Geology 



even to indicate the probable changes which took place 

 during its solidification. 



The magma consist chiefly of molten silicates of 

 various elements, together with smaller quantities of 

 oxides, sulphides, phosphates, and many other bodies, 

 and also water and various gases. 



The great masses of granite and other coarsely- 

 crystalline igneous rocks can be proved to have solidified 

 under a considerable cover of other rocks, and in some 

 cases to have been later laid bare by the removal of this 

 cover by denudation. It is with these vast masses of 

 "Plutonic" rocks that the greater number of mineral 

 veins are associated. 



Let us now consider some of the possibilities which 

 may occur during the cooling of a great magma at some 

 depth below the surface of the ground. In the first 

 place the cooling will be slow, as the thick cover of rocks 

 will prevent the rapid escape of the heat. We will 

 suppose the magma to be perfectly fluid, and at such a 

 temperature that its constituents are freely miscible in 

 all proportions. 



As the temperature slowly falls it may be that the 

 first action is a separation into two or more liquids (as 

 in the case of the phenol and water), and then, as the 

 temperature falls lower still, the various compounds of 

 which the magma consists will commence to crystallize 

 out in some definite order, dependent upon their relative 

 solubility. 



As the magma becomes solid the other substances 

 in solution the water, gases, and other volatile sub- 

 stances will also be thrown out of solution, and, as they 

 will be at temperatures far above their boiling points, 



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