NIAGARA. 151 
gan. The dam, moreover, was demonstrably of sufficient 
height to cause the river above it to submerge Goat Island; 
ana. this would perfectly account for the finding by Sir 
Charles Lyell, Mr. Hall, and others, in the sand and gravel 
of the island, the same fluviatile shells as are now found in 
the Niagara river higher up. It would also account for those 
deposits along the sides of the river, the discovery of which 
enabled Lyell, Hall, and Ramsay to reduce to demonstration 
the popular belief that the Niagara once flowed through 
a shallow valley. 
The physics of the problem of excavation, which I made 
clear to my mind before quitting Niagara, are revealed by 
a close inspection of the present Horseshoe Fall. We see 
evidently that the greatest weight of water bends over the 
very apex of the Horseshoe. In a passage in his excellent 
chapter on Niagara Falls, Mr. Hall alludes to this fact. 
Here we have the most copious and the most violent 
whirling of the shattered liquid; here the most powerful 
eddies recoil against the shale. From this portion of the 
fall, indeed, the spray sometimes rises wjthout solution of 
continuity to the region of clouds, becoming gradually 
more attenuated, and passing finally through the condition 
of true cloud into invisible vapor, which is sometimes re- 
precipitated higher up. All the phenomena point distinctly 
to the center of the river as the place of greatest mechanical 
energy, and from the center the vigor of the fall gradually 
dies away toward the sides. The Horseshoe form, with the 
concavity facing downward, is an obvious and necessary 
consequence of this action. Right along the middle of the 
river the apex of the curve pushes its way backward, cut- 
ting along the center a deep and comparatively narrow 
groove, and draining the sides as it passes them.* Hence 
the remarkable discrepancy between the widths of the 
Niagara above and below the Horseshoe. All along its 
course, from Lewiston Heights to its present position, the 
form of the fall was probably that of a horseshoe; for this 
is merely the expression of the greater depth, and conse- 
quently greater excavating power, of the center of (he river. 
The gorge, moreover, varies in width, as the depth of the 
* In the discourse the excavation of the center and drainage of 
the sides action was illustrated by a model devised by uy assistant, 
Mr. John Cottrell. 
