240 The American Geologist. October, looi. 
depth of about 150 feet above the Thousand Islands, but with 
its water level beneath the present surface of the west part 
of this lake. In like manner with the earlier lake Iroquois, the 
progressing northeastward uplift caused the level of the lake 
St. Lawrence and afterward of lake Ontario to rise upon the 
land in the southwest part of the Ontario basin. It was dur- 
ing these late stages of the lacustrine history of this region 
that the deep channel of the Niagara river at the mouth of its 
gorge was eroded, the channel being subsequently partially re- 
filled with water by the continuance of the northeastward land 
elevation. The river from Lewiston north to its mouth has a 
depth of 100 to 200 feet, which indicates almost as much rise 
of this part of lake Ontario, for no hieh waterfall existed to 
erode the very deep channel there. 
The vast country which had been ice-covered and de- 
pressed under the weight of the thick continental ice-sheet 
was gradually uplifted, and to a greater hight at the north 
than at the south, during the removal of the ice burden. 
While lakes Agassiz and Warren still existed the northern 
parts of their areas were raised, in comparison with their 
southern outlets, 300 to 400 feet or more. It is also found, by 
the present inclinations and relationship of the successively 
formed shorelines of these and the other associated glacial 
lakes, that this epeirogenic movement proceeded as a perma- 
nent wave of land elevation from the periphery of the old ice- 
sheet inward to its central area. 
Accurate maps of the crest line of the falls were made by 
Hall in 1842, by the U. S. Lake Survey in 1875, by Wood- 
ward in 1886, and by Kibbe in 1890. It is thus ascertained 
that in the forty-eight years following the first survey the 
lengthening of the gorge, by the recession of the central part 
of the Horseshoe fall, was 270 feet, the average rate being 
about five and a half feet yearly. But the central curve or 
apex of the cataract is worn back much faster than its sides, 
because the river has its maximum depth of fully twenty feet 
at its center and there makes a plunge of not merely the 160 
feet from the verge to the surface of the water at the foot of 
the fall, but of nearly 200 feet lower to the bottom of the river, 
working thus most effectively to undermine the horizontal 
rock strata and break down the thick limestone at the top. 
