GOHMISSION GEOLOGIQVE - Arkisto.gsf.fi

Suomen Geologinen Seura. N:o 12. Qeologiska Sallskapet i Finland. 17
a spectacular fashion. B is found some 50 m away from A (but there
may be missing links below sealevel in between). The enclosing rock '
is a kinaigite gneis giving evidence of strong deformation. But lacking
the dark ~eins one would not perceive that it must have been kneaded
as a chunck of iutty.
In none of my specimens could 1 detect even indications of the
structure shown in figure 3, D II. On the other hand several ceaes were
found with the structure of 1 (fig. 1; 2,A; and 5 B, and C), where the
wall rock is intensively deformed close up to the vein, but much less
at a very short distance away from it. In the latter case one could
easily be deceived into believing that the wa11 rocks show no deformation.
Often there must have been unconformity between the vein
and its wall to begin with. Especially with an irregularly shaped intrusion
it will be difficult to ascertain whether our theoretical case II
is actuaily represented. 1 admit that Suter's figure 39, although a
))Schema)) does suggest primary sinuous injection. 1 wilI return to this
case presently. On the other hand nobody can doubt that Milch's
drawing is accurate in this respeot and here the theoretical case of
fig. 3, D 1 is beautifully shown (fig. 5, A).
The case shown in fig. 3, E, that appears repeatedly in fig. 5, C
is also significant. It shows how a thin vein is deflected and given a
twist by the contortions of a thicker vein. The original cross-over
remaim at right angles to the less mobile thicker vein.
5. Presumably Read believes the magma to have flowed not up
along the strike of the folds, but across from one to another, but he
does not say so. In any case the magrna would have to follow a long
course, and a quantity would pass through each fold, yet the thin,
sometimes almost cut off tongues of country-rock between the meanders
have not been shifted out of place (fig. 2, C).
6. Fig. 1, C shows two veina separated by a thin screen. Straight
screens are common and could be folded subsequently, but the direct
formation without subsequent distortioIi is difficult to irnagine.
7. As Read already remarked: >>Time and again, veins have been
observed that are perfectly plane ))lits>) when parallel to the bedding
or foliation, but show marked tortuosity when they leave these controlling
guideso - He believes that: ))The absence of easily opened
plane channels for the injected materia1 thus affects the form of the
veins)).
In the direction of the ))controlling guideso plane fissures were
evidently readily formed. But why should the vein then continually
leave these easily formed fissures and force its way across the ~tough .
directionv? Fig. 1, A gives a good example.