The Botany of Iceland - 01.12.1914, Blaðsíða 60

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THORODDSEN
there upon the highest ridges and peaks, which cannot be reached
by the coarser grains of the drifting sand, are seen small patches
of soil supporting mosses and a few plianerogams.
In Iceland blown sand consists almost invariably of decomposed
volcanic rocks; quartz-sand does not occur in Iceland. The most
common blown sand is palagonite-dust usually of a yellowish-brown
colour, wliich when examined microscopicalhq is seen to consist
of glass-particles, tachylite, palagonite, plag'ioclase, augite and various
Fig. 15. Móhella in Króksdalur, not far from Sandmúladalsá. (Phot. Heinrich Erkes.)
finely decomposed zeolitic alteration-products. Volcanic ashes ofrecent
date often occur as blown sand especially in the interior of the
lava-wastes; they are heavier and less mobile, consequently, they
are not dispersed in quantities beyond the volcanic districts. In the
neighbourhood of the great glacier-bearing mountains, considerable
tracts of level land are often covered with glacial clav, which when
dried, crumbles into dust and drifts beyond the nearer surroundings.
Around Dyngjufjöll, especially south-east of Askja, large areas are
covered with blown sand, consisting of liparitic pumice-dust which
all dates from the eruption of 1875. Moreover, stretches of blown
sand consisting' of decomposed mussel shells, i. e. calcareous dust,
occur here and there along the coast of the north-western peninsula.