PLANETARY EVOLUTION 



comparatively late epoch ; although glaciation, 

 whether brought about by changes in the dis- 

 tribution of land and sea or by a variation in 

 the eccentricity of the earth's orbit, certainly 

 does seem to imply a progressive dependence 

 of the earth upon the supply of solar heat, due 

 to the lowering of its own proper temperature. 

 Such facts, however, are wholly inadequate to 

 describe the primitive heat of the earth. The 

 flattening of the poles being considerably greater 

 than could have been produced by the rotation 

 of a globe originally solid on the surface, it fol- 

 lows that the whole earth was formerly fluid. 

 And this conclusion, established by dynamical 

 principles, is uniformly corroborated by the ob- 

 served facts of geology. Now the fluidity of the 

 entire earth, with its rocks and metals, implies 

 a heat sufficient to have kept the planet incan- 

 descent, so that it must have shone with light 

 of its own, like the stars. Similar conclusions 

 are indicated by the observed geologic features 

 of Mars and Venus ; and in the case of the 

 moon we shall presently see what a prodigious 

 loss of heat is implied by the fact that the forces 

 which once upheaved its great volcanoes are 

 now quiescent. The sun, too, is pouring away 

 heat at such a rate that, according to Sir John 

 Herschel, if a cylinder of ice 184,000 miles in 

 length and 45 miles in diameter were darted 

 into the sun every second, it would be melted 



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