272 PROCEEDINGS OF SECTION 6. 
potash and oligoclase felspar + biotite and muscovite. The 
two micas occur together in some of the granite at Mt. 
Cameron and the Blue Tier, but that rock is tin-bearing, 
and consequently not in a normal state. As a matter of 
fact, when the miner sees white mica he looks for tin ore; 
and the nearer he is to the ore-deposit, the more abundant 
will be the muscovite. Where the granite is not tin-bearing, 
biotite is the only mica present, and it is often very plentiful. 
The biotite in the tin granite loses its iron, gives rise to the 
formation of talc, and is eventually converted into a colour- 
less non-pleochroic species. This change is evidently due to 
the agencies involved in the deposition of tin. At tha 
Silver Echo and Stony Ford mines, near St. Helens, 
golden-yellow mica occurs, probably lithia mica. Lithia 
bearing muscovite is common in the tin granites. At thi 
Crystal Hill Mine, near Lottah, there is an unusually fine 
development of apatite in such granite. 
As a rule the East Coast granite has a porphyritic struc- 
ture. Large crystals of orthoclase felspar, one and two 
inches in length, are developed in a matrix of ordinary grain. 
These felspars kaolinise, and present a striking appearance 
on the weathered surfaces of the rock. Apart from the 
titaniferous iron ore and magnetite, the mica was the first 
mineral to separate out, as may be seen by the numerous 
crystals of biotite enclosed in the felspars, then felspar, 
lastly quartz, which fills the interstices. The colours of the 
stone range from white through grey to pink. The granite 
at St. Helens differs from that at the Blue Tier and Mt. 
Cameron in being somewhat more quartzose. 
Cassiterite (tin ore) occurs very generally in the above 
granites, either in veins, rarely as lodes, and especially at 
the Blue Tier in floors and irregular zones of greisenised or 
otherwise altered country-rock (the so-called quartz-por- 
phyry). With regard to the alleged quartz-porphyry dykes, 
it should be noted that in the eastern workingsof the Anchor 
Mine the massive stanniferous rock underlies the normal 
country granite, from which it is parted by a horizontal 
quartz-felspar seam of pegmatitic character. The stan- 
niferous zone is too irregular in outline to justify the use of 
the term “dyke,” and its quartzose character and abundance 
of mica are characteristically the signs of the greisenisation 
and stanninisation of granite. Diffused fluorite is noticeable 
at the Anchor and Liberator mines. In lieu of the popular 
term quartz-porphyry for this rock, I suggest for discussion. 
tin-granite, stanniferous granite, greisenised granite. Tim- 
stone has been proposed, but is already a synonym for 
cassiterite. 
