333 



The mode of differentiation of these rocks can be con- 

 sidered under the following heads : — 



A. THE ORIGIN OF THE MICROCLINE — ALBITE APLITES.. 



The microcline-aplites and albite-pegmatites are the only- 

 intrusions within the confines of the granite as exposed. No 

 basic dyke-rocks were seen by the writer. In the country rock, 

 some miles from the granite contact, Prof. Howchin^ 20 ) has 

 reported a basic dyke of diabasic composition. The writer, 

 unfortunately, was unable to visit the locality during his visit. 



Despite the abundance of aplite associated with the 

 granite, there appears to be a scarcity of other satellitic types 

 of intrusion, such as those of lamprophyric type, which, if 

 present, might suggest complementary differentiation. The 

 field evidence must be taken as it stands, for we have no 

 warrant to assume that such lamprophyric types are present 

 but still uncovered. 



This evidence is therefore suggestive that the microcline 

 aplites are direct derivatives of the granite magma by a process, 

 of fractional crystallization. 



The aplites as now developed in the granite came into 

 their position during the cooling and contraction of the 

 crystalline granitic mass, viz., by intrusion into contraction 

 cracks and fissures. In this respect, therefore, the aplitic 

 intrusions resemble those characteristic of so many granitic 

 masses. 



The origin of the albite rich pegmatites which are 

 developed in minor amount within the granite remains to be 

 treated. 



B. THE ORIGIN OF THE ALBITITES. 



It has been noted in the previous discussion that aplites 

 associated with basic rocks were often highly sodic, but not 

 invariably so. In the case of the Willoughby pegmatites, the 

 sodic type is associated only with granite. Their sodic nature 

 — in some cases they consist almost wholly of albite — is, how- 

 ever, no reason for genetically connecting them with basic 

 rocks. Here again field evidence warrants no such assertion. 



Their mode of occurrence is essentially as small dykes 

 cutting the granite. Their possible modes of differentiation 

 can be considered under three heads : ■ — 



(a) They represent an immiscible liquid phase separating 

 from the residual magma. 



(20) W. Howchin: Trans. Roy. Soc. S. Austr., 1903, voL 

 xxvii., pt. i., p. 82. 



