Acta naturalia Islandica - 01.02.1946, Blaðsíða 55

ORIGIN OP THE BASIC TUFFS OF ICELAND
49
to (400b), containing grains of glass and worn crystals of plagioclase
and olivine. (457) from the dyke itself is somewhat different. We
find a considerable number of rounded grains of opaque glass in a
yellow translucent, faintly polarizing granular groundmass. But
it is difficult to discern whether this groundmass was originally
compact sideromelan or not. It is not possible to find sharply-
defined grains of sideromelan. The separate rounded grains
of opaque glass show that no ordinary hot lava was erupted
but either a colder fluid or perhaps more probably a wet mixture of
glass fragments.
In the southern slope of LjósavatnsskarQ we find also, as might
be expected, several layers of brown glass intercalated between the
basalt lavas, but here we find also layers of the usual grey conglo-
merates. They have been mentioned earlier and interpreted in the
traditional way as “obvious signs of glaciation”9).
There are also other features of interest to be found here,
I ascended to the edge of Stóradalsfjall at Kambur, the edge being
at about 850 m above sea level. A litle east of Kambur I found the
following section at the edge of the escarpment, Fig. 9:
1. Lavas of grey fine-grained basalt.
2. Wind-blown sand and gravel.
3. Grey basalt.
4. Sediments, 4—5 m thick.
5. Grey dolerite.
The sediments (4) consist of brown
mudstone at the top, then brown tuff
and at the bottom greyish conglomer-
ate with large boulders of basalt on
which I was not able to detect any striae. The matrix of the con-
glomerate consists of worn grains of basalt and fine basalt debris,
but no distinct fragments of glass are seen in the slide (438a). The
tuff (439) consists of perfectly rounded grains (at least down to a
diameter of 0,1 mm) sparsely cemented by palagonite. The grains are
of opaque glass, basalt, and yellow translucent glass in that order
of abundance and this material is most probably wind-blown.
Of interest is the fact that the conglomerate is traversed by a
number of more or less vertical veins of brown matter of a thickness
of several centimeters- These veins (slide 438b) which obviously
are of volcanic nature consist of a structureless mass of minute frag-
Origin of the basic tuffs of Iceland. 4
^ <d°
3 ,TnrmT]TrTT]iTrnfri
2 “o °0 0°^r,arf
/
Fig. 9. — Sediments in south
side of Ljósvatnsskarð.