146 THK R'K A(;K in CANADA. 



\vliii'li)Oul, ami liavu cluanud oiit an old cliaimt!! aliove 

 this, and cut back the present face of the fall some 

 distance since the close of the glacial j)erio(l, and the 

 careful observations of Dr. S]»encer* ha\e shown that 

 the existing relation.s of the Niagara escarpment and the 

 lakes were established antecedent to the time when the 

 present fall was established. Claypole-f* has also shown 

 that the terraces with fresh-water shells on the Niagara 

 river prove that the I'ctaining ridge between lakes Krie 

 and Ontario was then as high as now. As I have else- 

 where argued also, the thickness of the harder bed, the 

 Niagara limestone, which the river has to cut, has, owing 

 to the southerly dip of the rocks, been increasing as the 

 falls were cut back, and there is reason to believe that a 

 part of the gorge al>ove the whirlp(jol was formed in prc- 

 glacial times, and has merelv been c":'.?!'n'3d out bv the 

 modern river, 'i'licre is nlno some reason ro believe that 

 the amount of water in the fall may have been greater in 

 the early modern [)eriod than now. 



What, then, is the rate of recession of this great 

 cataract, and how long has it l)een cutting its gorge ? 

 The rate of cutting has been vari(jusiy estimated at from 

 one f(jot to three feet annually ; Itut the actual measui'cd 

 rate for the last fort}'-twt) years, as given on the authority 

 of Mr. 1!. 8. Woodward, of the U. 8. Geological Survey, 

 is 24 feet, or nearly two-and-a-half feet per year. This 

 will give, say 12,000 to 10,000 years for the time required, 

 and, making allowance for the deductions above stated, 

 we may confidently allirm that the great cataract began 

 its labour somewhere between seven and twelve thousand 



* Iroquois Beach. Traii.s. R. S. Can., 18S9. 



f American Xatura/ist, Oct., 1886. Truus. Ceol. Socy., 1888. 



