296 G. F. WRIGHT POSTGLACIAL EROSION AND OXIDATION 



Dr. J. W. Spexcer said : Professor Wright's determinations of the 

 rapid erosion by weathering, seen on the sides of the Niagara Gorge, is 

 based upon artificial conditions. The railway cut has steepened the natu- 

 ral slope covered with talus ; consequently the rate of lateral erosion now 

 found here is excessive. Furthermore, the lower levels of the gorge wall 

 became exposed only after the sinking of the waters to and below that of 

 the Iroquois beach, and, on account of the later exposure of the slopes 

 and the subsequent overdeepening of the gorge, when all the lake waters 

 were turned into Niagara, the lower slopes were excessively steep, and, 

 when exposed to weathering, such would favor very rapid removal of the 

 debris and shales. 



Professor Wright's reply : The movements of land level during the 

 Glacial period were abnormal and can not be judged by present move- 

 ments. The relief furnished by the removal of the ice-sheet led to a rapid 

 reelevation of the land, so that the 1,000 feet may easily have been 

 accomplished in 5,000 years. 



The Conewango was never filled with gravel to the height of 300 feet. 

 The deposit at East Warren is a dump in the lea of a rocky projection, 

 while on the west the glacial torrent scoured out the channel. 



