80 RELICS OF PRIMEVAL LIFE 



composed of the less dense and more oxidizable sub- 

 stances combined with oxygen, and similar in cha- 

 racter and appearance to the slag which forms on 

 the surface of some ores in the process of smelting. 

 Of this slaggy material there might, however, be 

 different layers more or less dense in proceeding 

 from the interior to the surface. This molten sur- 

 face would, of course, radiate heat into space ; and 

 as it would naturally consist of the least fusible 

 matters, these would begin to form a solid crust. 

 We may imagine this crust at first to be smooth 

 and unbroken, though such a condition could 

 scarcely exist for any length of time, as the hard- 

 ened crust would certainly be disturbed by ascend- 

 ing currents from within, and by tidal movements 

 without. Still, it might remain for ages as a spher- 

 oidal crust, presenting little difference of elevation 

 or depression in comparison with its extent. When 

 it became sufficiently thick and cool to allow water 

 to lie on its surface, new changes would begin. 

 The water so condensed would be charged with 

 acid substances which would begin to corrode the 

 rocky surface. Penetrating into crevices and flash- 

 ing into steam as it reached the heated interior, it 

 would blow up masses and fragments of stone, and 



