THE CRYSTALLINE ROCKS NEAR ST. JOHN, N. B. 21 



As the sinking continues, the heat and pressure increase, 

 and the remaining water has a more powerful influence, 

 enlarging the old crystal grains and changing the surface- 

 formed minerals into new ones, which are much more 

 crystalline and harder. Finally the rock passes into a 

 state in which it is only kept from melting by the 

 enormous pressure from above, which, by preventing the 

 slight expansion which rocks undergo in becoming fluid, 

 compels it to remain solid. 



The Origin of Igneous Rocks. 



If now this pressure be relieved, as it would be if a 

 crack should form at the surface and extend down into 

 the buried rock, the whole mass liquifies and squeezes 

 up through the crack, pushing apart the walls as much 

 as it can, part of it perhaps reaching the surface and 

 pouring out over it, part solidifying on its way up. This 

 molten magma contains the one or two per cent of water 

 that still remained in the sediment after it had progressed 

 far down into the region of internal heat ; and this water 

 greatly assists in keeping it fluid. 



The part that reaches the surface cools quickly, and 

 much of its contained water escapes as steam, being now 

 released from the pressure that had kept it liquid. This 

 steam blows up the lava into scoria and pumice, or 

 shatters it into fragments, which are thrown from the 

 mouth of the crater by the ascending steam, and scattered 

 far and wide over the country round. The coarse frag- 

 ments will fall near the opening, making a volcanic 

 breccia, so called ; the finer material will travel further 

 and form beds of volcanic ashes or tuff". Some may fall 

 into the sea — all indeed, if the volcano be sub-marine, — 



