22^ THE POPULAR SCIENCE MONTHLY. 



northward course. Finally, at about seven miles from the present 

 Falls, we come to the edge of a declivity which informs us that we 

 have been hitherto Avalking on table-land. At some hundreds of feet 

 below us is a comparatively level plain, which stretches to Lake On- 

 tario. The declivity marks the end of the precipitous gorge of the 

 Niagara. Here the river escapes from its steep, mural boundaries, 

 and, in a widened bed, pursues its way to the lake, which finally re- 

 ceives its waters. 



The fact that, in historic times, even within the memory of man, 

 the fall has sensibly receded, prompts the qitestion, How far has this 

 recession gone ? At what point did the ledge which thus continually 

 creeps backward begin its retrograde course ? To minds disciplined 

 in such researches the answer has been, and will be, at the precipitous 

 declivity which crossed the Niagara from Lewiston, on the American, 

 to Queenstown, on the Canadian side. Over this transverse barrier the 

 united affluents of all the upper lakes once poured their waters, and 

 here the work of erosion began. The dam, moreover, was demon- 

 strably of sufficient height to cause the river above it to submerge 

 Goat Island ; and this would perfectly account for the finding by Mr. 

 Hall, Sir Charles Lyell, and others, in the sand and gravel of the isl- 

 and, 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. Here we see evidently that the great- 

 est 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 whirl- 

 ing 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 repre- 

 cipitated 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 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, cutting along 

 the centre a deep and comparatively narrow groove, and draining the 

 sides as it passes them. 1 Hence the remarkable discrepancy between 

 the widths of the Niagara above and below the Horseshoe. All along 

 1 In the discourse this action was illustrated by a model. 



