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



sion. The four surveys naturally give data for superseding 

 the earlier estimates, and if the mean rate of retreat of the 

 falls during 48 years, be taken, its age would appear to be 

 9,000 years. The conjectures of the older geologists have 

 been set aside by recent writers who have endeavored to reduce 

 the age to 7,000 years by using the maximum rate of measured 

 recession. Substituting a measured rate of retreat for one 

 purely assumed was a step in the right direction, but without 

 knowing it, the later writers were farther astray than the earlier, 

 for they neglected to take into account the changing episodes 

 of the river, which was not known to the earliest observers. 

 Only one other geologist besides myself has called attention to 

 the varying forces which have made the Niagara canon, — and 

 this is Mr. G. K. Gilbert,* by whom and the writer the princi- 

 pal phenomena affecting the history of the river have been dis- 

 covered. The last question which had to be determined before 

 a computation of the age of the falls could be undertaken was 

 the approximate amount of work accomplished by the river 

 during each of the episodes in its history. This I was able to 

 estimate last fall. 



2. Modern Topography. 



For distance of 19 miles from Lake Erie (573 feet above 

 tide), the Niagara peninsula is a plain, with slight undulations, 

 rising from 15 to 30 or 40 feet above the lake. But three 

 features are notable : [a) a drift ridge trending westward from 

 the falls and surmounted by a beach (L, fig. 8) rising to 114: 

 feet above the lake, with a knob 30 feet higher, at Drummond- 

 ville ; (b) at the outlet of Lake Erie, the river cuts through 

 an escarpment of Devonian limestone, which there rises to 

 about 30 feet ; and (c) at a point about a mile north of the site 

 of the falls there is another limestone ridge here named 

 William Johnson's ridge in honor of the first settler (e, e, fig. 1) 

 with an elevation of 40 or 50 feet. Between these two rocky 

 ridges is the Tonawanda basin. From the northern margin of 

 the plain, the escarpment suddenly descends about 240 feet to 

 a lower plain which extends eight miles to the shores of Lake 

 Ontario (247 feet above the sea). Upon leaving Lake Erie 

 the river channel is only a quarter of a mile wide but reaches 

 a depth of 48 feet. After passing the Devonian escarpment,, 

 the river is broad, even a mile and a half above the fall, with 

 a depth of from 1 to 16 feet. The canon is about 36,500 feet 



*Mr. Gilbert writes thus: "You are aware that I am everywhere quoted as 

 estimating the age of the river (Niagara) as about 7,000 years. It was partly to 

 dispel this impression that I wrote .... In point of fact I have made no esti- 

 mate and my opinion, so far as I have one, is that the age of the river is much 

 greater than 7,000 years." 



