331 



VI. The Relations of the Rock Types. 



In the accompanying table the mineral constitution of 

 the rock types is shown. 



The sign + indicates the presence of the mineral as a 

 constant feature, and often in relative abundance. 



The sign — indicates that the mineral is sparingly pre- 

 sent, and may be absent. 



The two combined, + , indicate that varieties of the one 

 rock type may show the variation indicated: — 



Rock Type. 



BO 



0> 



u 

 O 



a 

 o 



M 



4) 



-+J 



'■*=> 



O 



« 



Oligoclase 

 - Andesine, 



or Basic 

 Oligoclase. 



6 



a • 



r— ( 

 O 



o 

 u 



o 



s 



N. 



u 



83 



3 



9) 



£ 



r— 1 



o 



C3 



CO 



O 



w 

 u 



■+3 



d 

 Pi 

 O 



• 



Granite ... 



+ 



+ 



+ 



+ 



+ 





- 



- 



- 





Microcline 

 Aplites 



— 



- 





+ 



+ 



± 



(Calcic) 



- 





- 





Albitites .. 











+ 



+ 



± 



+ 



+ 



+ 



The origin of the aplites, both potassic and sodic (albitite), 

 with the granite remains to be discussed. 



Aplites associated with granite rocks consist essentially of 

 alkali felspars and quartz, and occur as dykes or irregular 

 sheets. In distinction from pegmatites they are characteristi- 

 cally fine grained. This fine-grained texture may be inconstant, 

 and with a transition into a coarser type they grade into 

 pegmatites. 



Pegmatites appear to differ from aplites only in this, that 

 they are typically coarser grained, and often contain a wider 

 range of accessory minerals, these characteristics being 

 generally assignable to the greater concentration of mineral- 

 izers during their crystallization. 



These rocks represent the residual magma obtained by 

 fractional crystallization, whether by sinking of crystals, or 

 by a selective filter pressing, or squeezing out of the residual 

 liquid from the crystalline mass of granite composition. 



Such residual magma, on the contraction resulting from, 

 the cooling of the crystalline mass, is injected into cracks or 

 joints so formed. Where differentiation of the granitic magma 

 followed different lines, we have aplites associated with 

 lamprophyric rocks in complementary relationship. 



A review of the literature on granite aplites indicates that 

 these are dominantly potassic, or sodi-potassic. 



Of aplites associated with basic rocks our knowledge has 

 increased during the last few years. Such aplites may occur 



