IGNEOUS ROCKS OF TASMANIA. 273 
It has been reported that cassiterite occurs sporadically 
’n the country granite itself. Even if this be the case, it 
aeed not be regarded as a primary constituent. In such 
instances it may have been derived from adjacent pegmatite 
or quartz-veins traversing the rock. Beck, in his Lehre von 
- den Erzlagerstatten (1901), quotes cassiterite as primary 
in many granites, e.g., the tourmaline granite of Eibenstock, 
of Greifenstein, of Altenberg, in the Erzgebirge, but of no 
economic value. He says the only instance known where it 
is of any commercial importance is at Etta Knob, Black 
Hills, in Dakota, but discounts his example by saying that 
the rock is not normal granite, but pegmatite. 
The granite on the Schoutens is of the same porphyritic 
variety as at Mt. Cameron and elsewhere, and is likewise 
traversed by greisenised bands of rock and veins of quartz 
carrying tin ore. At Gladstone, the margin of the granite 
at its contact with the Silurian slates is marked by a zone 
of greisenised rock (at the Fly-by-Night and on the Esk 
property). The contact of the granite with the Silurian 
strata at St. Helens is greatly obscured by drift, but of 
interest where visible. In the road-cutting south of St. 
Helens Bridge one or two exposures are seen of chloritic 
schist lying in vertical lenses in the granite, and further 
west on the same strike is the tin mine at Stony Ford, where 
the ore occurs in a band of garnetiferous chlorite rock en- 
cased in granite. 
The East Coast granite was already exposed by denuda- 
tion in Permo-Carboniferous times, for the conglomerates 
and mudstones of that period contain fragments of it. 
Examples are to be seen on Maria and Bruni Islands and at 
Beltana. At Ross, a huge block firmly implanted in Lower 
Permo-Carboniferous sandstone at the foot of Macquarie 
Tier was mined a few years ago, under the impression that 
it was a protrusion of stanniferous bed-rock. Westof Lefroy 
also a granite boulder lying upon Silurian slates was ex- 
plored for gold. 
At Roy’s Hill, near Avoca, is a hard quartz-mica stan- 
niferous rock, which forms the margin of granite upon 
Silurian strata. The contact is concealed by Permo-Carbon- 
iferous sandstones and conglomerates, but the smoothed 
outlines of the underlying contact rock indicates a denuded 
surface prior to the Permo-Carboniferous period. Curiously 
enough this surface carries a concealed stanniferous wash of 
those days. Mr. W. F. Petterd was the first to point out 
that the dark patches in the tin ore of the Mt. Rex Mine on 
Ben Lomond are pseudomorphs of the large porphyritic 
- erystals of the country granite. 
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