22 The Make-Up and the Wordings of the Earth 



the reasonableness of the idea does not make it a fact. There 

 are, however, actual records kept since 1843, and from these 

 we know that the Falls have been cutting the gorge back 

 an average of 104 inches a year. Is it not reasonable to sup- 

 pose that many years before human records were made as to 

 the precise location of the Falls, at some very remote date, 

 the edge of the rock was still farther down the valley? We 

 have exactly the same confidence in the statement that the 

 Falls were farther north a thousand years ago than they are 

 today as we have in the statement that the Falls existed before 

 the discovery of America. 



Let us assume then that this process of water tumbling 

 over the rock has been going on in the past. Let us assume fur- 

 ther that this tumbling of water which today wears away rock 

 has in the past also worn away rock. We may then ask, How 

 long has it taken the discharge of water from Lake Erie into 

 Lake Ontario to wear back the gulley from its northern end 

 to the present location of the Falls? With such assumptions, 

 the performance which man has been able to observe as a 

 fact is extended backward in time. The result of the calcu- 

 lation is that it must have taken between thirty and forty 

 thousand years for the gorge to have been cut back the dis- 

 tance from Lake Ontario — about twenty miles. And this 

 calculation we accept not as an accurate statement of fact, 

 but as an approximation, since we realize that we have no 

 way of knowing whether the volume of water has been the 

 same year after year, and since we do know that the resistance 

 of the rock and soil has not been exactly the same throughout 

 the whole length of the distance. 



Sedimentation 



At a relatively early stage in the formation of the earth, 

 we conceive the temperature to have become sufficiently 

 lowered to condense most of the water vapor into fluid, and 

 to leave masses of the continents exposed above the ocean. 

 The action of the weather broke down this original crust and 



