THE EARTH 137 



Certainly it had a separate existence millions of 

 years ago. It has been calculated by Professor 

 G. H. Darwin that it is at least fifty-six million 

 years since the Earth bore her only child, the 

 Moon. ^ 



At the time the Earth bore the Moon it was a 

 molten mass, probably pear-shaped, for reasons we 

 have already explained, and surging with great 

 tidal waves produced by the attraction of the Sun ; 

 and the theory is that eventually the tugging of 

 the Sun tore away a great piece of molten metal 

 twenty-seven miles in depth, which since then has 

 rotated round the Earth as the Moon. The Earth 

 was spinning at a tremendous pace then, six or 

 twelve times as fast as now ; and since the Moon 

 revolved round it every three or four hours, and 

 was only ten or twenty thousand miles away, the 

 tides in the molten masses of the Earth and Moon 

 must have been very great. 



Soon after bearing the Moon, the Earth acquired 

 a solid crust, which must at first have had a 

 temperature of 1200° C. At this stage all the 

 water now on the face of the Earth may have 

 existed as a deep, dense atmosphere of a steam, 

 which must have exercised great pressure on the 

 thin crust of the Earth, and may have helped with 

 the Moon to form the beds for the coming oceans. 

 When the temperature of the crust fell to 370° C. 



