31 



call the rock a ternary eutectic of quartz, albite, and musco- 

 vite. The larger quartz grains associated with mica contain 

 abundant minute glassy and liquid inclusions, with fixed and 

 movable bubbles, whereas those of the granophyric inter- 

 growths are almost if not entirely devoid of such inclusions. 



It should be noted that the development of mica preceded 

 that of kaolin. Both types of alteration are in some degree 

 selective, as some of the felspars show very little alteration 

 either to muscovite or kaolin even when almost surrounded by 

 heavily kaolinized minerals. No reason can be assigned for 

 this; the attacked and unattacked felspars appear to be of 

 identical composition. 



Specimen B from the Middleton Beach end of the outcrop 

 is of a pinkish-grey colour and has the muscovite more evenly 

 distributed than it is in A. Under the microscope the rock 

 appears as a veritable confusion of muscovite, quartz, and 

 subordinate albite, the grainsize being medium and very 

 uneven. Granophyric fabric is of very much less account than 

 in the previous specimen. Quartz and mica are for the most 

 part in fairly large and very irregular grains up to 2 mm. in 

 length at times. The former contains numerous glassy and 

 liquid inclusions, and shows occasional traces of crystal out- 

 line, muscovite being then moulded on it. Some of the large 

 quartz grains contain scattered small grains of felspar, recall- 

 ing the quartz of certain rocks' from Broken Hill w^hich 

 have come under the writer's notice, and which are reasonably 

 supposed to have undergone silicification, in the course of 

 which the original quartz grains have become so enlarged by 

 accretion of silica as to enwrap crystals of other adjacent 

 minerals. 



The muscovite in the rock is at times very evidently 

 eating into the albite in irregular fashion, and one large 

 squarish section of mica appears to be pseudomorphous after 

 a tabular felspar. The albite is not as a rule nearly so kaolin- 

 ized as in Specimen A ; it is in tabular to prismatic crystals up 

 to 1 mm. long. Sometimes a crystal has been corroded until a 

 mere skeleton is left, the relics preserving optical continuity 

 while the deep embayments are filled with quartz or muscovite. 



Besides these coarser-grained minerals the rock contains a 

 good deal of finer-grained material, a confused aggregate of 

 quartz, albite, and muscovite, with some granophyric inter- 

 growth and a certain amount of replacement of the felspar. 



There seems no reason to doubt that in the rocks just 

 described a process of greisenisation has been taking place. 

 The original rock was a granophyric soda aplite ; in specimen 

 A the alteration has just started, in B it has proceeded so 



B 



