FIRE, ROCK, AND SEA 17 



may sometimes build up to the point of shattering the glass. 



The natural period of oscillation for tides on the earth is 

 four hours. The result of the sun's tidal influence about three 

 billion years ago was therefore to build up tidal resonance. 

 Partly due to this, it has been suggested that the tides became 

 ever higher, until the earth was pulled into the shape of a 

 rather long egg, and finally a portion was completely ejected as 

 a colossal drop of matter, to form the moon. The mutual tidal 

 action of earth and moon came into play immediately and 

 caused them to separate very rapidly, so that both were able to 

 reshape themselves into almost spherical bodies. Thus, some 

 say, the moon was born, and they place the time at about three 

 billion years ago. 



It has been suggested that when the moon was torn loose, 

 the earth was already losing its liquid state, so that in place of 

 the moon there remained for a while a great void in its sur- 

 face, a part of which is now the Pacific Ocean basin. It has also 

 been suggested that the remainder of the crust moved slowly 

 to fill this gap, and in drawing apart gave rise to the Atlantic 

 by a splitting process. If this actually took place there could 

 have been at the time little more than a thin skin of solid 

 crust. The perfect spherical shape of the moon could hardly 

 have been achieved if the moon had been torn away when the 

 surface of the earth was in its present solid state. The present 

 shape of the earth, a slightly flattened sphere, must have been 

 molded while all except the outermost crust was fairly liquid, 

 and still able to smooth out the effects of losing the moon. 



A more exact way of setting the date when the earth ceased 

 to be molten is based on the great advances made in recent 

 years by researches into radioactive substances. The element 

 uranium is one of these substances found today in the rocks of 

 the earth. It is continually giving off radiations of various kinds 

 at a steady rate. These radiations include alpha particles, which 

 are the positively charged nuclei of helium atoms, beta par- 



