THE LESSON OF EVOLUTION 17 



As with the Solar System, so also in the Earth itself, 

 we can trace distinctly a physical evolution. The 

 discovery of tidal friction gave an independent proof 

 that the Earth had had a beginning not infinitely 

 remote ; for, if that had been the case, the tidal friction 

 would have reduced the time of the Earth's rotation 

 on its axis to that of the moon. Also we have suffi- 

 cient geological evidence to show that not more than 

 one hundred millions of years ago the earth was in a 

 molten condition, and probably shone with its own 

 light. As cooling went on, the silicates crystallised 

 out, forming a solid crust over the still molten, metallic 

 interior, and the earth then became a dark body. 

 At that time all the water above the crust was in a 

 state of vapour, which subsequently fell as hot rain ; 

 forming a boiling ocean. With this rain the denuda- 

 tion of the primitive crystalline rocks commenced, and 

 their debris was deposited on the bed of the ocean as 

 sedimentary rocks. Gradually the continents were 

 formed, the new ranges of mountains following each 

 other in orderly succession ; the great oceans becoming 

 narrower and deeper as well as more and more salt. 

 These processes are still going on ; but, as the earth 

 is cooling, the internal energy which uplifts the moun- 

 tains must be diminishing, and in time it will be 

 insufficient to counteract the denudation. Then the 

 whole of the land will be swept into the sea, and 

 the waves of the ocean will roll over the surface of 

 the earth unopposed unless, indeed, before that time 

 arrives, the ocean should have been frozen into a mass 

 of ice, or should have sunk slowly into the ground. 

 All these things are approaching, but which of them 

 will come first it is impossible to say. 



B 



