Acta naturalia Islandica - 01.02.1946, Blaðsíða 54

48
TRAUSTI EINARSSON
The material of these tuff layers is clearly transported, probably
produced by explosive eruptions occurring somewhere else during the
piling up of these series of basalts. But primary sideromelan may
also be found here, i. e. eruptions of such a material also occurred
in this area.
At an elevation of 260 m I found a narrow vein of brown matter
traversing a basalt lava, this being no doubt of igneous origin. Yet, in
a thin section (450) it is seen to consist of perfectly worn grains of
translucent yellow glass, dark glass, and some crystals of plagioclase
and grains of lava, also rounded.
The similarity of this matter and the stratified tuffs in Staðar-
bygðarfjall, Hafrárdalur and those higher in the present section is
so great that one may feel some hesitation as to the secondary posi-
tion of these sediments. They might certainly be derived from a
material that was erupted in the worn condition.
At an elevation of about 420 m in our gully we find further primary
material. This is a 10 m thick dyke of the same compact brown
matter as that of the described vein. From this dyke apophyses are
sent into the walls sometimes surrounding blocks of basalt breaking
from the walls (Fig. 8). Towards the top the brown matter is
horizontally stratified and this part has probably been carried into
the fault from above. A thin section from this part is very similar