Full resolution (JPEG) - On this page / på denna sida - Sidor ...
<< prev. page << föreg. sida << >> nästa sida >> next page >>
Below is the raw OCR text
from the above scanned image.
Do you see an error? Proofread the page now!
Här nedan syns maskintolkade texten från faksimilbilden ovan.
Ser du något fel? Korrekturläs sidan nu!
This page has never been proofread. / Denna sida har aldrig korrekturlästs.
NEPHEI.INsBEARING PEGMATITIC DYKES ON SEILAND
In addition to the minerals mentioned above, a small amount of magnetite
is shown microscopically. The amphibole, showing intergrowth of parallel
lamellæ of ironore is greenish brown, and according to the colours of
absorption, similar to the brown amphibole in the peridotite.
The scheme of absorption is: a dark brown, |3 brownish yellow, y
golden yellow.
The plagioclase is labradorite (An60).
f) Hornblendite. The specimen shows a coarse-grained rock
connected with the amphibolegabbro«pegmatite, but contains no plagio«
clase. The minerals met with are brown amphibole in large individuals,
as usual elongated // c, and biotite. The rock occurs as dyke inter«
secting the peridotite.
4. Nepheline’gneiss, with «Augenstructure», from the south slope of
Tver fjeldet. According to Hoel, this rock occurs as dyke in the
peridotite, which as mentioned shows a structure quite normal for plu«
tonic rocks, with only feeble signs of affection by stress. The nepheline«
gneiss on the contrary, younger than and intersecting the peridotite, is
rather shistose with a prominent «Augenstructure» due to spheric and
ellipsoidal nodules of nepheline and more lens«shaped bodies of plagio«
clase. The nodules of nepheline occur in sizes from the size of a pea
to that of a walnut. The nepheline is fresh, glassy and subtrans«
lucent. The only visible sign of mechanical deformation of the nodules
are cracks in different directions through the substance of nepheline, gene«
rally normal to the parallel«structure of the rock. The curved surface of
the cleavage faces of the plagioclase indicates more evidently the mechanical
deformation of the rock.
The fine«grained groundmass of the rock shows a parallel arrangement
of light«coloured and dark«coloured stripes, with curvatures along the
borders of the «Augen».
As the dyke intersects the peridotite, showing the primary mag«
matic structure, and therefore solidified later than the former, it seems
probable that the shistose structure of the nepheline«gneiss is protoclastic
due to magmatic pressure during the intrusion of the dyke.
Microscopically, nepheline, plagioclase, biotite and magnetite are shown
as chief constituents, apatite, zircon and calcite as minor constituents.
The ellipsoidal sections of nepheline often show along the borders
intergrowth with grains of plagioclase. At the ends of the sections grains
of magnetite are sometimes met with, continued in tapering stripes of
biotite in small scales.
119
<< prev. page << föreg. sida << >> nästa sida >> next page >>
