ESTIMATING THE AGE OF NIAGARA FALLS. 153 



a new cycle of rapid disintegrations in the superincumbent strata 

 follow. 



An important point to be noticed, and which is evident from two 

 of the reproduced photographs (Figs. 3 and 4), is that the talus has 

 never reached up so high as to check the disintegration at the mouth 

 of the gorge of the jSTiagara shale and limestone which form the 

 upper one hundred feet of the face, and which exhibit the maximum 

 amount of enlargement which has taken place. The thickness of 

 the jSTiagara limestone is here so small that it has not been so im- 

 portant an element in forming the talus as it has been farther up the 

 stream, where it is two or three times as thick. Now, while our 

 original supposition was that one quarter of an inch annually was 

 eroded from the upper two hundred feet, this would involve the 

 erosion of a half inch per annum over the top of the gorge to bring 

 the calculation within the limit of ten thousand years. It certainly 

 is difficult for one who examines the facts upon the ground to believe 



Fio. 6. — Section, drawn to equal vertical and horizontal scale, showing enlargement of Niagara 



gorge on the east side at its mouth at Lewiston : 1, Niagara limestone, 20 to 30 feet; 



j^ 2, Niagara shale, 70 feet; 3, Clinton limestone, 20 to 30 feet; 4, Clinton and Medina 



■ shale, 70 feet ; 5, Quartzose Medina sandstone, 20 to 30 feet ; 6, softer Medina sandstone, 

 120 feet above water level. 



that the crumbling away of this exposed Niagara shale could have 

 been at any less rate than that; so that the estimate of about ten 

 thousand years for the date of that stage of the Glacial period in 

 which Niagara River first began its work of erosion at Lewiston (an 



VOL. LT — 12 



