462 J. W. Spencer — Duration of Niagara Falls. 



The measured recession has probably obtained, since the 

 cataract cut its way through Johnson's ridge, for beneath the 

 Tonawanda basin the limestones have a thickness of only 45-55 

 feet as the upper 90 feet had been removed in pre-Pleistocene 

 times. The capping limestone in Johnson's ridge was 140 feet 

 thick. To the north the thickness was reduced. Along those 

 portions of the chasm where the limestone is heavier and the 

 gorge narrower than in the pre-glacial depression, the stronger 

 arches must have arrested the maximum rate of retreat, and 

 on this account, I have reduced the measured mean rate of 

 recession by an estimated amount of ten per cent, or to 3'75 

 feet a year for the recession of the falls from the end of the 

 canon to Johnson's ridge, under conditions of the modern dis- 

 charge and descent. The mean descent of the river was from 

 the plain now at 340 feet above Lake Ontario ; but whilst 

 passing the rapids of Johnson's ridge, 25 feet must be added 

 to the declivity of the river. After the basin behind the 

 ridge was reached, the water plain was reduced to about 320 

 feet including 50 feet of descent above the falls in the form 

 of rapids. The surface of the country has been deformed 

 since the commencement of the cataract by a northward ter- 

 restrial uplift to the extent of 12 or 15 feet divided through- 

 out the length of the gorge where, as seen in the canon, the 

 character of the different strata is remarkably uniform except 

 in the described depressions, across Johnson's ridge, and at the 

 end of the chasm where the capping limestones were much 

 thinner but partly compensated for by the greater prominence 

 of the hard Clinton and Medina layers. 



The following computations are based upon the mean rate 

 of recession modified by the variations in the descent of the 

 waters and their changing volumes, which have been dis- 

 covered in the geological investigations of the Great Lakes. 



7. Sketch of the Lake History and the Nativity of the Falls. 



This outline is taken from the chapters on the Lake History 

 noted at the foot of the page.* At the commencement of the 

 Lacustrine epoch, Warren water covered most of the Lake 

 region, and Forest Beach was its last strand. Afterwards the 

 waters sank 150 feet, thereby dismembering Warren water into 

 Algonquin Lake (confining it to the basins of Superior, Michi- 



* " The Iroquois Beach, a chapter in the History of Lake Ontario," Trans. Roy. 

 Soc. Can. 1889, p. 132. '"Deformation of the Iroquois Beach and Birth of Lake 

 Ontario." this Jour., vol. xl, p. 443. 1890. •' Deformation of Algonquin Beach 

 and Birth of Lake Huron, Id ," vol. xli, p. 12, 1891. " High Level Beaches in the 

 region of the Great Lakes and their Deformation." Id., p. 201. " Deformation of 

 the Lundy Beach and the Birth of Lake Erie," Id., vol. xlvii, p. 207, 1894. All 

 by J. W. Spencer. " The History of Niagara River." by G. K. Gilbert, Six. Rep. 

 Com. State Res. N. Y., 1891. 



