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NEPHEI.INsBEARING PEGMATITIC DYKES ON SEILAND
1. Gneiss. One specimen from 01dervik*vand shows a well*marked
parallel structure. Megascopically, the following minerals are determinable:
quartz, and feldspar (altered) as prominent constituents, mica and grains
of red garnet in small amounts. Microscopically, quartz and feldspar are
met with as the chief minerals. The feldspar is entirely altered, the quartz
is sqeezed. The structure of the rock appears to be secondary. Minor con*
stituents are garnet, biotite in small scales, and a little iron*ore.
2. Pevidotite. (Lherzolite). The specimen collected from the south
slope of Tverfj eldet1 shows megascopically the usual weathered sur*
face of peridotitic rocks with rusty brown colour and the grains of the
other constituents, e. g. pyroxene and spinel, placed into relief. On a fresh
surface of fracture the colour of the rock is dark greenish brown. The
grains of pyroxene are easely visible due to the deeper colour. The rock
is of medium grain.
Microscopically, the following minerals are shown as the chief consti*
tuents: olivine, monoclinic and orthorombic pyroxene, and in smaller
amounts: plagioclase, dark brown amphibole, green spinel, traces of biotite,
chromite (?) and pyrite.
Olivine, only in part altered, in polarized ligh’t colourless. 2V
nearly 90°. Opt. neg.
Monoclinic pyroxene, often enclosing orientated lamellae of
orthorombic pyroxene, shows a feeble pleochroi’sme with colours of
absorption like those of bronzite. The angle of extinction 3: y is about
45° and indicates that this pyroxene belongs to diopside.
Orthorombic pyroxene belongs to bronzite. The extinction is //
with the cleavage*traces; the pleochroi’sme is feeble, but recognizable. Opt. pos.
Amphibole, occuring only in small amount, is strongly pleo*
chroic with the following scheme of absorption: a dark brown, [3 brownish
yellow, Y golden yellow. This amphibole is a characteristic mineral for
the series of rocks, connected with the peridotite and intersecting it as dykes.
Plagioclase, sparingly met with, is determined as labradorite
(An60-An65).
Spinel is of a deep green colour and occours rather abundant as
irregular grains.
3. Femic dyketrocks, in part olivine*bearing, connected with the plutonic
peridotite and intersecting it as dykes. The specimens described were
collected on the south slope of Tverfjeldet.
a) 01ivine*gabbro of medium grain shows megascopically light*
1 See the map, Fig. 2.
117
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