PJeistocenp Papcrx <it tln^YdsJiiiuiton Mectuxjx. 233 
F. AVright. The ice dam wliicli Wright and others supposed to 
have turned the Ohio temporarily out of its valley, so as to flow 
around the ice-front, producing a lake above Cincinnati, was not 
the cause of this silt deposit, nor of the conspicuous terraces of 
the Ohio, Monongahela, and Allegheny rivers, which are of 
fluvial origin, sloping in the same directions as the present 
streams. 
Prof. J. W. Spencer, in discussion, stated his belief that de- 
pression of the land to the sea level accounts for the silts of these 
valleys and for the Pleistocene shore lines about the great lakes 
tributary to the St. Lawrence. 
President T. C. Ciiamberlin replied that in basins sloping 
northAvard the receding ice-sheet, pausing at many stages which 
are marked l)y moraines, was a barrier holding these lakes at the 
higher levels of their ancient beaches. 
The iitiltudf of the eastern (vnd central jjortiuns 0/ the Inited 
States during the Glacial 2^<ii'iod. By T. C. CuAMBERLIN. The 
deposition of loess in the lower Mississippi valley was contem- 
poraneous with the maximum extension of the ice-sheet in this 
liasin, diu'ing the earlier great epoch of glaciation. The attitude 
of the land was then low, with very slack drainage, allowing this 
fine silt to 1)e spread l)y broad river floods or in shallow lakes. 
A long interglacial epoch followed, in which the Mississippi basin 
was moderately uplifted, with increasing amount toward the north. 
Great erosion of the loess and of the underlying gravel and sand, 
formerly called Orange Sand but now named the Lafayette forma- 
tion, took place during this interglacial time. The waters flowing 
from the ice-sheet and terminal moraines of the later glaciation 
carried much gravel, sand, and fine silt into the channels of trib- 
utaries of the ^Iississii)pi, and along the V)road, deeply eroded 
valley of this river, showing conditions of drainage similar to 
those of the present time. On the Atlantic coast marine clays 
overlying the till in southern Maine, along the St. Lawrence, and 
in the basin of lake riuuni)lain, proved that during the maximum 
advance of the ice-sheet and its recession the land was depressed 
somewhat lower than now. President Ciiamberlin therefore con- 
cludes that the Ice age was not attended In- any high elevation of 
these regions. [See this jjaper in the Nov. fxEorxKUST. ] 
Mr. "Warren Ui'HA.m. in discussion, noticed that this jjaper 
treated only of the closing part of the epochs of glaciation, when 
