Niagara Falls 

 1889 learn the relations of those variations to each other, and so we 



Gilbert , 



ask: 



Question 1 5. How have the variations of rock section, the 

 variations of cataract height, the variations of form of channel 

 and the variations of volume been related to one another in point 

 of time? What have been their actual combinations? 



Question 1 6. How have the various temporary combinations 

 of factors affected the process of retreat and the rate of 

 recession ? 



The tale of questions is not exhausted, but no more are needed 

 if only it has been shown that the subject is not in reality simple, 

 as many have assumed, but highly complex. Some of the ques- 

 tions are, indeed, easily answered. It may be possible to show 

 that others are of small moment. It may even be that careful 

 study of the local features will enable the investigator to infer 

 the process of cataract work at each point from the existing con- 

 dition of the gorge, and thus relieve him from the necessity of 

 considering such remote questions as the nature of glacial climate 

 and the history of glacial retreat. But after all paring and 

 pruning what remains of the problem will be no bagatelle. It 

 is not to be solved by a few figures on a slate, nor yet by the 

 writing of many essays. It is not to be solved by the cunning 

 discussion of our scant, yet too puzzling knowledge — smooth- 

 ing away inconvenient doubts with convenient assumptions, and 

 cancelling out, as though compensatory, terms of unknown value 

 that happen to stand on opposite sides of the equation. It is a 

 problem of nature, and like other natural problems demands the 

 patient gathering of facts, of facts of many kinds, of categories 

 of facts suggested by the tentative theories of to-day, and of new 

 categories of facts to be suggested by new theories. 



I have said our problem is but the stepping stone to another 

 problem, the discovery of common units for earth history and 

 human history. The Niagara bridges the chasm in another way, 

 or more strictly, in another sense, for the term of its life belongs 



582 



