THE HISTORY OF THE NIAGARA RIVER. 255 



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 aflected, and yet there might be an important dif- 

 erence. 



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 itiver began its work ? 



Question 5. Does the rock section beneath the limestone — the shale 

 series with its imbedded harder layers — does this vary in diiferent parts 

 of the gorge ? 



Question 0. Through what distance were the several members 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 con- 

 sideration 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 sur- 

 face 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 re-filled, either by the 

 falling in of fragments from the cliffs or by contributions of debris 

 brought by the current? 



Question 0. What has been the form of the channel at the crest of 

 the fall from point to point during the recession '? Wherever the chan- 

 has been broad, and the water of uniform depth from side to side, the 

 force of the falling water has been applied disadvantageously ; wher- 

 ever the channel has been narrow, or has been much deeper in some 

 parts than in others, the force of the water has been applied advanta- 

 geously. 



There are many ways in which it is possible that the volume of the 

 river was made to differ at early dates from its present volume. Dur- 

 ing the presence of the ice there was a different climate, and there 

 were different drainage systems. 



Question 10. During the early history of the river was the annual 

 rainfall on which its water supply depended greater or less than now ? 



Question 11. Was the evaporation from the basin at that time greater 

 or less than now ? It is believed that at the present time the Niagara 

 River receives less than half the water that falls upon its basin in rain 

 and snow, the remainder being returned to the air by evaporation from 

 the lakes, from the surface of the land, and from vegetation. 



Question 12. Was the water supply increased by ablation ! There 



