The Botany of Iceland - 01.12.1914, Side 74

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THORODDSEN
mixed with a large quantity of tuff-dust (móhella). The surface is
divided into squares or more or less regularly formed polygons, by
bands of small stones or gravel, while the clay of the interior of the
squares or polygons is destitute of stones. The surface resembles a
net — the ineshes of which are formed by irregular bands of gravel.
Usually it is only the gravel which supports plant-growth; there
the plants can find shelter between the stones, while the middle of
tlie clay-cakes is too wet for plant-growth. But when the “rude-
mark” becomes drier, vegetation may gradually extend over the
Fig. 18. Portion of a “rudemark” in the neighbourhood of Reykjavík.
showing the position of the stone and gravel bands. (Drawn by Th. Thoroddsen.)
cakes of clay, first forming a scattered growth upon them and ulti-
mately soil and a plant-carpet, especially when tuff-dust and drifted
soil liave settled on the surface. The polygonal cakes vary greatly
in form and size, but generally they have a diameter of only V*—
1 metre. The knolls (þúfa, pl. þúfur; see Fig. 17), whicli play sucli
an important part with reference to Icelandic vegetation and agri-
culture, stand in close genetic relation to the “rudemarks” and we
will therefore try to give an account of the way in which thev both
have probably originated, but questions bearing on this point require
to be elucidated by the experimental investigations of persons living
on the spot. My investigations in Iceland have confirmed me in
my opinion that the Icelandic þúfur — as already mentioned —
stand in a close genetic relation to the “rudemarks” and that sub-
surface ice is an essential condition for the formation of both;