PHYSICAL EVOLUTION 9 



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 denudation 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 succes- 

 sion ; 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 mountains 

 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. 



