860 The American Naturalist. [October, 
cession now known to be wholly erroneous. Again, within the 
last eight years, there have been several writers who have been 
using corrected céefficients of retreat, still their results are 
more inaccurate than the guesses, as to the age of the falls, 
made a hundred years ago, yet they may be said to have ap- 
proximated the truth within their observations, but the obser- 
vations have become enlarged. 
A hundred years ago, Andrew Ellicott estimated the age of 
the falls at 55,000 years. Forty years later, Bakewell made 
the falls about 12,000 years old. Over fifty years ago, Lyell 
conjectured the age at 35,000 years, and this estimate was 
commonly accepted until about a decade ago. The founda- 
tions for the measurements of the retreat of the cataract were 
laid by Professor James Hall, when he made the first preserved 
instrumental survey of the cataract in 1842. Since then, 
measurements have been repeated in 1875 by the Lake Survey? 
in 1886 by-Professor W. S. Woodward, and in 1890 by Mr. 
Aug. 5. Kibbe. From these surveys the mean rate of modern 
recession of the falls is found much more rapid than was form- 
erly supposed, as it amounts to 4.175 feet a year, and if the 
history of the falls had been uniform, then the age would 
have been only 9,000 years—not so different from the guess of 
half a dozen years ago, which took the maximum medial re- 
treat of the cataract, and made the age only 7,000 years. Had 
the gentlemen taken the mean rate as then known, which the 
scientific, methods dictated and since supported by the action 
of the river, they should have made the age of the falls 11,000 
years, near which estimate some did. This point is noticed on 
account of many secondary writers finding the number 7,000 
years as agreeable to their theories. 
Owing to some structural variations, I have taken 3.75 
feet a year as the mean rate to be adopted for the retreat 
_ of the falls mechanically applied to the different conditions of 
the river. These have been occasioned by the changing 
heights of the falls and the volume of the water. With regard 
to the latter point, it has been found that for three-fourths of 
the duration of the river, the drainage of Lake Huron and the 
upper lakes was by way of the Ottawa River, and not by way 
B tes Oa ee eae eS 
