Acta naturalia Islandica - 01.02.1946, Síða 23

ORIGIN OF THE BASIC TUFFS OF ICELAND
17
There yet remains to be mentioned one of the strata, not thicker
than 1 m, a dark-grey conglomerate with small rounded pebbles of
basalt. This conglomerate has all the characteristics of Pjeturss’
moraines, with the exception that no striated pebbles were found. Its
position suggests moreover that it might correspond to Pjeturss’
higher moraine at Barkarstaðir.
Studying a thin section (256) I was, however, rather surprised to
find that the groundmass of this conglomerate consists mainly of
worn fragments of palagonite and dark glass, although worn grains
of basaltic lavas and an abundance of minute fragments of plagioclase
and augite are also present.
The material of this conglomerate, so markedly morainic in appear-
ance, is thus mainly of a glassy nature, not exactly what one should
expect to find in a groundmoraine embedded in a series of basaltic
lavas. Furthermore a consideration of these thin glassy sediments
as a whole does not suggest the existence of a glacier as a creative
agency.
I think it is more natural to consider the conglomerate and the sedi-
rnents as a whole as a link in the volcanic series rather than a break,
especially a break of such magnitude as a glaciation. In my view the
conglomerate may represent a volcanic mud flow but this question
will be discussed in more detail later. This assumption is in accord-
ance with the absence of structure, the wearing of the material, and
its composition, and even the eventual presence of a few more or less
indistinctly scratched stones would, I think, hardly be very surpris-
ing.
Walking up along the brook we again meet with a grey basaltic lava
and a similar conglomerate as before, and this is probably higher
in the series as no dislocations could be seen. Yet this is not quite
clear. At last we arrive at a deep terminal gorge eroded by the brook
into an enormous basaltic layer of a thickness no less than 100 m.
It forms the edge of the main plateau. This lava is a dark, extreme-
ly fine-grained basalt, which by consolidation has been jointed in
such a way that it is almost wholly built up of innumerable cubes
or polyhedrons with an average size of about 1 — 2 dm3. Only near
the top of the lava is this peculiar jointing replaced by vertical
columns.
At a few places the lava encloses patches of brown palagonite.
This lava is obviously very easily broken up and we have already
Origin of the basic tuffs of Iceland 2