THE FIRE-HARDENED ROCKS 



earth's surface. The only thing we can urge is that they 

 do not seem big enough for the purpose, if the earth 

 were indeed all molten except for a thin crust thirty 

 miles thick. For that would leave a molten ocean more 

 than 7900 miles across any way it was measured : 7900 

 miles deep, 7900 miles broad, 7900 miles long, if we take 

 the diameter of the earth to be 8000 miles. We all 

 know what great tides the Moon and Sun by their attrac- 

 tion raise in the earth's outer ocean of water. Think what 

 tides they would raise in this inner ocean of molten rock 

 and metal. The earth's crust would not be able to hold 

 such tides in. The molten stuff would be always break- 

 ing through the flimsy thirty miles of outer solid rock as 

 if it were egg-shell. Twice a day there would be out- 

 breaks of lava vast enough to submerge continents. 



No, that will not do. We will not confuse our readers 

 by telling them all the theories that have been formed, 

 but will only state what the late Lord Kelvin believed, 

 and most of the present generation of geologists believe. 

 It is that the heat of the earth's crust continues to in- 

 crease only for a certain distance of the way down, and 

 that owing to pressure the earth is solid (though very hot 

 except towards the surface) for two thousand miles down. 

 There remains a thickness of another four thousand miles 

 on either side of the earth's centre to be considered. 

 That might be molten, but the pressure would be so great 

 that it would behave as if it were a solid. We know the 

 earth cannot be solid all through because it does not 

 weigh enough. The earth cannot, of course, be weighed 

 in any scales, but there are methods of weighing it never- 



8? 



