Acta naturalia Islandica - 01.02.1946, Page 32

Acta naturalia Islandica - 01.02.1946, Page 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.

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Acta naturalia Islandica

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