BENEATH THE EARTH'S SURFACE 503 



thick, as the interior heat would melt rocks under ordinary 

 pressure. But the earth is too rigid for its interior to be 

 otherwise than solid. So great is the pressure to which it 

 is subjected that probably none of the material deep down 

 in the interior of the earth is in a molten condition. 



If the pressure near the surface should be decreased, 

 or if the normal amount of heat at any place should be 

 increased, the material might become fused, and under 



AN HAWAIIAN CRATER 



certain conditions might find its way to the surface. We 

 know that heated material from below does rise toward the 

 surface and intrude itself into the surface rocks and in 

 some places pour forth over the surface. 



What causes the uprising and outpouring of this molten 

 material from below the surface of the earth, and how and 

 why it 'reaches the surface are questions which as yet are 

 unanswerable. But as soon as this igneous material comes 

 within the range of observation, its properties and actions 



