The Botany of Iceland - 01.12.1914, Blaðsíða 44

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THORODDSKN
the island; one of the most active volcanoes on Reykjanes is called
Trölladyngja. The best-known volcano near Mývatn is Leirhnúkur,
but besides this, many smaller crater-rows and separate craters
occur; the volcanoes near Mývatn were particularly active during
tlie years 1724—30. In the centre of Odáðahraun rises the volcanic
mountain-group Dyngjufjöll with the crater-valley of Askja; it is
one of the largest volcanoes of Iceland. The crater-valley, which is
surrounded by circular mountain- walls, covers an area of about
55 square km. In the south-eastern corner of it there is a deep
volcanic depression with a lake; at the edge of the latter a new
crater opened on March 29, 1875, and discharged an enormous
quantity ol' pumice over the eastern part of Iceland and, as men-
tioned above, the dust was carried as far as to Scandinavia. The
greatest eruption which has taken place during historic times in
Iceland was the eruption of the above-mentioned crater-rows of Laki
in 1783. The lava which poured fortli filled valleys, altered the
course of rivers and destroyed several farmsteads, fertile meadows
and extensive pastures. North of Iceland submarine volcanic erup-
tions have occasionally taken place.
Almost all tlie volcanoes of Iceland are associated with fissures
in the tuff and breccia areas of tlie palagonite formation. In the
southern part of lceland all the mountain-ridges, valleys and rivers
exhibit a decided dependence on tectonic lines of deeply situated
fracture from SW. to NE. Open íissures in the surface, all the
numerous crater-rows, and the lines joining the volcanoes, have a
similar direction. Moreover, hot springs — both alkaline springs
and sulphur springs — are arranged along the same lines. In North
Iceland, on the other hand, the tectonic lines and the fissures and
volcanoes, have generally a direction from S. to N. Both these
directions probably are combined in a curving band of fracture-
lines which lies across the island. In the basalt plateaus of the
west coast there are several cauldron-like fissures and concentric
fractures, and along the southernmost of these depressions, which
extend over both the tuff and breccia areas around Faxaflói, the
volcanoes and hot springs are arranged in a semicircle.
Earthquakes are very frequent in Iceland not only in con-
nection with volcanic eruptions, but also apart from them; in the
latter case thev are chiefly confined to three districts with well-marked
natural boundaries. All the greater earthquake shocks are tectonic