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Suomen Geologinen Seura. N: 0 35. Geologiska Sällskapet i Finland. 89
However, it could also be assumed that during the formation of the
Karelidic mountain zone or soon after, the Karelidic earth movements gave
rise to the formation of the crack zones westward of them, which were filled
with the magmas of the hypersthene rocks.
7) An age determination of two of the hypersthene rocks, pyroxenegranite
from Vaarslahti, NE of Pielavesi, and pyroxene granite from Petäjävesi,
has recently been published by Wetherill, Kouvo, Tilton and Gast
(1962), according to which the age of the hypersthene rocks is 1730-1760
million years. The age of the Svecofennides is given as 1 800 million years.
In an earlier publication by Kouvo (1958) the age of the rapakivi has
been given as 1 620 ± 20 million years.
The age of the Karelides is given by Wetherill, Kouvo, Tilton and Gast
(1962) as 1 800 million years.
Earlier all Finnish geologists considered the age of the Karelidic movement
as less than that of the Svecofennian movement. On account of the
age determinations which give ab out the same age for the Svecofennides
and the Karelides, some authors have considered both the Svecofennian
and the KareIian mountain chains as belonging to the same period of mountain
formation; the Kalevian, JatuIian, and Onegian formations (the whole
of the KareIian) would then appear to correspond to a Flysch-period of the
Svecofennian.
This is not the place for a discussion of these important age problems.
Here I wish only to point out that the hypersthene rocks are about 100
million years younger than the principal part of the remai~ing rocks of
the Svecofennidian movement, and that there are indications that they are
older than the Karelidic movement.
Also it must be remembered that an uncertainty of 10 to 15 per cent
in the age determination of rocks as old as the Svecofennian would me an
an uncertainty of about 180-270 million years, which is a longer period
of time than that required by some great mountain chains to form.
It therefore seems premature to state, considering of present methods of
making age determinations, that there had not been aperiod of 200 to 300
million years between the formation of the Svecofennian and the Karelian
mountain chains and that the belt of the hypersthene rocks was not formed
du ring this interval.
Besides the hypersthene rocks dealt with above, there occur in Central
FinIand two other series of igneous rocks from the same period, aseries of
mnakites» and aseries of apIitic granites, which are described in the following.
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