THE ABYSS OF CENTURIES. 291 



to sustain so high a figure except the general guesses of cer- 

 tain people who "entertain no doubt that man has been on 

 the earth one or two hundred thousand years." 



The problem may be attacked in quite a different way. 

 We have noted vast erosions which have taken place during 

 the history of the world. If we could only obtain a clew to 

 the rate of any one of these erosions, we could soon calculate / 

 how much time was required for the work. Now, the Niagara 

 gorge presents a specific and measurable amount of work ; and, 

 if we can determine how much of that work is accomplished 

 yearly, we shall have the age of the gorge. This problem is 

 so intelligible that it has received the attention of many in- 

 vestigators. But their results have been widely divergent. / 

 The whole gorge is seven miles long. Mr. Robert Bake well ' 

 made its age 12,300 years. Messrs. Lyell and Hall made it 

 35,000 years. Mr. E. Desor, who assumed much too low a 

 rate of erosion, assigned 1,232,000 years. Mr. Jules Marcou 

 found data to make it 64,842 years. These wide discrepancies 

 arise from the difficulty of ascertaining the rate of recession. 

 Within a few years that has been well settled at about three I 

 feet a year. Professor James Hall, in 1842, caused to be made 

 a trigonometrical survey of the falls, and fixed permanent land- 

 marks. In 1875, Mr. James Gardner, Director of the New 

 York State Trigonometrical Survey, repeated the survey of the 

 Falls, and ascertained that there had been a recession of at 

 least one hundred feet in thirty-three years. This rate would 

 give 12,300 years as the age of the whole gorge, from Lewis- 

 ton. The three miles below the "whirlpool," where the new 

 gorge intercepted the old one, would have required 5,280 

 years. As we may fairly assume that the old gorge extended 

 some distance above the whirlpool, we may conclude that the 

 work of the modern river has extended over a period between 

 5,280 and 12,300 years. From what I have stated in Talk 

 XL VIII, you will understand that the work of the modern 

 river is post-glacial work, and these numbers concern the 

 duration of the post-glacial period. 



I have said that the old gorge from St. David's was the 



