The Botany of Iceland - 01.12.1928, Side 34

358
JOHS. BOYE PETERSEN
influences its vegetation. I have visited the littoral meadow at Borg
especially referred to by Jónsson, and, like hhn, I find that Vau-
cheria sphœrospora is the most widespread species of alga in the
meadow itself and particularly »wliere the carpet of Gramineæ
abutted on the clay, wliich was partly overgrown with Triglochm
maritimumi.. I found that, especially on the clayey banks of tlie
stream mentioned at p. 4 (H. Jónsson 1. c.), it formed very large
tufted growths (Fig. 1). Among its filaments and, I think, covering
the whole surface of the meadow, there were numerous species of
Diatoms, wliich can hardly, liowever, be assumed to thrive parli-
cularly well on the surface. More probably they must be regarded
as halophilous species. Hence I shall make no furlher mention of
them here. On littoral meadows there will likewise occur washed
up marine algæ, especially green algæ, in small depressions.
2. Rocks and Loose Stones.
The bed-rocks of Iceland consists for the most part of basalt
and palagonite-breccia. There is no essential difference in the chenr-
ical composition of these two kinds of rock, the breccia being formed
of fragments of basalt rnixed with volcanic tuff and other volcanic
rocks of more recent date. There is, however, a difference in the
plrysical composition, the basalt being much more compact than
the breccia. The basalt contains some calcium, but bound in such
a way that it is not easily soluble, and it may therefore be supposed
that the plants that grow on it do not derive much benefit frorn
its content of calcium. Finally, a considerable part of the surface
of the island is covered by lava-streams of earlier or later date.
As regards the algal vegetation on these, which is on the whole very
sparse, I can only give little information, as will appear from the
succeeding part.
In Nature rocks wúll no doubt always contain a greater or less
amount of moislure wliich may be deiáved directly from tlie rain
partly absorbed by the rock. Another part of the rain runs down
the rock, especially in certain places where furrows rnake a way
for it. ln many places a considerable amount of water is produced
throughout the summer by the snow melting on the highest peaks,
and finally, in many places, water issues from the mountain sides
in smaller or larger springs. Thus there will often be found places
in the rocks, pai'ticularly the basaltic rocks, where the moisture is
considerable. For the algal vegetation the amount of moisture is