OCCURRENCE OF MICA IN CENTRAL AUSTRALIA. 
43 
These problems are dealt with separately in the following sections. 
( a ) Cause of the different nature of the granite pegmatite-dykes. 
A large intrusion of igneous molten matter contains a considerable 
amount of water and various gases which become saturated with mineral 
matter. As the molten mass cools and crystallises out, the gases and 
watery solutions are squeezed into cavities formed within, or near, the 
apex of the cooling mass. If the enclosing rocks are sufficiently cool to 
develop fault cracks the gases may escape through these ; with the gases 
some of the hydrous solutions may also get away. If the intruded rocks 
are devoid of cracks, as would be the case in that deep zone, the zone of 
metamorphism, where temperature and pressure keep the minerals in a 
plastic condition, the liquids and gases of the intrusion, together with a 
certain amount of mineral in solution, are kept confined until cooling 
has proceeded far enough in the enclosing rocks to allow the latter to 
crack or fault. When cracking does take place, whether through general 
earth movements, or through internal gas pressure, or through expansion 
caused by crystallisation, not only can the gases and liquids escape, but 
the release of pressure also liquifies part of the consolidated igneous rocks 
near by, and that too is squeezed into the crack. 
Assuming that a granite mass is suddenly thrust into a well-cooled 
zone capable of cracking and faulting, normal granite and aplite dykes 
and grano-pegmatitic (very coarse granite) dykes are thrust into the 
cracks after the escape of solutions and vapours. The solutions and 
vapours would effect mineralisation thousands of feet and even 
miles away from the granite body. The less mobile dykes would 
gradually close up the fissures and vents. If the cooling granite 
body were very large there would still be pockets of mother-liquor, i.e., 
gases, liquids and mineral matter in solution, in parts of the intrusion. 
This liquor, being the last to solidify, would again be squeezed to the 
apex of the mass, where it would remain in a liquid and gaseous con- 
dition until further earth-movements caused further cracking and fault- 
ing, thus permitting its escape. But when the enveloping rocks are still 
plastic and undergoing the process of metamorphism the mother-liquor 
eats or stopes its way up through the plastic metamorphie formations, 
forming large irregular pegmatite dykes, until it reaches a zone which 
has attained a stage of cooling that has produced joints and faults. 
Then the mother-liquor will follow the channels of easy passage. 
Thus we can from the same large granite mass get an earlier series 
of granite, aplite, and grano-pegmatitic dykes, and a later series of 
pneumatolytic pegmatite dykes formed from the residual magma. If 
release of pressure by great earth-movements should liquify a portion 
of the already cooled granite mass, there can be a still later series of 
porphyritic granite dykes. 
The dykes containing books of sheet mica belong to the true pneuma- 
tolytic series formed by the invasion of the enveloping metamorphie 
rocks by masses of squeezed-out mother-liquor or residual magma. For 
the production of muscovite the mother-liquor must be highly potassic. 
Naturally this would most frequently occur in the case of a highly 
potassic granite magma ; sometimes also in a normal granite magma in 
which differentiation and chemical combination have left potash dominant 
in the residual liquor. 
