196 SPARKS FROM A GEOLOGIST'S HAMMER. 



initiation by mathematical analysis.* The results of cal- 

 culation show that a period of high eccentricity termi- 

 nated about 80,000 years ago, and another period about 

 720,000 years ago. To which of these shall we refer the 

 Glacial Period of Post-Tertiary time? Certain geologists, 

 impressed by the vastness of geological intervals, would 

 decide promptly in favor of the remoter epoch. But, as 

 we have stratigraphies! evidence of the occurrence of an 

 earlier glacial period in Miocene time, the date of this 

 would be removed back to the next preceding period of 

 high eccentricity, 2,500,000 years ago. The admission of 

 such an interval since Miocene time would set back the 

 commencement of sedimentation beyond 100,000,000 years, 

 which, as Sir William Thomson has demonstrated, is the 

 largest interval which can be admitted, according to the 

 laws of cooling, since the commencement of terrestrial in- 

 crustation.! 



We have then to examine whether an interval of 80,- 

 000 years is sufficient for the whole amount of denuda- 

 tion which the continents have suffered since the Glacial 

 Period. An ingenious investigation, instituted by Mr. Croll, 

 shows that the actual denudation is not less than one 

 foot in six thousand years. If we assume the Glacial 



* See especially Stockwell, Smithsonian Contributions to Knowledge, xviii ; 

 Croll, Climate and Time, ch. iv, xix; R. W. McFarland, Amer. Jour. Sci., Ill, 

 xi, 456. 



tThe interval since the last decline of continental glaciers, judging from 

 the comparative amount of sedimentation and other geological results, is not 

 over four tenths of one per cent of the whole time since the beginning of in- 

 crustation. Authorities differ widely as to the possible length of that time. 

 Professor Newcomb says the total mass of the sun would cool from its present 

 condition to a body as dense as the earth in twelve million years; and that not 

 over ten million years can have elapsed since the heat of the sun was too great 

 to permit water to exist on our planet. With such views it is improbable that 

 Post Tertiary time amounts to more than 61,000 years, or Post Glacial time to 

 more than 30,000 years. 



