Acta naturalia Islandica - 01.02.1946, Page 21

ORIGIN OF THE BASIC TUFFS OF ICELAND
15
2. TINDAFJÖLL
We begin with a brief orientation of the Tindafjöll area which
lies between the rivers Markarfljót and Eystri-Rangá in Southern
Iceland. It may be broadly described as a very gently dipping plateau
or a part of a dome with grey basalts at the surface, rising from
600 m at the southern edge above the farms Barkarstaðir and Múla-
kot to 750 m. in the centre, where many irregular mountain peaks,
mostly of palagonite breccia, rise above the plain.
This plateau has been cut by the Markarfljót whereby good pro-
files have been opened in the steep slope of Fljótshlíð. In a profile
at Barkarstaðir Thoroddsen has mentioned alternating lavas and
palagonite tuffs and also two main layers of conglomerate, indi-
cating considerable erosion and rest of volcanic activity. Accord-
ing to Pjeturss striated boulders are easily found in these structure-
less conglomerates and accordingly he considers them to be moraines.
Thus the whole massif should be of Quaternary age.
I ascended the plateau at Múlakot 5 km. west of Barkarstaðir.
The lowest horizon is here a small outcrop of a rather loose brown
tuff. In a thin section (243) it is seen to consist of angular fragments
of unaltered sideromelan with an average diameter of 0,5 mm. Some
of the fragments enclose phenocrysts of plagioclase. Separate frag-
ments of bytownite are also fairly abundant and olivine occurs
sparsely. These crystals were most probably originally enclosed in
the glass. Dark glass is also abundant, containing a greater number
of phenocrysts than the translucent variety. The whole mass is ce-
mented by zeolites. This tuff can have suffered no or very little trans-
portation, except through the air.
Covering this tuff we find 4 lavas of a porphyritic grey basalt
with thin partings of scoriae, the total thickness being 15—18 m.
A thin section from the second lowest lava (245) is very rich
in phenocrysts of bytownite and augite but olivine is sparse. The
base is very fine-grained. The other lavas are nearly identical with
this one, and the magmas of the lavas and the tuffs seem thus to be
closely related.
Covering these lavas is a thick layer of a structureless conglomer-
ate with an abundance of rounded blocks, large and small, in a
compact dark-grey matrix. It has all the appearance of Pjeturss’