The Botany of Iceland - 01.12.1942, Síða 23

THE PTERIDOPHYTA AND SPERMATOPHYTA OF ICELAND
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large tertiary woods have been destroyed by pumice-eruptions and
lava-streams.
At the time of these tertiary woods the mean temperature of the
ycar in north-west Iceland was at least 9°C—probably somewhat more
(11°—12°) ; now the average temperature for the year in this part of
the country is but 2°C. In the tertiary woods, with quite a different
aspect than thc present ones, there grcw comparatively many species of
trees (cp. Thoroddsen, 1914, p. 213) remains of which are well pre-
served in the clay-layers.
The palagonite formation is composed of different kinds of tufa
and breccia, and in its upper division contains much morainic material
and scattered glacial gravel, as also some ice-striated lava-streams. The
tufas are usually of a brownish-yellow colour owing to the intermixture
of palagonite (cp. Thoroddsen, loc. cit. p. 214), and consist of lava-
dust and lava-fragments, with an abundance of volcanic glass-pieces
(palagonite, trachyte), slag and bombs, and other constituents.
The breccias are distingushed from the tufas by being more coarse-
grained and by their content of larger, angular pieces of lava; the
fragments consist of compact basalt, dolerite, pumice, slaggy lava and
volcanic bombs, and are often covered with a glassy crust. The tufas
and breccias originate from volcanic eruptions which chiefly ejected
ashes and lava-fragments and produced only a few lava-streams.
All around Iceland, both in the basalt and the tufa formations, small
patches of liparite occur, easily discernible from the darker basalt by
its lighter colour. In colour and structure the liparites vary very much,
and are often accompanied by othcr closely allied rocks. The liparites
and allied rocks cover an area of about 800 sq.km.
In some places, e.g. on both sides of the bay Lón in south-eastern
Iceland, the rocky promontories consist of gabbro, probably of terti-
ary age.
Marine deposits, with abundant shells of mussels and snails from
the end of the tertiary epoch, are found in northern Iceland, in thc
neighbourhood of ísavík.
In Central Iceland where tufas and breccia form the foundation,
large areas arc covered with ice-striated lava-streams. This ice-striated
Explanation of fig. 1 : 1. Basalt. 2. Diluvial and Alluvial Accumulations in the
Highland. 3. Post-glacial Basaltic Lava. 4. Doleritic Lava (pre-glacial and glacial).
5. Palagonitic Breccia, Tuffs and Conglomerates. 6. Diluvial and Alluvial Accumula-
tions in Vallcys and Lowlands. 7. Sandar. 8. Volcanic Ash, Blown Sand, “Móhclla”,
9. Pliocene (Crag). 10. Liparite and Granophyre. 11. Glaciers.