PHYSICAL AND GLIM AT AL CONDITIONS. 145 



of the last cold period must have been at least 100,000 

 years ago, and a period of 80,000 years may have elapsed 

 since the ice age began to give way to the present condi- 

 tion of things. If, on the other hand, we suppose that the 

 climatal change depended on variations in the heat of 

 the sun, we have no measure of time, for if these occur to 

 the extent required we do not know their periods or if 

 these have any regularity. We can only infer from the 

 fixity of solar heat within very narrow limits in historical 

 times that any material change must have occurred very 

 long ago. 



Lastly, if with Lyell we have recourse to changes of 

 elevation and depression leading to different amounts of 

 heating surface and different distribution of oceanic and 

 atmospheric currents on the earth itself, geologists may 

 assign less or more time to such changes according as they 

 prefer to regard them as the results of secular or cata- 

 clysmic changes. Thus if we adopt the astronomical 

 theory we are shut up to a very ancient date. If we can 

 explain the facts by merely geological changes the date 

 becomes uncertain. 



I have in previous publications * on this subject argued 

 that the amount of denudation which has occurred since 

 the glacial period is very small, that animal and vegetable 

 life have remained unchanged since the ice age, and that 

 such facts as we can measure in river erosion and changes 

 related to this, indicate but a short time. We may here 

 look at the last of these and cite a few facts. 



In the case of the falls of Niagara,, we know that these 

 have cut the present gorge from lake Ontario back to the 



* Notes on Pleistocene of Canada, 1872, and later papers in Canad. 

 Record of Science,. 

 11 



