GOHMISSION GEOLOGIQVE - Arkisto.gsf.fi

Suomen Geologinen Seura. N:o 12. Geologiska Sallskapet i Finland. 35
gradually more indistinct and its cementing mass less quartzite-like,
whereas the pebbles decrease in number. At the point exposed farthest
north of the contact-line the cement is gneissic in appearance and
contains, besides quartz, light-coloured hornblende, rather anorthiterich
plagioclase (An,,), light-coloured or almost colourless biotite
(y=/?= 1.5~12)~ microcline, magnetite, titanite, and apatite. The
comparatively large amount of titanite crystals arranged in rows
parallel to the schistosity is especially striking.
South of Tyhnynmäki numerous big Glacial boulders of conglomerate
occur. They have apparently been derived from the conglomerate
horizon of the north side. Among these pebbles the greater
part are drawn out and highly sheared, but in some conglomerate
boulders they show well rounded shapes, and some ovoidal pebbles
were seen lying across the general schistosity. Most pebbles consist
of quartzite of the Tiirismaa type, containing much sillimanite, others,
again, of pure quartz, like vein-quartz. The cementing matrix also
is mostly very rich in quartz. One boulder was found in which well
flattened quartzite pebbles, from one to three centimeters long, are
evenly distributed in a reddish feldspax-bearing gneiss cement.
On close examination the conglomeratic structure is quite visible
but, owing to the highly metamorphic character of both the matrix
and the pebbles, it is not distinct enough to allow, for instance, the
structure to be clearly reproduced in photographs. Some critical reader
might therefore question the conglomerate character and suggest a
*tectonic conglomerate>). This possible explanation has been considered,
but as might be gathered from the above description it does not
seem probable.
Conglomerate is also exposed on Rautakankara southeast of the
quartzite at the east end of the area, where the contact-he nuis in
a northeasterly direction. In outcrops 24 and 16 the immediate country-rock
of the quartzite is conglomeratic, with pebbles almost exclusively
of quartz .within a narrow-boundary-zone. The cementing
mass is quartzitic but rich in epidote like the epidote-quartzite following
next to it in the same outcrops. The boundary is gradual, the
number of pebbles decreasing outwards from the quartzite line.
EPIDOTE-QUARTZITE, QUARTZiTIC ONEISS (INUL. CORDIERITE-ONEISS),
AND BIOTITE-GNEISS FORMINO OUTER &UNTLES
AROUND THE QUARTZITE LENS.
The epidote-quarkzite is megascopically green in colour and
banded, due to an unequal distribution of iron ore grains, especially