335 



blue opalescent quartz so characteristic a feature of the 

 normal granite. They are further related by the presence of 

 albite, which, while the subordinate felspar in the potassic- 

 aplite, is the dominant felspar in the albitites. 



The albite-pegmatites are to be distinguished from the 

 potassic-aplites : — 



(i.) By their relatively minor development — being limited 

 to a few dyke or pipe-like masses. 



(ii.) By their coarse-grained texture — the potassic aplites 

 being predominantly fine grained. 



(iii). By the presence of accessory minerals as apatite y 

 zircon, and rutile in relative abundance. Such 

 accessory minerals are practically absent from the 

 microcline aplites. 



(iv.) By the absence of biotite. 



(v). By the absence of microcline. 



Varieties of Albitites. — The predominant type is a coarse- 

 grained quartz-albitite, which may pass into veins of pure 

 albite. The quartz occurs in blue opalescent grains and the 

 albite in Carlsbad twins — also twinned on the albite law. 

 The remaining type is a muscovite-albitite, in which mus- 

 covite is associated with albite. The accessory minerals are 

 developed in all types. 



The Mechanism of Differentiation. — The residual magma, 

 dominantly potassic in composition, was derived by the funda- 

 mental process of differentiation — fractional crystallization. 

 This liquid, by a process of straining off from the crystalline 

 granite, is regarded as occupying subsidiary pooTs or cham- 

 bers ( 24 > within the granitic mass. Consequent on such fractional 

 crystallization the residual liquid was enriched in mineralizers, 

 chiefly water. 



In the main granite the crystallization of plagioclase was 

 early initiated, and occurred with marked zoning, the varying 

 composition being from andesine to oligoclase. The residual 

 magma was thus enriched in albite molecules relatively to 



(24) In granitic masses the evidences of the existence of such 

 magma pools, as stipulated, are principally provided by the 

 occurrence of aplitic or pegmatitic phases with distinctly blended 

 contacts with regard to the granite mass. Crystallization, in 

 situ, is therefore demanded. Where contacts between the aplitic 

 or pegmatitic phase and the granite are sharp and well defined 

 crystallization occurred after intrusion from such a magma pool. 

 Many granitic masses show the evidences of two such types of 

 satellitic phases. 



