On the Falls of Niagara. 335 



then be lowered by nearly three hundred feet and the low countries 

 between lake Ontario and the Atlantic can scarcely escape from be- 

 ing swept at once into the ocean."* 



To prove that this is too alarming a picture, it is only necessary to 

 add to the fact that no barrier, properly speaking, intervenes between 

 the present cataract and lake Erie, the bottom of which shelves 

 very gently from the shore, and is nowhere deep, averaging only one 

 hundred and twenty feet. The action of discharge must therefore 

 at all times, even long after the cataract shall have entered the lake, 

 be extremely gradual as Mr. De la Beche has amply shown.f 



Besides, even supposing it possible for lake Erie to empty itself 

 thus suddenly, no similar discharge could take place from the other 

 lakes, inasmuch as lake Huron and Michigan are cut off from Erie 

 by a river eighty nine miles in length and by a difference of elevation 

 of fifty two and a half feet. 



As to the future rate of recession of the Falls I believe it to be 

 equally difficult to speculate. It is very possible that long before 

 this vast wall of water, advances to the brink of lake Erie, it will 

 have totally altered its shape and aspect. As the falls retreat, they 

 also rise, obeying the general ascent of the land towards lake Erie. 

 This circumstance combined with the horizontal position of the 

 rocky beds which they intersect, gradually reduces the thickness 

 of the underlying section of shale, and augments that of the over- 

 lying limestone. 



The diagram here annexed will elucidate the arrangement of the 

 several strata along the river, and assist in displaying the future po- 

 sitions which the falls must occupy as they enter the uppermost beds. 



O Lake Ontario. E Lake Erie. Q Queenstown. G termina- 

 tion of the ravine, a the falls seven miles from G. b the probable 



* The distance being twenty one miles from the present cataract to lake Erie 

 and the rate of action being- about four feet fcr anvnm the time necessary for this 

 great natural operation will be 27,7"20 years. As the fall will however be higher 

 than it now is when it reaches the top of the rapids, the action cannot be calcula- 

 ted at so much as it now is, and the United States on the coast may therefore safely 

 reckon on a lease of from 30,000 to 40,000 years. — Fairkolmc. 



t Third edition of Manual, page ICO. 



