THE STORY OF THE EARTH 31 



Thus we have in modern astronomy, the first two 

 pages in the story of the earth : first the incandescent 

 phase and the formation of our chemical elements, then 

 the birth of the moon. We have now to trace in greater 

 detail the transformations that gave us our planet from 

 the small, fiery mass of hundreds of millions of years ago. 



That the story was essentially one of cooling needs no 

 emphasis. Surrounded on all sides by the absolute cold 

 of space, the planet shed its heat prodigally about it. 

 That this cooling would lead of itself to the rotation 

 of the planet on its axis is proved by mathematical 

 considerations into which we cannot enter here. That, 

 further, the heavier elements would sink deeper into the 

 mass and the lighter elements remain at the fringe is a 

 simple consequence of gravitation. But this prepares 

 us fully for the succeeding phases of development. The 

 time would come when the liquid incandescent ball would 

 lose so much of its heat that a skin or scum would begin 

 to form on its surface. One must not imagine the 

 formation of the crust of the earth as a tranquil and even 

 freezing of the surface such as we observe on the pond 

 in winter, or even as the cooling of a vessel of molten 

 iron. Titanic energies convulsed the mass of the molten 

 planet ; tidal action raised its responsive wave as long as 

 this was possible; and tornadoes in the heavy atmosphere 

 lashed and tore the forming skin. For ages the new 

 film would struggle with the enormously high tempera- 

 ture below and the storms above ; it would fall in slabs 

 or masses, as it formed, deep into the liquid mass. 

 There are authorities even who think that the solidifica- 

 tion must have begun at the centre. But the general 

 feeling is that a film cooled first at the surface. It 

 would crack like the film on a basin of cooling paste, and 

 its great fragments sink some distance below the surface. 

 The skin of the earth would be too tight for its body in 



