Acta naturalia Islandica - 01.02.1946, Side 53

ORIGIN OF THE BASIC TUFFS OF ICELAND
47
faintly polarizing palagonite containing again fragments of crystals
ancl dark glass which in some cases is very heavily charged with
phenocrysts of plagioclase.
These occurrences of brown translucent glass can, I think, in no
way be connected with glaciers, and also in another respect they are
of great interest.
They show that sideromelan is found down to the lower parts of
the basalt plateau, but the higher parts of the plateau enclose Terti-
ary lignite and petrified tree trunks. A short distance to the east of
Kirkjufell, at Glerárdalur, these fossils are found in the basaltic
series at an elevation of 1050—1100 m, and here, in the ridge be-
tween Reithólar and Hlíðarskál, 1 found several separate layers of
sideromelan tuff in the series directly underlying the fossils. Thus,
already in Tertiary times, yerhays early in that period, sideromelan
ivas formed in Icelqnd.
LjósavatnsslcarS. In the 800 m high northern slope of the pass of
Ljósavatnsskarð, following the brook near Sigríðarstaðir, we have
the following layers of brown tuff which is probably mainly wind-
blown sideromelan.
1. At 500 m, thickness 10 m.
2. — 540 — several meters. In a thin section (464) we
find perfectly rounded grains of trans-
lucent yellow glass mostly altered, opaque
glass, and a few rounded grains of lava
and crystals of plagioclase, all cemented
by a yellow birefringent alteration pro-
duct.
several m. This tuff rests on a fresh lava
surface with very clear flow structure —
an indication of the rapidity with which
the series was built up22). A thin section
(466) of this tuff is very similar to (464)
except that no lava grains occur.
Yellow mudstone.
Brown tuff.
Brown-yejlow tuff.
3. — 580
4. — 615
5. —- 650
6. — 710