NIAGARA. 151 



gan. The dam, moreover, was demonstrably of sufficient 

 height to cause the river above it to submerge Gout Island; 

 and 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 Bamsay 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 without 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 Lew is ton 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 the 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 ii.y assistant, 

 Mr. John Cottrell. 



