INTERPRETATION OF PHENOMENA AS TO UPPER GREAT LAKES. 00 
small volume has cut scarcely more than a notch in the old ledge of the 
falls, 4 or 5 feet deep and 8 or 10 feet wide, and it is doubtful whether 
all of this is the work of the modern river. 
As to the age of most of this gorge there seems to be little room for 
uncertainty. The greater part is in all probability preglacial or inter- 
elacial, but for an estimated distance of 500 or 600 feet, or to a point a 
short distance below the lower end of the island around which the gorge 
now divides at the falls. it appears to be of postelacial age. This partis 
narrower, has steeper walls, and is in a general way fresher in appear- 
ance than the parts below. It is also shallower, as might be expected 
from the circumstances of gorge-making at this place, for on account of 
the large volume and the relatively slight descent the work of erosion 
was accomplished by a process of scour by swift flow, which is quite 
different from the pounding of a vertical fall. A gorge made in this way 
tends to be narrower and shallower than it would be if made by a ver- 
tical fall of the same stream. The writer’s observations at this place were 
not exhaustive, but so far as they went, the exact place where postglacial 
gorge-making began was not made out. The influence of the bed of lime- 
stone is also an element of uncertainty. Hence it seems impossible on 
present information to draw a definite estimate of the time or duration 
of the outlet river from this gorge. Its value as a basis for a time esti- 
mate is less than that of the gorge of the Whirlpool rapids at Niagara. 
Duration of the northern outlet—It might be supposed, however, that if 
the Nipissing-Mattawa river flowed for a relatively long time in this 
valley it would have accomplished a much greater work of erosion, 
especially in the removal of the bouldery drift obstructions, but it is be- 
lieved that such a conclusion would be a mistake. No just idea of the 
history of this river can be formed without taking full account of the 
peculiar conditions of its existence. The geolozical and physiographical 
conditions favored the minimum of erosion. With the probable excep- 
tion of Talon chute, there was no place in the whole course of the river 
where its descent gave it sufficient power to spin boulders even of small 
size. Being the outlet of a great lake, the water was clear at the start. 
The bed and banks were mostly unyielding granite and gneiss or boulder- 
packed bars, and afforded very little sediment. ‘There were no tributaries 
of any importance—none that brought it more than a trifline quantity 
of detritus—and nearly two-thirds of its course lay through lake basins, 
where it could neither gather nor carry sediment and where it dropped 
whatever small quantity it had picked up from any source above. 
Another element of stability under such conditions as obtained in this 
valley was the steadiness of volume and flow. Once begun, the river 
never changed appreciably, and because the bed was substantially un- 
