The Botany of Iceland - 01.12.1914, Blaðsíða 49

II. CONDITIONS PERTAINING TO SURFACE
AND SOIL.
fter having tlius given a brief, general survey of the orographical
and geological conditions and having described the substratum
and general structure of the island, we will now pass on to a de-
scription of the surface itself, with which plant-growth is more parti-
cularly associated. As mentioned above, Iceland is built up of basalt,
tuffs and breccias, but basalt is the fundamental rock; the tuffs
and breccias are, for the most part, nothing else but basalt split
and pulverized. The mineralogical and chemical composition of the
soil is therefore essentially tlie same over the entire island, provided
the siliceous liparites are excepted which have no effect of any
importance to the whole.
Seen from a distance, the basalt mountains usually appear to
be steeper than they are in reality, and the small terraces or
steps of the layers of basalt are not distinguishable in the higher
part of the mountain from a distance except when tliey are snow7-
covered or when, as rarely happens, a scanty vegetation (especially
mosses) has been able to gain foothold upon the narrow ledges.
The rule is that the steps of the basalt mountains become broader
as the base is approached. At the top the separate layers project
as a narrow ledge which is only half, one, or two metres broad,
but lowest down in the valleys, and nearest to the sea the separate
layers form enormous terraces which may attain a breadth of V*—
V2 km. or more. The upper surface of these broad terraces is covered
with gravel and clay, and sometimes with a scattered plant-growth,
or sometimes with a continuous vegetation, with bogs or swamps;
there, enormous, elongated snow-wreaths may persist far into the
summer. On basalt mountains erosion is more active on the sunny
side, therefore the other side is steeper and more sparsely covered
with plants. On the sunny side the average inclination is usually