GOHMISSION GEOLOGIQVE - Arkisto.gsf.fi

Suomen Geologinen Seura. N:o 12. Geologiska Sallskapet i Finland. 37
high relief. The colourless amphibole shows comparatively large extinction
angles, y Ac = 27". Its indices of refraction determined by
immersion were found to be y' = 1.647, a' - 1.628. It has been
partly epidotized. From its short-prismatic habit it does not seem
to be tremolitic but rather a pale-coloured hornblende, while its low
refringence points to a very low iron-content. The rock is traversed
by numerous dikes of pegmatite.
The next zone farther southeast of the epidote-quart~ite and amphibolite
zone on the slopes of Rautakankara consists of quartzitic
gneiss, exposed in outcrop 31. It is composed of quartz, highly anorthitic
plagioclase, pale-brown biotite ( y = 1.5 s 1 ), cordierite, oxidic
iron ore, muscovite, epidote, apatite, and chlorite. The cordierite is
unevenly distributed, being enriched in bands parallel to the schistosity,
and has been much altered.
Let us now return to the rocks bounding the quartzite on the northern
side. At the west end of Tiirismaa, in outcrop 29, only conglomerate
is exposed, but farther north there is an outcrop of quartzitic
gneiss (outcrop 30). It is a light-coloured banded and fine-grained
rock. The banding is due to the arrangement of the dark constituents
in rows parallel to the schistosity. Quartz is the most abundant constituent
and shows a strongly undulose extinction. The other constituents
are microcline, pale-brown biotite (y = 1.594), anorthiterich
plagioclase (a> e of quartz), abundant oxidic iron ore, titanite,
apatite, and muscovite. Dikes of pegmatite are numerous.
In outcrop 14 a fine-grained gneissose rock is seen in direct contact
with the conglomerate and appears to be very similar to the
cement of the latter (cf. above p. 35), as though formed from the same
sediment but without boulders. Its minerals are plagioclase (An,,),
quartz, pale hornblende, oxidic iron ore, titanite, epidote, and apatite.
The rather large quantity of titanite is characteristic, as in all the
other rocks from the northern boundary zones and, to a certain extent,
also in the rocks of Rautakankara in the east. The rock is granoblastic,
but with serrated outlines of the grains. Towards the north
this hornblende-bearing gneiss is followed by a biotite-plagioclasegneiss
which still occurs in outcrop 15 farther north. It is fine-grained,
megascopicslly almost leptite-like, and composed of plagioclase (An,,),
quartz, rnicrocline, biotite, oxidic iron ore, chlorite, apatite, and zircon.
In structure it is granoblastic and fairly distinctly schistose, the quartz
showing strain shadows. The biotite is greenish brown and darker in
colour than any of the varieties found in the immediate country-rocks
of the quartzite. In this gneiss are intruded very numerous dikes and