760 TE JOCRNAL, OF VGHOLOGVA 
subject upon which differences of opinion have arisen among 
the several geologists who have investigated the region; it being 
maintained by some, among whom the present writer is included, 
that in consequence of the enlargement of the drainage area by 
a glacial invasion, a considerable part of the valley has been 
deepened below the limits of the preglacial rock floor. Others 
maintain that the deepening occurred shortly before the first ice 
invasion, during a supposed brief period of higher elevation 
than the present and is therefore preglacial. We can only refer 
the reader to the literature of this subject as presented by Carll, 
White, Stephenson and Chance, in the reports of the Pennsyl- 
vania Geological Survey, and more recently by Chamberlin, 
Wright, Foshay, Hise and Leverett, in the American Journal of 
Science. 
Reviewing the data of the entire field we find that the valley 
floors of the Mississippi, Illinois and Wabash were excavated, 
prior to the first ice invasion, to a depth 50 to 200 feet below the 
present streams. The southern portion of these valley floors 
apparently slope southward, with a gradient not greatly different 
from that of the present streams, while the northern portions are 
considerably depressed. It seems probable, therefore, that the 
north part of the Mississippi basin stood relatively, if not abso- 
lutely, higher than now prior to the first ice invasion. The por- 
tion near the southern border of the glaciated district may have 
stood no higher than at present. The region drained by the 
Missouri, on the other hand, seems to have had, up to the time 
of the first ice invasion, a lower altitude than the present. 
There seems, on the whole, little evidence that much of the 
Ohio drainage basin has stood higher. The complications in 
this valley, resulting from the great increase of the size of the 
drainage basin, make it difficult to estimate the preglacial alti- 
tude, for the enlarged stream would require, independent of 
altitude, a deeper valley. 
AGE AND STAGE OF DEVELOPMENT OF THE PREGLACIAL VALLEYS, 
In closing this discussion a few remarks seem necessary con- 
cerning the date of the uplift which inaugurated the channeling 
