776 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



Expulsion of Residual Magma. — Harker has suggested a third way in which 

 gravity may affect differentiation.* He writes: — 



' Any differentiation which depends on the sinking of crystals under 

 gravity belongs necessarily to a somewhat early stage of crystallization, 

 when the bulk of the magma was still in a liquid condition. At a later stage, 

 when the crystals formed are so numerous or so large as to touch and support 

 one another, the condition may be likened to that of a sponge full of water; 

 and it is easy to picture a partial separation being effected by the straining 

 off or squeezing out of the residual fluid magma from the portion already 

 crystallized. That such a process does in fact take place is amply proved 

 by the phenomena of pegmatites, which represent the final residual magma 

 of plutonic intrusions.' 



The squeezing out is regarded as specially noteworthy if the freezing magma 

 is subject to pressure from movements of the earth's crust. 



4. Effect of Solution of Foreign Eock. — The fourth of the primary law3 

 affecting differentiation has been stated with unusual force by Loewinson-Lessing.f 

 He holds that in many instances magmatic differentiation is induced by the absorp- 

 tion of foreign rock. This exotic material may bring about liquation in the 

 original magma which, as a whole, may have suffered little chemical change by 

 the assimilation. Here, as in many other points, Loewinson-Lessing's summary 

 of petrogenic theory shows keenness, profundity, and breadth of view, which 

 are seldom rivalled in other general works on igneous rocks. 



*A. Harker, Natural History of the Igneous Eocte, 1909, pp. 323-27. 

 t F. Loewinson-Lessing, Compte Rendu, Seventh Session, Congres geol. interaat., 

 1899, p. 380, etc. 



