HAMILTON SCIENTIFIC ASSOCIATION 141 



planet was profoundly disturbed by tides produced by the sun, for 

 there was yet no moon; it has, indeed, been suggested that the 

 moon was severed from the earth by a tidal wave of great height. 

 Be this as it may, the event of the separation of the moon from the 

 earth is to be regarded as marking tlie first critical period in the 

 history of our planet. 



According to Professor Wiechert, the outer envelope of the 

 earth's interior nucleus consists of silicates possessing a density of 

 3.2. It was from this envelope, when molten, that the moon was 

 removed, twenty-seven miles in depth going to its formation. The 

 density of our satellite, allowing for differences in temperature, is 

 the same as that of the material from which it originated. Under 

 circumstances of diminution of atmospheric pressure, as the moon 

 drew away from the earth our satellite would become as explosive 

 as a charged bomb, and the resultant explosions may have caused 

 the existing features on the moon's surface. 



The solidification of the earth probably became completed soon 

 after the birth of the moon and marks the second critical period in 

 the world's histor5^ 



On the crust becoming solid, definite areas of high and low 

 pressures might have been established, causing depressions and ele- 

 vations on the surface. When the temperature of the globe's at- 

 mosphere in cooling had fallen to 370° C, that part of the atmos" 

 pliere consisting of steam would begin to liquify and the huge 

 dimples on the earth's surface would become filled with super- 

 heated water, deepening till they formed the oceans. This is the 

 third critical period in the earth's history. The course of events 

 now becomes somewhat obscure, but sooner or later the processes 

 of denudation and deposition commenced and have been acting un- 

 interruptedly ever since. 



The nebular hypothesis states that our earth was once a glow- 

 ing star which in process of time cooled down to a liquid form and 

 assumed a spherical shape^ its atmosphere charged with water and 

 mineral matter vapourised into steam by the intense heat. As the 

 process of cooling continued, a thin crust was formed, gradually 



