The Botany of Iceland - 01.12.1914, Blaðsíða 67

PHYSICAL (iEOGRAPHY
251
formed by the rivers, and converted into terraces, which form the
substratum of bogs and grassland. The glacial moraine-gravel often
extends far up the mountain-sides and forms here a substratum
for soil and plant-growth. In other places in the valleys are steep
rock-faces, stony slopes, heaps of Iarge fragments of rock (urd),
and the conical heaps of finer and coarser gravel brought down
by tbe mountain-streams, which all help to give variety to the
plant-formations. While basalt-mountains are slightly and slowly
disintegrated, tuff-mountains are extremely liable lo disintegration,
hence the products of the latter, combined with the action of wind,
glaciers and rivers, play a more important part. The conlribution
of the basalt towards soil-formation dates mainly from the Glacial
period. As we have seen from the above, the substrata which
support plant-growth are (1) firm ground, having a rocky base
(basalt, liparite, breccia and lava); (2) loose soil, consisting of mo-
raines, river-gravel, sand, clay, blown sand, volcanic ashes and tulF-
dust (móhella); and (3) the products of the plants themselves: boggy
soil composed of peat and humus.
The characler of the subsoil below the humus-layer and the
plant-covering is consequently in close relation to the chemical and
mineralogical composition of the underlying rock. Over the greater
part of Iceland the inorganic soil consists of decomposed basaltic
rocks, the main mineralogical constituents of which are plagioclase
(especially lime-felspars) and augite, but magnetite and olivine also
occur, often in great quantilies, and apatite and a small quantity
of titanic iron. Tbe chemical composition of the Icelandic basalts
is rather uniform. On an average they contain 43—53 % of silica,
11—18 % of alumina, 11—22 % of iron (Fe 0 and Fe2 ()3), 8—13 %
of lime, 2—9% of magnesia, 0.2—2% of potash and 1—4% of
soda. Because anorthite, of the plagioclases, is very largely distri-
buted in tlie Icelandic rocks, not only in the basalt, but also in
the recent lavas and tuffs, these Icelandic rocks often contain a
comparatively small amount of silica and a very considerable amount
of lime and also alumina. The reason why the Icelandic soil is
nevertheless poor in carbonate of lime may be found in tlie faet
that the lime can only with difficulty be separated from its siliceous
compounds, and because in the whole of the island, no sedimentary
calcareous rocks are found, though such are of common occurrence
in other countries. In districts where sulphurous acids sent out
from fumaroles have affected the rocks, as is common in tuff-districts,
The Botany of Iceland. I. 17