DIFFERENTIATION PHENOMENA OF THE PROSPECT INTRUSION. 113 



The Methods of Origin of Igneous Magmas and Hocks, 



(1) Unequal incidence of oxidising agencies on the primor- 



dial metallic substratum, which is assumed to have 

 been itself already differentiated by gravity. 



(2) Extensive melting and incorporation, due : — 



(a) to release of pressure through earth movements. 



(b) to high temperature of intruding magma. 



The incorporation may be (a) complete or (b) 

 partial, the fusible constituents being separated 

 either by being squeezed out by earth movement 

 or in other ways. 



(3) Emulsation (Liquation in the sense of Durocher and 



Backstrom). 



(4) Separation of earlier products of crystallisation in the 



following ways: — 



(a) Sinking of crystals. 



(b) Fractional crystallisation on cooling surfaces. 



(c) Mechanical separation of mother liquor : — 



(i) by earth movements squeezing it out into 



cracks, 

 (ii) by contraction of a solidified crystal 



network. 



It is obvious that no one process of differentiation will 

 account for all the different rocks in existence ; and, in my 

 opinion, everyone of the modes of formation of igneous 

 magmas and rocks above enumerated can be proved to have 

 been actually operative in one case or another. The 

 principal questions to be considered here will be the con- 

 ditions under which the processes operate. I have purposely 

 omitted reference to hypotheses which are now generally 

 discredited, such as Bunsen's theory of mixture of normal 

 pyroxenic and trachytic magmas, and Soret's diffusion 

 principle. 



H-July3,1912 



