Acta naturalia Islandica - 01.02.1946, Page 47

ORIGIN OF THE BASIC TUFFS OF ICELAND
41
servation and here the main
interest concentrates on the lay-
ers f, g and h. According to
Pjeturss g is a grey conglomer-
ate containing worn and striat-
ed stones and resting on the
striated surface of the lava f,
and it is hence assumed to be an
especially clear case of a mo-
raine.
Now in a thin section (374)
from the bottom of this con-
glomerate the hard groundmass
is seen to consist mainly of
angular fragments of isotropic
brown glass, only occasionally
altered. Fragments of olivine,
augite and plagioclase are pre-
sent, probably phenocrysts, as
they also occur as such in the
glass fragments. The brown
stratified tuff h (375) which
rests directly on the conglomer-
ate consists almost exclusively of small fragments of yellow glass,
only very few fragments of crystals being seen.
These layers are certainly no ordinary products of a glaciation and
a very strong evidence would be needed to prove their morainic
nature.
As to glacially striated pebbles I cannot endorse Pjeturss’ state-
ment that these are easily found. There are scratched, rounded
stones which, however, prove nothing. In a mud flow, as e.g. in an
avalanche, the pebbles might be scratched like this.
There is, however, the striation of the surface of the lava f, which
might be a strong evidence of glaciation.
This striation is very clear, but it is not of any great magnitude
as the original flow structure of the lava surface is at many places
clearly seen and it is further of interest to note that the flow
direction of the lava was the same as the direction of the striae.
The surface is in spite of the filing rather rough and there is only
Fig. 5. — The gorge of Glymur (After
Pjeturss, 1. c. Fig. 4.)