DIFFERENTIATON PHENOMENA OF THE PROSPECT INTRUSION. 115 



ranges is good evidence that there never was an intimate 

 mixing of the upper and lower layers of the crust. 



At some considerable depth there is, probably, a like 

 predominance of basic rocks and magmas, which have only 

 here and there, through exceptionally deep orogenic move- 

 ments, been forced up to the surface and upper parts of 

 the crust. I do not, therefore, believe that there has ever 

 been a primordial magma of uniform composition from 

 which granitic and gabbroic types were derived by differ- 

 entiation, but there was a primitive magma of granitic 

 composition which, by admixture and selective reaction 

 with varying proportions of the molten metallic substratum 

 (also itself probaly varying somewhat in composition from 

 place to place), formed various basic and intermediate 

 magmas. By "selective reaction" I mean that those 

 metals having the greatest affinity for oxygen and silica 

 w r ould be silicified first and absorbed into the magma, 

 whilst, except under peculiar circumstances, such as may 

 have caused native iron to remain in certain basalts, the 

 unoxidised metal would sink through the magma like molten 

 iron through the slag in a blast furnace, and be left behind 

 when the magma was extruded. 



It will be remarked that I postulate, throughout the 

 whole of this discussion, a high degree of fluidity for the 

 magmas considered, which some authors, having in view 

 the experiments of Joly, Fonque and Levy, and others, in 

 melting silicates, are not disposed to admit. I grant that 

 there is much geological evidence that magmas containing 

 free silica are usually very viscous when intruded near the 

 surface, or are erupted. Aplite veins, and the apophyses 

 of granite masses, on the contrary, have every appearance 

 of having been much more fluid, doubtless owing to their 

 high temperature, and to the water- vapour and other gases 

 they retained under the high pressure existing at great 



