78 BF. B. TAYLOR—ORIGIN OF GORGE OF WHIRLPOOL RAPIDS. 
\ 
yielding, the direction and force of the current remained almost perfectly 
constant. In each of the bouldery rapids the boulders formed a closely 
compact floor entirely covering the finer sediments beneath them. No 
doubt a considerable quantity of these sediments was washed away when 
the river first began to flow, but as these disappeared the boulders gradu- 
ally settled down closer together into positions of more effective resist- 
ance, and when the protection of the finer sediments beneath had once 
become complete, the unvarying volume and steadfast direction of the 
current left them henceforth undisturbed. Under conditions so extremely 
uniform it seems impossible to set any definite limit to the endurance of 
a boulder-packed bar in rapids of moderate velocity ; and so, in spite 
of the apparently small work of erosion accomplished, the evidences, on 
the whole, indicate a long duration for the Nipissing-Mattawa river. The 
Nipissing beach suggests the same conclusion even more strongly. 
ScHEME OF CORRELATION 
With these facts before us we can hardly fail to perceive the almost 
obvious correlation which they suggest, namely, that Nipissing Great 
lake, with the Nipissing-Mattawa river as its outlet, is the equivalent in 
time of lake Erie, with the comparatively small Erie-Niagara river as its 
outlet.* On this basis the amount of erosion accomplished by the one 
river in this time must be the exact correlative of that accomplished by 
the other. Nipissing Great lake represents one complete episode in the 
lake history, and one in which Niagara did not have the discharge of 
the three upper lakes. The reduction of the morainic obstructions in the 
path of the Nipissing-Mattawa river to compact boulder-covered bars, the 
boring of potholes in the hard boulders, and the making of some part of 
the Talon gorge stand as the correlative of something done by the 
smaller river in the gorge of Niagara. In other words, from the stand- 
point of the lake history, we are bound to suppose a large reduction of 
the volume of Niagara fora relatively long time; while, on the other 
hand, from the standpoint of Niagara gorge history, the stronger of two 
alternative hypotheses leads us to the same inference. Going backward 
from the present in the lake history, we find that the North Bay outlet 
was active next before the present one at Port Huron, but the old beach 
at the head of that outlet is now 120 feet above the level of Georgian bay. 
Changes of such magnitude would seem to require a considerable dura- 
tion of time, and we should expect therefore that the present episode of 
*The name Brie-Niagara is suggested for the Niagara river as it was when it carried only the 
discharge of lake Erie. The name combines elements having both historical and geographical 
significance, as does also the name Nipissing-Mattawa as applied to the former great outlet river of 
the north. 
