PHYSICAL AND CLIMATAL CONDITIONS. I45 



of tlie last cold period iiiust have Leen at least 100,000 

 years aj^o, and a period of «S0,000 years may have ela^jsed 

 since the ice a^'e he^^an to give way to the ))reseiit coiidi- 

 I'on of things. If, on the other hand, we snppose that the 

 cliniatal change de[)ended on variations in the heat of 

 the sun, we have no nieasuvc of time, for if the.se occur to 

 the extent re(|uired we do not know their jjcriods or if 

 lh(!se have anv regularitv. AVe can only infer from the 

 lixity of .solar heat within very narrow limits in historical 

 times tluit any material clumge mu.st liave occurred very 

 long ago. 



Lastly, if with Lyell we have recourse to changes of 

 elevation and de])ression leading to dill'erent amounts of 

 lieating surface and dillerent distrihution of oceanic and 

 atmos])heric currents on the earth itself, geologists nuiy 

 assign less or more time to such changes according as tiiey 

 ])refer to regard them as the results of .secular or cata- 

 clysmic changes. Thus if we adopt the astronomical 

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

 explain the facts by merely geological changes the date 

 Ijcconjes uncertain. 



1 have in previous publications * on this subject argued 

 that the amount of denudation which has occurred since 

 tlie 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 List of these and cite a few facts. 



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

 iiave cut the present gorge from lake Ontario back to the 



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

 Record of Science. 

 II 



