GOHMISSION GEOLOGIQVE - Arkisto.gsf.fi

Suomen Geologinen Seura. N:o 12. Geologiska Sallskapet i Finland. 15
at hand and most of the significant modes of occurrence must have
been noted.
That normal folding of resistant rocks is ruled out is obvious when
the figures of Sederholm, Read and this paper are examined. The absence
of correlation between two neighbouring and roughly parallel
dykes and between the dykes and surrounding country rock is obviow
and excludes the possibility, that dykes and the enveloping rock reaoted
to the folding in the same manner.
First the arguments opposing Read's hypothesis of primary shape
will be given.
1. Holmquist gives an example of a basic vein that has been first
stretched and drawn out into separate parts, the uiterstices having
been filled by aplitic ichor trickling in from the surroundings. This
composite vein is folded by compression in the plane of the original
stretching. The basic part must have been solid and in situ during the
stretching, but it has partaken in the folding. The aplite was emplaced
during the stretching of the basic dyke and is thus proved to have been
contorted after the injection. For this case the original plane shape
and subsequent folding are established beyond doubt.
2. There is a clear correlation between the width of the vein and
the wavelength of the folds. The thicker the vein the larger the folds.
Sederholm's figure 34, p. 73, bibl. 10 offers an example with 3 wavelengths,
each restricted to a different width of vein. (See also my figures
1, D, and F; 2,B; and Ei, B). When the magma is forcing its way
into the rock the width that the consolidated intrusion will ultimately
attain is not yet fixed, because at the time of consolidation the magma
that first opened it at a given point, has flowed further on. In Read's
conception the wavelength would therefore have to determine the extent
of the subsequent intrusion, an entirely unwarranted assumption.
3. In some cases straight and folded veins are found in the same
homogenous rock, for instance granite. The rocks were plane fissuring
and yet they contain ptygmatic folds.
4. The veins are generally of uniform width over long distances
and also in their sharpest bends. As a consequence the walls do not
fit each other (see fig. 3, D 1). When we try to pull the parts of a jigsaw
puzzle apart, we do not obtain a shape like frg. 3, D 1 but II, yet 1 is
the shape of a typical ptygmatic vein.
There is one observation that would be very strong evidence in
favour of Read's theory. That would be if the structure of the wall
rock depicted in fig. 3, D II could be found. It would definitely de-