The Botany of Iceland - 01.12.1914, Blaðsíða 58

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THOKODDSEN
where numerous glacier-rivers branch out upon the plateau, there
are flats of rolled gravel and clay which are sometimes so steeped
in water from the melting glaciers that they cannot be traversed.
On the iníerior plateau large areas (about 6500 square km.) are
occupied by lava-streams — usually sheet-lava, with intervening tracts
of slaggy lava. These lava-deserts are very poor in plant-life, and
in the most highly-situated districts, they are almost entirely destitute
of vegetation. Water is also very scarce, as the rain-water and the
melting snow from the snow-drifts penetrate into the lava and do
not reappear until far away as springs. The surface of these deserts
consists of a hard, stiffened stony mass without a vestige of soil-
covering, the hollows often íilled with volcanic ashes and blown
sand. The vegetation upon the lava-streams ditfers greatly on the
plateau from what is found in the low-lying district, even if the lava-
streams are of the same age; in the latter locality the oldest post-
glacial lava-streams are often covered by a thick layer of soil, sup-
porting heather-moors and coppice-woods; while on the plateau*
lavas of the same age are quite bare. From Trölladyngja, a volcano
in Odádahraun, an enormous lava-stream (Frambruni), 110 km. in
length, has flowed down through Bárdardalur to Ullarfoss. Up at
the volcano itself the lava, to a height of 1000 metres above sea-
level, is entirely destitute of vegetation; lower down from Dyngju-
fjöll to Bárdardalur (800—200 metres), the vegetation becomes denser
eacli step. The blown sand which has accumulated in the low-Iying
tracts, is here and there covered with lyme grass, which is soon
followed by dwarf willows and heather. Down in Bárdardalur (160
—100 metres) the lava is entirely covered with greensward and
river-gravel so that only a few lava-peaks protrude. Near Lunda-
brekka a rather tliick layer of peat is formed on the surface of the
lava; otherwise the large lava-waste of Odádahraun (600—1200 metres)
is almost destitute of plant-growth. The few plants which occur are
especially met with where hollows in the low-lying tracts are filled
with blown sand; most frequently some tufts of Elymus arenarius
can be seen, and here and there a few specimens of Silene maritima,
Cerastium alpinum and Armeria maritima. In places where the moun-
tain streams from Dyngjufjöll have carried down a little clay, soil-
formation has taken place to a small extent and extremely small
specimens of Salix herbacea and Polygonnm viviparum exist there.
Lichens, which occur so frequently on low-lying lava-streams, are
very rarely met with on Odádahraun, so the lava-surface is usually