The Botany of Iceland - 01.12.1914, Qupperneq 31

The Botany of Iceland - 01.12.1914, Qupperneq 31
PHYSICAL GEOGRAPHY 215 covered on the exterior with a crust of tachylyte. Tlie tufF-forniation appears to consist of several divisions the mutual relation of which has not, however, yet been elucidated. AIl round Iceland, in both the basalt and the tuff formalions, small patches of lipariie occur. This rock occurs in small intrusive beds and dykes which, on account of their light colour, are dis- tinguishable from the dark basalt and can therefore often be seen from a distance. The liparites vary very greatly both in colour and in structure, and in places where larger sections are exposed, the colouring is often richly variegated. Liparites are almost alwTays accompanied by many closely allied, glassy rocks, especially pitch- stones, which occur as dykes, perlites, sphærulites, obsidian and pumice. In south-east Iceland and on Snæfellsnes, veins of grano- phyre are found in some places. The rocky promontories on each side of the Bay of Lón in south-east Iceland consist of gabbro, probably of Tertiary age; this rock is also found under the névés of Vatnajökull, because many of the glacier-rivers carry down an abundance of pebbles of gabbro. The liparites and allied rocks, wrhich on the w'hole cover an area of about 800 square km., are distributed all over the island, but most frequently in larger quan- tities in East Iceland. Liparite eruptions have taken place at all periods from the earlier Tertiary times to the present day; some volcanoes w'hich have been active within historic times have ejected liparitic pumice. In tlie neighbourhood of Húsavík in North Iceland are found, near Hallbjarnarstadir, marine deposits with abundant shells of mussels and snails dating from the end of the Tertiary epoch, from the period called in England the Red Crag; these for- mations are found nowhere else in Iceland. In central Iceland, where tuff and breccia form the foundation, large areas are covered by old ice-striated lava-streams. These lava- streams are distinguished from the lavas of the present day by their colour and structure. The modern lavas are usually dark in colour with a compact basaltic structure, while these ice-striated lavas have lighter colours and a doleritic structure. These dolerite- lavas, during the Glacial period and immediately after, tlowed from dome-shaped lava-cones with large crater-openings, here and there still extant; or sometimes from large “Bedded Volcanoes” (Strato- Vulkane) the ruins of wrhich — half destroyed by erosion — are still to be found here and there within the area of the palagonite forma- tion. These ice striated lavas are of different ages; some of them
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The Botany of Iceland

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