OCCURRENCE OF MICA IN CENTRAL AUSTRALIA. 
49 
(c) Indications for the prospector. 
From the foregoing considerations it appears that the pegmatite 
dykes which deserve most attention are : — 
(1) Principally pegmatite dykes intruding coarse-grained basic- 
gneisses. On the other hand, large coarse granite (grano- 
pegmatitic) dykes do not merit much investigation. 
(2) All pegmatites which are very rich in muscovite, especially 
those which show some moderate or large books on the out- 
crop. 
(3) Pegmatitic dykes which have a zone or selvage of all-mica 
rock, resembling a minette, lining their walls. That indicates 
a potash-rich mother-liquor which has acted on the flucan 
and wall-rock, producing mica. 
(4) Pegmatitic dykes with intermittent quartz blows along the 
outcrop (like the Billy Hughes) and pipes with a quartz out- 
crop centrally situated (like the Caruso). 
(5) Pegmatite dykes which have produced an extensive develop- 
ment of garnet in the country-rock traversed by them. 
From the considerations outlined in section (a) and (&) it can also 
be seen that the scarcity of large book-mica is due to a great number of 
special conditions which are necessary for its production. In addition 
it might be stated that a large proportion of the book-mica from most 
mica mines is only fit for grinding up as scrap mica, because many books 
have defects or flaws which wholly spoil the mica for use in sheets. The 
principal defects which condemn mica to the scrap class are (1) inclusions 
of other minerals, especially those which are fair conductors of heat and 
electricity, (2) the fishtail or herring-bone structure, and (3) ruling. 
In fishtail or herring-bone mica the sheets are traversed by a number 
of radiating ribs, like those of a fan. This appears to be due to twinning, 
resulting from crystallisation pressure or strain at the time of formation. 
This type of structure predominates in large books developed around 
irregular quartz-felspar blows, instead of in parallelism to the lode 
direction. 
Ruled mica is mica in which all the sheets are traversed by equi- 
distant lines which look as if they were made with a parallel ruler. The 
ruling passes through every sheet of the book and is a plane of fracture. 
As the lines are usually less than half an inch apart they spoil the mica. 
The rulings represent a secondary cleavage parallel to a pinacoidal face, 
developed by tectonic pressure. This structure is very common in the 
mica of north-south pegmatite dykes near the western end of the Hart’s 
Range where the strike of the country commences to change from east- 
west to west-north-west. 
It has been pointed out that payable book-mica is often confined to 
particular lenses, or shoots, within the dyke, or plug. Mr. Archibald 
A. C. Dickson, M.E. (U.S.A.) and A.I.M.M. (London), in a small work 
called “The Mica Miners’ Gfuide,” which is based on his long experiences 
in India as a manager of mica mines, states that these lenses or shoots 
are always connected by a “bryle” or “indicator” which can be picked 
up by an experienced mica miner. Such indicator floors or veins have 
also been observed in several of the Territory mica mines, notably in 
those which (like the Kismet) have their best mica in greisen-like lenses. 
