32 EVOLUTION 



that early stage, and volcanic eruptions through it would 

 be a constant occurrence. 



After ages of this conflict of solid scum and a 

 refractory liquid fire, sufficient solid matter would be 

 formed to encircle the entire globe. A crust of solid but 

 white-hot rock would now confine the fire below, with 

 constant eruptions through it to ease the pressure. No 

 water could settle on it, and the tempestuous atmosphere 

 lay heavily upon it. Lower and lower sank the tempera- 

 ture of the crust, as its heat radiated into space. A 

 distant observer would see the little white star turn 

 yellow, then red, and at last, when the temperature sank 

 to 500C., lose its light altogether, and henceforth be lit 

 up only from the central luminary. Later the great 

 masses of oxygen and hydrogen, that had combined into 

 molecules of water in the atmosphere, began to settle on 

 the crust. In the enormous pressure of that primitive 

 atmosphere water could lie as liquid on the surface 

 at a much higher temperature than now, and a new 

 struggle of the elements would ensue. The heated crust 

 would send the water hissing back into the atmosphere 

 in clouds of steam, but its heat would now pass away 

 more rapidly, and the contest would be comparatively 

 short. Before long a boiling ocean would cover nearly 

 the whole of the earth's crust, and above this was an 

 atmosphere still fifty times heavier than the atmosphere 

 of to-day, because of the great volume of carbonic acid 

 gas that it contained. 



Thus we get the triple zone of matter that encircles 

 our planet the lithosphere, or girdle of rock ; the hydro- 

 sphere, or belt of water ; and the atmosphere, or mantle 

 of respirable air. Naturally, their features were very 

 different at that remote date from those they present 

 to-day, and it is very important in view of the later 

 chapters to bear well in mind the physical evolution of 



