NIAGARA. 199 



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

 moreover, was demonstrably of sufficient height to 

 cause the river above it to submerge Goat 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 

 Ramsay to reduce to demonstration the popular be- 

 lief 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 copi- 

 ous 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 attenu- 

 ated, and passing finally through the condition of true 

 cloud into invisible vapour, which is sometimes repre- 

 cipitated higher up. All the phenomena pointdistinctly 

 to the centre of the river as the place of greatest me- 

 chanical energy, and fron\ the centre the vigour of the 

 fall gradually dies away towards the sides. The Horse- 

 shoe 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 

 14 



