Niagara Gorge and Postglacial Time. — UpJiain. 241 
The entire extent of the Horseshoe fall is found by these sur- 
veys to be worn away at an average rate of about two feet 
yearlv; but during- the four years from 1886 to i8qo the aver- 
age annual rate of erosion for the whole was live feet. Along 
the more shallow American fall, northeast of Goat island, the 
mean yearly erosion is about two-thirds of a foot, but from 
1886 to 1890 it averaged one and two-thirds feet. The energy 
of this part of the Niagara cataract is not sufficient to remove 
its hup-e fallen blocks of limestone, 011 which the water strikes 
along all the base of the precipice. 
Only the center of the Horseshoe fall plunges deeply into 
the river beneath, and its concentrated and intensified erosion 
tends at present to lengthen the gorge beyond its normal rate, 
which appears probably to be about three or four feet yearly. 
With such average erosion, the recession of the falls and pro- 
longation of the gorge would amount to one mile in about 
1,500 years ; and the action of the great cataract along the two 
miles of deep water south of the railway bridges would have 
begun about a thousand years before the Christian era. The 
entire erosion of the six and a half miles of gorge between 
Lewiston and the present falls would require about 10,000 
years, excepting that this period would be diminished prob- 
ably about a third, to 7,000 years, more or less, by the pre- 
glacial erosion of the St. David's stream and its northeastern 
tributary. 
In 1841 Sir Charles Lyell estimated the rate of lengthening 
of the gorge to be about a foot yearly, and its age therefore 
about 35,000 years. Gilbert, at the Buffalo meeting of the 
American Association for the Advancement of Science in 1886, 
comparing the exact surveys, estimated the recent rate of re- 
cession of the falls to be about five feet yearly, giving, if this 
were the average for the whole gorge, about 7,000 years for its 
erosion. Pohlman, in 1888, considered the recent recession to 
be at the rate of a mile in 2,000 years ; but the whole period, 
as he first showed, was much diminished on account of the 
preglacial erosion. Later, in consideration of a supposed loss 
of the outflow of the upper great lakes, leaving to the Niagara 
river, during a very long time, only a small part of its former 
and its present volume of water, the age of the river and 
