NORTH AMERICAN GEOLOGY. 195 



lake slightly depressed the earth's crust, atul that on its evaporation 

 the normal conditions were resumed and the lake bed and terraces 

 elevated.* The same writer called the attention of the American As- 

 sociation to several small anticlinal ridges in the rocks of western 

 New York, which are believed to have resulted from horizontal expan- 

 sion of superficial strata consequent on post-glacial amelioration of 

 climate.t 



21. Niagara Falls. — At the Buffalo meeting of the American Associa- 

 tion there was a general discussiofi of the various questions connected 

 with the chronology of Niagara Falls, which was taken part in by Gil- 

 bert, Chamberlin, Hall, Pohlman, Claypole, Woodward, Wright, Davis, 

 Comstock, and Holly. It was shown by Gilbert that the Niagara River 

 probably began its work at the close of the glacial period, when the 

 retreating ice opened the St. Lawrence Valley and separated the two 

 lakes, Erie being held back by the Niagara escarpment. Consequently 

 the age of the Niagara River is a measure of Post quaternary time. 

 The rate of recession determined by comparison of a recent survey by 

 Woodward with those of Hall in 1842 and the United States Engi- 

 neers in 1875 would require seven thousand years for the excavation 

 of the G miles of gorge. A number of considerations, however, qualify 

 this estimate, and they are summarized as follows: 



" At stages of recession earlier than the present there was a thinner 

 body of limestone to be undermined and removed ; there was a deeper 

 exposed body of shale; the water plunged from a greater height; the 

 water was concentrated in a narrower channel; it carried more floating 

 ice ; and all these differences tended to make the rate of recession faster. 

 The rate may also have been influenced by variations in the amount of 

 detrital load (a tool of erosion), by variations in the solvent i)0wer of 

 the water, and by variations of its volume due to changes of climate or 

 catchment basin. The catchment basin was formerly extended by in- 

 cluding part of the area of the ice sheet, but it may have been abridged 

 by the partial diversion of Laurentian drainage to other courses. 



"The problem admits of expression in an equation : 



Li \ e Length of gorge 

 "Age of gorge = ^ ?' — ■ 



Rate of recession of falls 



— effect of antecedent drainage 



— " " thinner limestone 



— " " thicker shale 



— " " higher fall 



— " " narrower cross section 



— " " more floating ice 



± " " variations of detrital load 



± " " chemical changes 



± " " changes of river volume." J 



*Aiu..Jonr. Sci., in, vol. 31, pp. 284-209, and pi. 



t Proceedings, vol. 35, p. 227. 



tAm. Assoc. Proc, vol. 35, pp. 222, 223, 



