46 
PROCEEDINGS OF THE ROYAL SOCIETY OF QUEENSLAND. 
(e) In the crystallisation of the final residual portion of the magma, 
formation of mica is favoured in preference to orthoclase, because its 
molecular volume is 140 to 142 as against orthoclase (98-7 to 109-4) ; and 
also through the fact that the cooling of the dyke has proceeded so far as 
to bring it into the zone of stress. Orthoclase is a deepest zone mineral, 
muscovite a middle zone or stress mineral. Mica is usually oriented in 
definite directions in regard to the wall of the dyke, indicating stress at 
the time of its deposition. 
Normal orthoclase consists of Si0 2 64-7 per cent., A1 2 0 3 18-4 per 
cent., K 2 0 16-9 per cent. ; and ordinary muscovite Si0 2 45-2 per cent., 
A1 2 0 3 38-5 per cent., K 2 0 11-8 per cent., H 2 0 4-5 per cent. Since pres- 
sure increases solubility and release of pressure increases crystallisation, 
the escape of much of the C0 2 causes rapid deposition of felspar crystals 
as well as of excess silica, but when the escape vent is again sealed by 
quartz or quartz-felspar deposition pressure again increases. The lower- 
ing of temperature and pressure in the first place by escape of gas is 
practically equivalent to changing crystallisation conditions from those 
of the Lower Zone of Grubenman to those of the Middle Zone. That 
change favours the formation of mica instead of felspar. When pressure 
then again increases somewhat by the sealing up of vents of escape of 
liquid, and solubility increases, the remaining potassic mother-liquor, 
consisting largely of potash and water (caustic potash), will redissolve 
already formed felspar to form mica and quartz. The final solutions 
of redissolved felspar in potassic mother-liquor actually do not crystal- 
lise out into book-mica and quartz before pressure is again reduced by 
the cooling down of the enclosing wall rocks of the dyke. Magmatic 
water combined with potash also plays a very important part in the 
development of mica instead of orthoclase at the close of the consolidation 
period. 
Wherever the texture of the dykes is fairly open and miarolitic, the 
mica may be somewhat evenly mixed with quartz and felspar in the last 
part to consolidate ; but where the dykes are nearly flat, and crystallisa- 
tion proceeded under great pressure, the final potash-rich solutions to 
enter into combination are squeezed towards the walls, near which the 
best book-mica occurs. The mica books often grow at the expense of 
already consolidated felspar, when the residual magma has become too 
low in aluminium even to form mica. Usually the hanging-wall side 
of the dyke has the best mica ; sometimes both walls have good books. 
In the flat-dipping' dyke of the Sister Mine at Carrara, on the south side 
of the ITart ’s Range, the best books lay in saucer-like depressions in the 
footwall, corroded out by the potassic liquors which formed the mica. 
In some mines there are also lines of small lenses of greisen-like com- 
position, forming expansions of narrow greiseny bands crossing from 
hanging-wall to footwall (as in the Kismet Mine, fig. 4) and these con- 
tain excellent book-mica. They suggest a fracturing of the consolidated 
portions of the dyke, and the squeezing of the potassic residual magma 
into the fractures. In some of the mines up Eldorado Creek (e.g. 
Oscari’s Mine) there is clear evidence of felspar having been broken 
down, and mica developed at its expense, a greisen-like mixture of quartz 
and mica having formed by the action of the potassic solutions on 
already crystallised orthoclase. 
