Acta naturalia Islandica - 01.02.1946, Síða 32

TRAUSTI EINARSSON
i!6
The breccia is some distance west of the farm covered with a layer
of a grey conglomerate, closely resembling the old “moraines”. Yet
no striated stones were observed, and the thin section (283) reveals
that this layer is not different from the previously described grey
conglomerates. The matrix consists mostly of glassy fragments, some
lai'ge but most of them very small, and a few fractured crystals
of plagioclase, olivine and augite, and this is in my view most natur-
ally explained as a mud flow.
If we compare (283) on the one hand, with (281) and (282) on
the other, the main difference is that in the former the large
mass consists of very small glass fragments whereas in the latter
the average size of the fragments is much larger. This is easily under-
stood when it is considered that in one case we have most probably
a flow of a wet mixture of fragments, in which grinding and com-
minution of the fragments must be assumed, whereas in the other case
the conclusion seems inevitable that a (very thick) lava flow consoli-
dated partly as a mass of glass which afterwards crumbled into
innumerable fragments — because of inevitable tension in the g'lass.
In (283) we see the effect of a long transport of a mass of glass
fragments, an effect which is absent in (281) and (282).
A characteristic feature of the volcanic. series here in Síða is the
vast extent of the layers of breccia. It appears as if we had the
same thick layer of breccia and lava of block jointing all the dis-
tance from Þverárgnúpur to Kirkjubæjarklaustur or even to Skál,
over a distance of 25 km. At least the extension is so large that an
extrusion beneath an ice-sheet seems to be quite out of the question.
This extension is however easily understood if a fluid magma flowed
over the area and consolidated in this peculiar way. —
Above the edge of the scarpment of Síða the land rises gently in-
wards and the rough oblique plain is surmounted by two high roughly
dome-shaped mountains, the Kaldbakur and the Geirlandshraun.
Two rivers, Geirlandsá and Þverá have cut deep gorges into the
roots of Kaldbakur. The roots of the mountain are here similar to
the section already described. In the lower gully of Þverá we see
volcanic breccias such as that described before, lavas with block
jointing and columnar grey porphyritic basalt. Higher up along this
river, below the so-called Berjafit a succession of many such porp-
hyritic lavas is seen. Above these lavas is a thin layer of a grey
conglomerate and then comes a typical block lava.