337- 



The scheme of differentiation can be summarized in the 

 appended chart : — 



Residual Magma ^ Albite Pegmatites, 

 (dominantly sodic). 



Chonolitic 



Mass. 



Differentiation 



in 



situ. 



t 



Microcline Albite Aplites 



(differentiation may cease 



here). 



t 



Salic 



differentiate 



from a 



batholitic 



Magma. 



Residual Magma 

 (dominantly potassic). 



t 



Granite [Adamellite]. 



t 



Granitic Magma. 



t 



The very minor amount of albitite in comparison to the 

 development of potassic-aplite is here again emphasized. If 

 this relation be preserved at depth, it follows that the com- 

 position of the residual magma lies very close to that of the 

 potassic-aplites. 



In this connection the writer would point out that the 

 differentiation of the residual magma may be controlled by a 

 mechanical factor. Where the residual magma has been 

 forced into fissures and caused to rapidly cool, further 

 differentiation may be inhibited and the magma solidify as a 

 crystalline aggregate of quartz, microcline, and subordinate 

 albite. 



On the other hand, the filtering of the residual magma 

 into a subsidiary pool without rapid change of temperature 

 and its slow crystallization undisturbed, then fractional 

 crystallization may take place with the production of a small 

 amount of residual liquid, enriched in mineralizers and of 

 composition markedly different from that of the original 

 residual magma. 



Movement at this stage would result in the straining off 

 of the small amount of residual liquid, giving rise to intru- 

 sions of highly sodic-pegmatite. 



In composition, the potassic-aplites so derived would 

 differ but slightly from that representing the composition of 

 the residual magma. 



