Niagara Falls 



1889 and might lead to a slow rate of recession. If the thickness now 

 seen at the cataract were slightly increased or slightly diminished, 

 it is not at once apparent how the rate of recession would be 

 affected, and yet there might be an important difference. 



We have seen that the pre-glacial stream whose channel is 

 betrayed at the Whirlpool, removed the Niagara limestone 

 through a portion of the gorge, and 



Question 4 asks : Through what portion of the gorge was the 

 Niagara limestone absent when the Niagara river began its 

 work ? 



Question 5. Does the rock section beneath the limestone — 

 the shale series with its imbedded harder layers — does this vary 

 in different parts of the gorge? 



Question 6. Through what distance were the several mem- 

 bers of the underlying rock series removed by the action of the 

 pre-glacial stream? 



Coming now to consider the force of the falling water, a little 

 consideration serves to show that the force depends on at least 

 three things: The height through which the water falls, the 

 degree of concentration of the stream, and the volume of the 

 river. 



The height of the fall is the vertical distance from its crest to 

 the surface of the pool below. 



Question 7 asks: How has the height of the crest of the fall 

 varied during the history of recession? 



Question 8. How has the height of the base of the fall 

 varied? And this involves a subsidiary question, — to what 

 extent has the excavated gorge, as left by the retreating cataract, 

 been refilled, either by the falling in of fragments from the cliffs, 

 or by contributions of debris brought by the current? 



Question 9. What has been the form of the channel at the 

 crest of the fall, from point to point, during the recession? 

 Wherever the channel has been broad, and the water of uniform 

 depth from side to side, the force of the falling water has been 

 applied disadvantageously ; wherever the channel has been 



580 



