The Botany of Iceland - 01.12.1914, Blaðsíða 59

PHYSICAL GEOGRAPHY
243
quite destitute of covering; on the other hand, on lava-streams near
Mývatn (above 300 metres above sea-level) lichens are very frequent.
There, many species of phanerogams have also made their appear-
ance, and both there and in Kelduhverfi the lava is often found to
be covered by a thick carpet of moss. On Reykjanes scores of
square km. of lava are entirelv hidden beneath a soft greyish carpet
of Grimmias.
Between Jökulsá á Fjöllum and Jökulsá á Brú, and between
Kaldakvist and Skaftá, more recent tufls appear upon the surface
of very considerable tracts of the plateau, not covered by ice-striated
dolerite-lavas or by modern, basaltic lavas. Where this tufl' does
not appear as bare peaks or steep mountain-chains, the surface is
covered by lava-gravel disintegrated from the breccia, or else it is
covered by blown sand, which is widely distributed, not only on
the interior plateau but also in the lowlands. Blown sand is of
varied quality and origin. It majr be coarse or finer; it is some-
times so íine that it penetrates everywhere. During violent storms
in sandy districts the fme dust is carried to the most remote quar-
ters of the island and is deposited as a fine layer all over the sur-
face: it even falls on vessels in mid-Atlantic. But naturally most
dust falls in districts bordering on the tuff-belt or situated in it, as
the dust chiefly originates from the tuft'. The atmosphere in distant
regions is often yellowish-brown because of the fine dust suspended
in the air, and this dust-cloud is known in Iceland as “mislur.”
This tulf-dust has played a very important part in the formation
of the Icelandic soil and subsoil, and it can be demonstrated almost
everywhere. In the blown-sand districts proper, and in the neigh-
bouring regions, where also larger particles of stone are put into
motion, wind-blown sand has a great mechanical influence and is
a mighty geological factor; its denudating eflect upon the tuff moun-
tains has been very great. Harder rocks are also affected bjT the
sand; dolerites acquire a dull polish with irregular depressions,
striations and furrows, while basalt is likewise sand-polished though
to a less extent. Tuft and breccia mountains are always more highly
disintegrated on the windward side, and llie isolated fragments of
basalt embedded in the breccia project further from the ground-
mass on that side than to leeward. When the wind is stormy, great
masses of blown sand are constantly driven through the narrow
valleys, which occur between the numerous sharp tuff-ridges east
of Túngná, so that no vegetation can thrive there; onIjT here and