238 The American Geologist. October, 1001. 
great rapids. Here the greater 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 St. David's 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. 
The action of a high waterfall, with great volume of 
water, precipitated over a hard stratum of which large blocks 
give way and fall because they are gradually undermined, as 
in the Horseshoe or Canadian falls of Niagara, is well com- 
pared by McGee to the deep wearing of potholes. The fallen 
blocks are moved under the powerful impact of the high cat- 
aract and wear a deep channel, attaining near the foot of the 
present falls the depth of almost 200 feet under the river 
level. Such cataract action of deep channel wearing may be 
also supposed to have produced the g-reat depth (96 feet) of 
the Niagara river at the mouth of the gorge ; but I think that 
this is better attributed to the usual process of stream cutting 
at the time of depressed level of this part of lake (7)ntario, 
which is otherwise known by its lower inclined beaches ex- 
tending here under the lake. 
Among the conditions which might 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 onlv very slow erosion of the 
gorge. This hypothetical factor in our problem, which has 
been assumed bv Gilbert, Spencer, Taylor, and Hitchcock, to 
consideraljly prolong' the time of the gorge erosion, is the 
diversion of the outflow from the basins of the three lakes 
above lake Erie, 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 ]\Iattawa river to the Ottawa. 
But differential elevation of the land from its Late Gla- 
cial or Champlain depression took place here, as on the area 
