32 



far that granophyric fabric is almost obliterated and the rock 

 is to all intents a greisen. The evidence for these conclusions 

 may be summarized : — 



(1) The corrosion and embaym^nt of the felspars and 



their partial replacement by mica and quartz. 



(2) The coarse-grained quartz mica aggregates, with 



traces of idiomorphism in the quartz, irregularly 

 distributed through the fine-grained rock. 



(3) The secondary enlargement of the quartz grains, and 



the presence of liquid inclusions, evidently not 

 characteristic of the unaltered rock. 

 Kaolinisation and the development of mica have been 

 quite independent, for mica is occasionally found replacing 

 felspar which is free from kaolin, and indeed in specimen B 

 there is much less kaolin visible than in specimen A. It is 

 not improbable, indeed, that the kaolin was the result of the 

 continuation of pneumatolytic processes into the hydrothermal 

 stage ; the fact tliat one part of the rock-mass is so very much 

 more altered than another, although under similar weathering 

 conditions, is all against kaolinisation by weathering. 



With regard to the mechanics of greisenisation it appears 

 as if partial solution of the original felspars must have been 

 effected in places by the mineralizers, which thereafter filled 

 the cavities, and from which quartz and muscovite crystallized. 

 The substitution of these minerals for albite would require 

 some accession of silica to the rock, as the amount required 

 to make up the deficiency in volume (consequent on the change 

 from albite to muscovite) would be more than that set free 

 from the felspar. (24) 



Partial analyses were made of the two specimens, with 

 the following results: — 



A B 



SiO. 78-25 78-60 



AUO.,, etc 

 Na^..6 .. 



Sp. gr. .. 



n.d. 14-64 



5-00 1-74 



0-90 3-21 



0-88 1-37 



2-635 2*696 



(24) Assuming ALO., to remain constant, and H,0 and KjO 

 in some combination to be supplied bj' the mineralizers, we may 

 represent the reaction thus: — 



3(N.a.O. Al.O,. 6SiCX) + K..O-h2H,0 = 

 A!l>it« (mol. vol. 601 S) 



K..0. 3AI..O3. 6SiO,.. 2H.,0-|-12SiO.,-h3Na..O. 

 Muscovite (282-2) (273-6) 



There is thus a volume of 46-0 to be made up by SiOj. 



