25i BULLETIN OF THE NATURAL HISTORY SOCIETY. 



and will then be worked over by waves and currents 

 like ordinary sediments. The solid lavas will have cooled 

 at their surface, too quickly for crj-stals to form, and 

 will be glassy rocks or obsidiajis. Away from the surface, 

 if the lava stream be thick enough, crystals will have 

 had time to form, and the rock will be full of small 

 crystals. 



The part of the magma which failed to reach the 

 surface will cool very slowly, so that its crystals will be 

 comparatively large. They will finally interfere with 

 each other's growth, and their boundaries will not be 

 smooth faces, but irregular surfaces, the rock being made 

 up of more or less roundish grains of different minerals. 

 This structure is called granitic^ as being best shown in 

 the granites. But suppose the rock is interrupted while 

 it is cooling, when some large crystals have formed, but 

 before they have begun to interfere with each other ; and 

 pushing up to the surface, cools there. The still liquid 

 part will solidify as a mass of minute crystals, or a glass, 

 and throuo^h this will be scattered large ones. It has 

 then a jporphyritic structure, an extremely common one in 

 volcanic rocks. 



It is evident that a large mass of molten matter cool- 

 ing slowly at a considerable depth will have great 

 influence on the surrounding rocks, not by its heat alone, 

 but especially on account of the highly heated water it 

 contains. As it crystallizes, much of this water will 

 escape, and, intensely heated and loaded with dissolved 

 material, will work out into the surrounding beds, pro- 

 ducing new minerals and re-crystallizing old ones. 

 These contact effects will be most conspicuous next the 

 granitic rock, and will gradually decrease as we go away 

 from it. 



