Acta naturalia Islandica - 01.02.1946, Page 30

24
TRAUSTI BINARSSON
4. SÍÐA AND FLJÓTSHVERFI
The volcanic series of Tindafjöll, Eyjafjöll, and Mýrdalur are con-
tinued to the east and north. They extend to the Skeiðarárjökull in
the east and at least beyond Þórisvatn in the north. In all this area
previous authors speak of the Palagonite Formation, and it is de-
scribed as being in many ways quite similar to Tindafjöll and Eyja-
fjöll.
The formation is cut by the coastal plain in an escarpment of some-
times appreciable height and here many excellent sections can be
studied and the section of the series followed uninterruptedly over
large distances. The character of the series is everywhere the same,
a succession of very fine-grained lavas with columnar and poly-
hedrous or block jointing and brown breccia to fine brown and green-
ish tuffs, and the brown material is always 'composed of translucent
brown or yellow glass and its alteration products: the darker pala-
gonite, and the white zeolites. As far as I have been able to detect
no signs of noteworthy interruptions of the volcanism are present
nor are there any unmistakable signs of ice as an important factor
in the formation of the brown glass. But here we are able to make
many interesting observations of sideromelan on an original laver
which lead to a fuller understanding of its formation.
I shall now describe some observations more closely.
In Síða, between Skál and Þverárnúpur the edge of the escarpment
is made up of lava with block jointing, then below follows a brown
layer of breccia and brown matter which further down turns into
a lava of block jointing. Protuberances, not apophyses, extend from
the lava up into the brown matter in such a way as to show clearly
the genetic unity of both. We have here clearly a lava flow that con-
solidated both as a very fine-grained lava and as a brown compact
matter containing a varying amount of lava blocks.
The brown matrix of the breccia is in a thin section (286) seen to
consist of marginally altered angular fragments of sideromelan en-
closing a few phenocrysts of plagioclase and olivine. A great abund-
ance of zeolites and a little calcite occur as cement. There are both
large and small fragments of glass, a common size being 5 mm, and I
think there can be no doubt that they have resulted at this place from
the crumbling of a compact mass of glass, but did not creep forth as a