53 



and a fall of this description would be an active gorge- 

 making agent, whereas the American fall in its present 

 state hardly bears that distinction. 



A cataract of three times the volume of the American fall, 

 such as were the falls in the two Erie sections, would be 

 more powerful, but, other conditions being the same, the 

 rate of its recession in gorge making would be conditioned 

 in the same way, according as the water on its crest was 

 concentrated or attenuated beyond the average. For 

 an average depth of between 5 and 10 feet (1 -5 and 3 m.) 

 on the crest, which would signify neither extreme concen- 

 tration, nor extreme attenuation, and where the capping 

 limestone was thin or of moderate thickness, Spencer's 

 rate of -42 of a foot (• 128 m.) per year seems a little slow. 

 But where the capping limestone had great thickness it 

 is probably too rapid. It seems certain that the cataracts 

 in both of the Erie gorges probably underwent con- 

 siderable variations of this kind. Indeed, on account of 

 the number and magnitudes of the variable and uncertain 

 elements affecting the rate of recession, especially in the 

 older parts of the gorge, it does not seem possible to 

 state in precise terms the total duration of the falls. It 

 appears to have been somewhere between 20,000 and 

 30,000 years, possibly even 35,000 years. It is believed 

 that an attempt to set the estimate between narrower limits 

 would add nothing to its value. 



The Rapids above the Falls. — From Table Rock 

 house the car runs southward along the shore of the 

 rapids, affording a good view at several places of the 

 powerful current as it rushes swiftly towards the falls. 

 It is easily seen that a deep current flows close along the 

 shore, while farther out there are many rocks projecting 

 above the water. Sixty or seventy years ago there was 

 a low island covered with bushes and small trees in that 

 part of the rapids. There is considerable area lying 

 south and southeast from the apex over which the water 

 is now shallow. A deep, strong current passes north of 

 the shallows and falls over the brink at and north of the 

 apex of the fall and another larger current passes south 

 and west of it and falls over the brink west of the apex. 

 If this arrangement of the deeper currents continues 

 for sufficient length of time an island will appear between 

 them, as Goat island is now between the divided parts 

 of the river. It seems certain also that by the time the 



