The Botany of Iceland - 01.12.1914, Síða 31

PHYSICAL GEOGRAPHY
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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