THE FIRE-HARDENED ROCKS 



ing pitch, or very thick gum. In fact, at depths of about 

 six, seven, or eight miles, it is supposed by many geolo- 

 gists that if the lower rocks had room to move they 

 would have a tendency to flow. 1 



Suppose, however, they cannot flow, that there is no 

 room for them to flow, and that the pressure is not merely 

 thirteen or fifteen tons to the square inch, as it would be 

 at depths between five and six miles, but a hundred times 

 that amount, as it might be between five and six hundred 

 miles down. What would happen then ? We can only 

 imagine what does happen by stating what does not 

 happen. It used to be supposed as late as half a century 

 ago that the earth consisted of a crust of hard rocks per- 

 haps thirty to fifty miles in thickness, and that below 

 this crust the whole earth was a mass of red-hot or white- 

 hot molten stuff with flaming gases mixed with it. If 

 that were the case it would explain a good deal of what 

 we see around us. It would explain the volcanoes, for 

 instance, which belch out fire and lava and ashes and 

 molten rock, and sometimes great fragments of rock. 

 Perhaps some of our readers may remember the great 

 eruption of Mount Pelee, which took place in Martinique 

 some years ago. At one stage of the eruption a great 

 obelisk of rock a thousand feet high was pushed upwards 

 out of the crater, and eventually sank back again. It 

 came out of the depths of the earth. It was like a vent- 

 peg plugging some boiling mass below. Similarly we 

 might suppose that all volcanoes were vent-holes for the 

 tremendous commotion of boiling fiery rocks below the 



1 Geology : Earth History, p. 127. Chamberlin and Salisbury. 

 86 



