The Botany of Iceland - 01.12.1914, Síða 65

PHYSICAL GEOGRAPHY
249
is situated upon the same lava-stream, but as the level above the
sea is somewhat higher, the water most frequently penetrates into
the lava, and the soil is sandy, hard and dry and eovered with a
good and vigorous growtli of grass. The sedges disappear or retreat
to small patches where the local conditions allow tlie accumulation
of a greater amount of moisture. The thicker soil-layers upon the
lava-streams, usually originate from tuff-dust (móhella), whicli has
been carried thither and has gradually filled up all the depressions,
and from glacial clay deposited by rivers. Several lava-streams with
a thinner layer of soil support coppice woods — e. g. on Thingvalla-
hraun, and Hvitárhraun — and heather, as on Reykjaheiði near Keldu-
hyeríi, and others. In the lowlands the quantity of the plant-growth
upon the lava-streams is closely connected with their age, and by
the end of a century, a number of species has already settled down
on a lava-stream, as may easily be seen upon the Skaftá-lavas of
1783 and the Leirhnúk-lavas of 1724—30; on the other hand, the
lava-streams of Sveinagjá, which date from 1875, are still very poor
in phanerogams.
In tlie most thickly inhabited districts the substratum of the
soil generally consists of older and more recent glacial and alluvial
formations, very often in connection with “móhella,” volcanic ashes
and lava-gravel. In the lowlands, which were covered by the sea
at the close of the Glacial period, marine sand and clay layers are
most frequently found immediately upon the basal rock. The clay,
which was [deposited by tlie glacial rivers of the Ice Age‘ often
occurs in layers of considerable thickness; it is most frequently
bluish-grey in colour and turns blackish-blue on being wetted; it is
very tough and dense, and can often become rather hard. The clay
contains a very insigniiicant amount of carbonate of lime, usually
only 0.1—0.2 per cent, while the Danish Yoldia-clay, according to
Johnstrup, contains 5—15 per cent of carbonate of lime. Along
the rivers the banks of clay may often be traced for several kilo-
metres without any disturbance being observed in the position of
the layers, which is extremely regular and nearly alwaj's horizontal.
The thickness of the clay-formations varies greatly; in the most
highly situated parts of the lowland area and in the valleys, it is
sometimes as much as 20—30 metres, further down from 5 to 15
metres. The thickness diminishes the nearer the coast is approached;
but it may vary greatly. The clay occurs not only where rivers cut
through, but also as a substratum below morasses. Marine sand