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NEPHEI.INsBEARING PEGMATITIC DYKES ON SEILAND
line and of plagioclase. The prominent minerals are: nepheline, oligoclase,
biotite; minor constituents zircon, magnetite and apatite.
On the north slope of Tverfjeldet on the flat mountain side there
was a little mound of a light pegmatitic rock. — It was about 50 meters
in diameter and 8—10 meters high. The chief constituents of the rock
were white feldspar and biotite. The light coloured hill in the background
of the dark peridotite was visible at a long distance. This hill must be the
remains of denudation
of a horizontal dyke.
The p e g m a t i*
tic rocks inter*
secting the gneiss.
Immediately to the
west of Oldervik there
occur several dykes,
Vs—1 meter thick, the
chief constituents of
which are a red micro*
perthitic alcali*feldspar
showing microscopi*
cally a well*marked
eutectic structure, ne*
pheline, biotite and
some iron*ore. The
strike of these dykes
is from N—S and the
dip is vertical.
Pegmatitic dyke at Skarvebergbugten. The dyke is situated on
the north slope of the hill, to the north of Skarvevand. Its strike is E—W,
its dip vertical, and its thickness is about 10 meters. The dyke was mined
for black mica. Schetelig determined the following minerals: zircon,
tourmaline, apatite, biotite; the last named was found in mine*
able quantities only in some restricted parties of the dyke.^and when
these were emptied it was difficult to find new ones. Muscovite,
albite (aventurine feldspar), calcite occur as granular masses
of several meters in diameter.
Pegmatitic dyke in Ørnetind south of Skarvevand.
In this dyke also there was a mica*quarry, but there it was the muscovite
Fig. 4. Mica*quarry in the pegmatitic dyke of Ørnetind.
115
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