EROSION EFFECTS: 107 
Above the Whirlpool it seems very clear to my mind that the gorge 
erosion was much aided by the preglacial Saint Davids stream for the 
distance of one mile occupied by the great rapids. Here the major part 
of the depth and width of the gorge had probably been already eroded 
before the Ice age, being then filled with drift, which the postglacial 
river easily removed as soon as its gorge toward Lewiston was sufficiently 
deepened. No powerful falls have there cut a deep channel, and the 
river consequently has a constricted and very rapid course. Above the 
old Saint Davids ravine, however, a massive waterfall has operated along 
the latest distance of nearly two miles of the gorge, giving to the river 
there its great depth. 
EFFECT OF THE LAURENTIAN GLACIAL LAKES ON THE NIAGARA GoRGE 
HRosIon 
GLACIAL LAKES ABOVE NIAGARA RIVER 
Among the conditions which have been supposed to cause the Niagara 
river to vary from its present size, only one would produce a great and 
long-continued diminution of the river, so giving for a large part of its 
history only very slow erosion of the gorge. This hypothetical factor in 
our problem, which has been assumed by Gilbert, Wright, Spencer, and 
Taylor to considerably prolong the time of the gorge erosion, is the di- 
version of the outflow from the basins of the three lakes above lake Hrie, 
then confluent and forming the glacial lake Algonquin, to forsake its 
present course and pass eastward from Georgian bay, at first by the way 
of lake Simcoe and the Trent river to lake Ontario, and later by lake 
Nipissing and the Mattawa river to the Ottawa. 
But differential elevation of the land from its Late Glacial or Champlain 
depression took place here, as on the area of lake Agassiz, as soon as the 
land was unburdened by the glacial retreat. This northward uplift was 
in progress while yet the ice-burrier remained farther north and no1th- 
east, holding in succession the glacial lakes Warren and Algonquin, be- 
sides several earlier and smaller glacial lakes which became merged in 
lake Warren, on the upper part of the Saint Lawrence River basin. In 
the areas of lake Agassiz and of the Laurentian lakes alike, the uplift 
was nearly completed during the existence of the glacial lakes, as is 
known by the almost undisturbed horizontality of the latest and lowest 
glacial lake beaches. Finally lake Algonquin, by the northeastward land 
elevation, became divided into its successors, lakes Huron, Michigan, 
and Superior. 
Instead of the hypothesis of a long continued eastward outflow from 
lake Algonquin, my studies convince me that the Trent and Mattawa 
