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In certain of our slides the substance, as well as the form 

 of the felspar, has survived, and we are then able to diagnose 

 the original rock as containing porpbyritic crystals of quartz, 

 felspar, and mica floating in a ground-mass, which is some- 

 times composed of granular allotriomorphic quartz, some- 

 times of crypto-crystalline or felsitic matter, but usually 

 profusely besprinkled with scales of talc, derived from felspar 

 and mica. Where the dykes contain less topaz, as on the 

 North Valley side, we have detected a felsitic ground-mass. 

 We may here mention that the survival of felspar is a rare 

 occurrence ; what penological observers have seen hitherto 

 have been crystal for (lis only; and what is pointed out to 

 visitors at the Mount as kaolin is really a white decom- 

 position product of pseudomorphous topaz and tourma- 

 line. The quartz phenocrysts are idiomorphic, sometimes 

 with perfect outlines, or with rounded coiners and indent- 

 ations. Fluid inclusions are preseut with fixed and moving 

 bubbles. 



In quartz-porphyry primary muscovite phenocrysts are not 

 admitted by the best authorities as constituents, and it there- 

 fore becomes of some importance to establish the nature of 

 the mica, especially as muscovite is a characteristic mineral 

 of the elvan group. In our slices we have found numerous 

 porpbyritic forms indicating the presence of a mineral of the 

 mica group, but in most instances the substance is so altered 

 as to admit of a doubt whether it was a potash or 

 magnesian mica. However, in one case we have detected a 

 comparatively unaltered muscovite. Such an occurrence is 

 excessively rare. This mineral has for the most part been 

 converted into its alteration product pyrophyllite, aluminous 

 talc, which has also spread through the substance of the 

 felspar phenocrysts. It is likewise generally diffused through 

 the ground mass in formless scales, affecting a larger size 

 preferentially in the neighbourhood of phenocrysts, and 

 showing vivid interference colours like those of muscovite. 

 Whether the muscovite is a lithia-mica or not has not been 

 determined. Dr. Sommerlad's analyses appear to have 

 referred only to the dense topazised variety of the rock. 



Topaz. — This is a constituent of stanniferous granites all 

 over the world, but in the Mount Bischoff dykes it has taken 

 possession of the rock substance to an extent which expels 

 the original constituents. It occurs in three forms, columnar, 

 prismatic, and amorphous. 



The dense rock from the White Face, usually shown to 

 visitors as the topaz rock, consists mainly of the columnar 

 form with radiating crystals, known by the name pycnite. 

 The radial aggregates give a dark cross in polarised light. 

 Isolated crystals of quartz, sometimes with good hexagonal 



