THE FIRE-HARDENED ROCKS 



theless. One of two methods is by seeing how strongly 

 it attracts bodies to itself. But these things belong rather 

 to the romance of astronomy than to that of geology. 

 We need only trouble ourselves at present about the 

 results. 



One word more about the deep interior of the earth. 

 Dr. J. J. See, an American astronomer, has found how 

 heavy and how hard the earth is, taken as a whole. He 

 finds that if it were built from surface to surface of 

 hardened steel it would be just about as heavy and as 

 hard or as rigid. The steel would be like that used 

 for the armour - plate of battleships. Dr. See is not 

 prepared, however, to discard the idea that the earth has 

 a large fluid interior. If it were fluid, yet it would be 

 subjected to such enormous pressure by its own weight, 

 that if there were a moderately thick earth-crust, its 

 tidal surgings would be so " cabined, cribbed, confined," 

 that they would be comparatively ineffectual. We must 

 not run away with the idea (against which Dr. See 

 specially warns us), that there is any free circulation of 

 currents within the fluid interior. The rigidity produced 

 by pressure (or weight) is too great for that. Indeed, 

 this pressure is so great that, as another scientific 

 authority, Professor Arrhemus, has pointed out, the 

 matter at the core of the earth might even be gaseous ; 

 and yet would be so compressed by pressure that it 

 would possess a rigidity equal to the hardest steel. The 

 earth may be partly solid, partly liquid, partly gaseous, 

 but for all practical purposes Professor See would have 

 us regard it as a solid sphere having an average hard- 



