368 PRACTICAL LESSONS IN SCIENCE. 



can be given to the first or the second questions, but many at- 

 tempts have been made to answer the third. 



It is generally supposed that the gorge of the Niagara has 

 been formed since the retreat of the ice, and that if we knew the 

 age of the gorge we would know how long it has been since the 

 glacier. Early estimates of the age of the gorge varied from 

 35,000 to 100,000 years, but later ones vary from 4,000 to 

 10,000 years. The river at the present time is cutting back at 

 the rate of about 2.4 feet per year, or a mile in about 2,200 years, 

 and the gorge in about 15, 400 years. But there are evidences 

 that the river found considerable work already done on the 

 gorge when it began to flow, so that perhaps 7,000 years would 

 be a safe estimate for the age of the Niagara gorge, and for the 

 time that has elapsed since the ice retreated from the St. Law- 

 rence valley. 



The glacier was a very effective erosive agent, but whether 

 more effective or rapid than air and water is very doubtful. 



