362 MAN AND THE GLACIAL PERIOD. 



would sweep around the world every four hours. Such a 

 condition of things would evidently be incompatible with 

 geological life, and geology must limit itself to a period 

 which is inside of 100,000,000 years. Sir William Thom- 

 son and Professor Tait, of Great Britain, and Professor 

 Newcomb, of the United States Naval Observatory, ap- 

 proaching the question from another point of view, seem 

 to demonstrate that the radiation of heat from the sun 

 is diminishing at a rate such that ten or twelve million 

 years ago it must have been so hot upon the earth's sur- 

 face as to vaporise all the water, and thus render impos- 

 sible the beginning of geological life until later than that 

 period. Indeed, they seem to prove by rigorous mathe- 

 matical calculations that the total amount of heat origi- 

 nally possessed by the nebula out of which the sun has 

 been condensed would only be sufficient to keep up the 

 present amount of radiation for 18,000,000 years. 



The late Dr. Croll, feeling the force of these astro- 

 nomical conclusions, thought it possible to add sufficiently 

 to the sun's heat to extend its rule backwards approxi- 

 mately 100,000,000 years by the supposition of a col- 

 lision with it of another moving body of near its own 

 size. Professor Young and others have thought that pos- 

 sibly the heat of the sun might have been kept up by the 

 aid of the impact of asteroids and meteorites for a period 

 of 30,000,000 years. Mr. Wallace obtains similar fig- 

 ures by estimating the time required for the deposition of 

 the stratified rocks open to examination upon the land 

 surface of the globe. As a result of his estimates, it 

 would appear that 28,000,000 years is all the time re- 

 quired for the formation of the geological strata. From 

 all this it is evident that geologists are much more re- 

 stricted in their speculations involving time than they 

 thought themselves to be a half-century ago. Taking as 

 our standard the medium results attained by Wallace, we 

 shall find it profitable to see how this time can be por- 



