Rit (Vísindafélag Íslendinga) - 01.07.1962, Blaðsíða 109

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matrix where rich in basalt debris. From this it is clear that
the mass was very hot (above 500—600°C) when it stopped
moving, i.e. the brecciation took place during the original
flow of the lava mass. At the bottom of the flow the breccia-
tion was more fargoing than in the main mass; the glass
coating was here completely worn off and we have here clear-
ly worn basalt fragments of sizes 5—30 cm and even up to
1 m in diameter. Here indistinct stratification with slight
southward dip is seen. The breccia thins out to the south
with a sloping surface.
The brecciation process seems to have been the following.
An unusually thick lava flow, probably of low temperature
and high viscosity, has flowed in a plastic way down a slight-
ly dipping slope. By advancing crystallization it became fra-
gile and with the lubricating effect of the developed gas, i.e.
essentially steam, it remained mobile. Brecciation went far,
especially in the deeper part, until hy the escape of steam the
mobility decreased and the mass came to rest.
We have here a proof of the possibility of hrecciation in a
thick flowing mass. In an earlier paper (Einarsson 1946) I
stressed this possibility and pointed out the occurrence of ex-
tended layers that in part are lavas and in part a breccia of
various grades. I also assumed that brecciation might occur
already in the volcanic funnel, but I shall not discuss this
further here.
On the sloping surface of the breccia there is first a 4—5 m
thick succession of secondary tuff and conglomerate layers,
intercalated with more primary thin tuff strata. The second-
ary layers are probably mainly of eolian origin but parts con-
sist of rounded river gravel, and this clearly suggests sub-
aerial conditions at least in the neighbourhood. At one place
the light-grey colour suggests glacial clay and may have been
transported by glacial meltwaters. The secondary material
has been transported down a south facing slope and this to-
gether with the general southward dip of the elements shows