NIAGARA. 251 



waters, and here the work of erosion began. The dam, 

 moreover, was demonstrably of sufficient height to cause 

 the river above it to submerge Groat 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 Eiver higher up. It would also account for 

 those deposits along the sides of the river, the discovery 

 of which enabled Lyell, Hall, and Kamsay 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 re- 

 vealed 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 

 vapour, which is sometimes reprecipitated higher up. 

 All the phenomena point distinctly to the centre of 

 the river as the place of greatest mechanical energy, 

 and from the centre the vigour of the fall gradually 

 dies away towards the sides. The Horseshoe form, with 

 the concavity facing downwards, is an obvious and 

 necessary consequence of this action. Right along 

 the middle of the river the apex of the curve pushes 

 its way backwards, cutting along the centre a deep 

 and comparatively narrow groove, and draining the 



