26 LIFE IN THE PRIMITIVE OCEAN 



on the earth would also contain great quantities of 

 acids and chemicals. 



Here, then, we have three features of the earth 

 explained: its molten interior, its solid crust, and its 

 oceans.^ A globe of molten metal naturally cools 

 first at its surface, and forms a crust or "slag," as 

 molten iron does in the foundry. It is now at least 

 a hundred million years, and may be very much more, 

 since the earth began to cool. The solid crust has 

 become from fifty to seventy-five miles in thickness, 

 and the great mass inside remains at a terrific tempera- 

 ture. The pressure is too great to allow it to be fluid, 

 but there are leaks and irregularities in the crust, and 

 at times it oozes through as white-hot lava. There 

 you have the explanation of volcanoes, and partly of 

 earthquakes. 



At first the crust would be a fairly equal scum or 



slag all round the globe, and the ocean would be 



fairly equally distributed over it. At the most a few 



ridges of land might peep out of the hot ocean. But 



there would be for ages, probably for millions of 



years, a mighty battle between the crust and the 



molten matter below it. In cooling, the surface or 



crust would naturally shrink. The skin, so to say, 



would always tend to be too small for the globe, and 



* I ought to warn the reader that many geologists now follow 

 the Planetesimal Hypothesis, which I mentioned on p. i6. Ac- 

 cording to this, the earth was formed with less heat, and was 

 never all molten. The accoimt I am following is the one generally 

 received, and most probably correct. 



