12 POPULAR SCIENCE MONTHLY. 



Modern Recession of the Falls.— The modern rate of re- 

 treat of the cataract during forty-eight years has been determined 

 by comparing the crest of the falls, carefully mapped by four 

 surveys— the first by Prof. James Hall in 1842 and the last by Mr. 



Fig. 12. — The Four Surveys of the Canadian Falls, showing the Eetreat of the 

 (Cataract, in which some Inaccuracies are Apparent. (Kil)be.) 



Augustus S. Kibbe in 1890, Between the times of these surveys 

 not merely the historic Table Rock, but six acres of rocks form- 

 ing the floor of the river fell away by the undermining action of 

 the falls, and the center of the cataract moved upward for a dis- 

 tance of two hundred and twenty feet. From the 275,400 square 

 feet thus removed it is found that the mean annual recession has 

 been four feet and a sixth a year for the Canadian falls and two 

 thirds of a foot for the American cataract. The recession is 

 shown in Fig. 12. 



The work of the falls is not uniform, for there are years of 

 rapid central recession and slow lateral expansion, followed by 

 even a total central rest and rapid lateral enlargement of the 

 curve. From an approximate estimate of the variation in the 

 amount of work due to the physical and geological structure of 

 the district, the mean rate of recession of the falls under existing 

 volume of water and descent of the river may perhaps be reduced 

 to ;3"75 feet a year, wliich factor alone would indicate the age of 

 the cataract to be ten thousand years. But this simple story 

 would leave out of consideration the variability of the volume of 

 Niagara River and the descent of the cascades. 



TiiK Sroin- of tiik Lakks and the Bikth of Niacjara 

 Falls. — At the clo.se of the ice age, and after the geological 



