The Botany of Iceland - 01.12.1942, Blaðsíða 24

22
JOHS. GRONTVED
lava is of a lighter colour and of doleritic structure, while the recent
lavas are usually dark in colour and have a compact, basaltic structure.
During the glacial epoch Iceland was entirely covered by a sheet
of inland-ice, through which most likely only a few peaks projected
here and there near the edges. The jökulls (snow-fields) of the main-
land probably extended down to the sea in all directions, inasmuch as
the bottom-moraines from that time are found everywhere both in the
lowlands and on the plateau. The lateral and terminal moraines now
occurring in the valleys and lowlands date from a later period, when
the ice was retreating. Ice-striated rocks occur all over the island, both
in the highland, in the valleys, and in the lowlands, as also on islands
and skerries; and erratic blocks are found by thousands scattered over
the whole country.
It is assumed that at the beginning of the tertiary epoch Iceland
was connected with Greenland, the Færoes and Scotland by a broad
land-bridge. This land-bridge was a volcanic highland or plateau-
land formed by innumerable lava-streams coming principally from rows
of craters and from fissures. The plateau which is presumed to have
attained a height of from 3000 to 4000 metres above sea level, was
broken up and depressed towards the end of the miocene period; by
this subsidence, perhaps in conjunction with abrasion, the countries
were separated, and have never since been connected with each other.
Iceland was, however, considerably larger after the separation than it
is now, the land extending 50—100 km further out on all sides. The
subsidence continued through pliocene times, perhaps at a slower rate
than before, and the submarine coastal platform which occurs around
the whole island, and is limited by the 100 fathom line, was formed by
denudation. In the pliocene period Iceland was transversely fissured by
numerous lines of fracture causing violent volcanic action, by which
the tuffs and breccias of the palagonite formation were produced.
Along the same lines the volcanoes of the glacial period and of
the present time appeared.
From the end of the pliocene period to the present time the coast
line has been subject to considerable changes, the boundary-values of
which appear to be a positive displacement of 150 metres and a negative
displacement of 250 metres of the position of the sea surface relative
to the land. At the end of the pliocene, or during the earliest part of
the glacial period the coast-line was situated about 250 metres below
the present level, and in the broad coastal platform which thus be-