36 EVOLUTION OF THE EARTH 



gravel size were swept up by the earth many millions of times 

 more abundantly than are meteors at the present time. Those 

 meeting the earth with the higher velocities were consumed by 

 impact. Over the hemisphere of night the otherwise invisible 

 atmosphere above the cloud canopy scintillated with incessant 

 flashes of light and trails of luminous dust. Bodies of larger 

 size gave in their dissolution a still more brilliant display and 

 penetrated to greater depths. At longer intervals, with titanic 

 rush and roar, a greater projectile, tens or even hundreds of 

 miles in diameter, cleaved through the canopy of cloud, leaving 

 a tumultuous maelstrom behind, drove almost unchecked 

 through the deep and dense atmosphere below, and, with 

 worldwide commotion, was engulfed, with development of 

 fervid heat, within the molten sea. 



THE PASSAGE OF THE MOLTEN INTO THE RIGID EARTH 



Density stratification by fractional crystallization. The 

 central parts of the earth were compressed during the growth 

 stages by the increasing load above. This compression 

 developed heat, but also raised the fusion point and made for 

 a greater rigidity. It is not known, consequently, whether 

 during earth-growth the center tended toward a liquid or 

 solid state. The outer part, however, with a thickness of 

 perhaps the outer quarter of the radius, comprising about one- 

 half of the volume of the sphere, seems to have passed into 

 a truly molten condition. 



The heavy atmosphere and canopy of cloud prevented a 

 rapid radiation from the molten surface, probably sufficient 

 during the highly liquid stage to prevent a crusting over of 

 frozen rock. The method of solidification approached nearer 

 to that which occurs in a large reservoir of magma intruded 

 into the crust than to the freezing of a modern lava stream in 

 contact with the air. 



