THE CHILDHOOD OF THE EARTH 



rather recent, though not less than fifty million years 

 ago. The Moon is therefore one of the younger mem- 

 bers of the Solar System. But no other catastrophe, 

 either before or since, has occurred on the earth to 

 compare with its prodigious birth. Five thousand 

 million cubic miles of material left the earth's surface 

 never again to return to it. Whether it all left at 

 once or whether the action was prolonged we do not 

 know, but we may try in vain to imagine the awful 

 uproar and fearful volcanic phenomena exhibited when 

 a planet was cleft in twain and a new moon was born 

 into the Solar System. 



Then, life still being absent from the earth, the 

 Oceanic Era began. The waters condensed into an 

 ocean over the earth, or else collected in some great 

 oceanic depression. Lands presently emerged from it. 

 It was a hot ocean, steaming no doubt, for its tem- 

 perature was perhaps about 500 F. Some one may 

 ask Why, then, did it not steam away into clouds? The 

 answer is that the atmosphere was still very heavy in 

 that past era, probably still exerting a pressure as much 

 as fifty times as great as to-day. The pressure of the 

 atmosphere at the earth's surface to-day is usually about 

 fifteen pounds to the square inch. In such circum- 

 stances water boils rapidly away at the temperature of 

 212 F. But if the water be taken up to the top of 

 Mont Blanc, where the air pressure is less than that at 

 the sea-level (or if, which amounts to the same thing, 

 we reduce the pressure on the water's surface by placing 

 it under the receiver of an air-pump and partially ex- 



