wide and 3000 feet deep, belongs to the Pliocene; 
while the inner gorge, 3000 feet deep but very narrow, belongs 
to the post-Pliocene, 2. ¢., to the Ozarkian and the present. 
I said the Tertiary work is sharply marked off from the 
Ozarkian but the Ozarkian grades insensibly into ‘present. The 
evidence of this is given by Dutton, as follows: 
Between the Pliocene work of the formation of the outer 
canyon, and the subsequent work—Ozarkian to present—the 
cutting of the inner gorge, there was an interval of rest marked 
by a wide shelf between the two. The rising and the down- 
cutting was continuous during the Pliocene until finally the river 
reached its base level of erosion and rested from further down- 
ward cutting and commenced to sweep from side to side widen- 
ing its channel, until the canyon walls were nearly ten miles 
apart. Then began the Ozarkian rise and the beginning of the 
