The Botany of Iceland - 01.12.1914, Qupperneq 53

PHYSICAL GEOGRAPHY
237
carry the íiner palagonite-dust down into the valleys or to distant
quarters of the island where it is deposited and retained by the
vegetation, frequently forming thick layers (móhella). The heavier
lava-fragments which have tlius been deprived of matrix are left
hehind. On stretches of level ground the lava-gravel, thus loosened,
sometimes attains a thickness of several metres. As a general rule
the surface of the tuff-mountains is much alfected by the action of
water and air and along the sides of the íissures the effects may
be traced far down. Sometimes these fissures are filled with zeolites,
calcite or gypsum; and sometimes the mass which fills the fissures
is harder than the surrounding rock so that the surface presents
the appearance of a network of raised Iines, while the tuff in the
intervening spaces has been disintegrated and carried away. On
the ridges and peaks of several tuff inountains the surface is, as
it were, pock-marked with numerous small irregular hollows, chan-
nels and pot-holes whicli are probably due to tlie combined action of
water and drifting sand. There are often a great number of clefts
and flssures in the tuff-mountains which can sometimes be seen
from a great distance because of the plant-growth which retreats
to them to find shelter from the storm. On the whole the varied
forms of surface in the tuff districts greatly influence the details of
the distribution of the plants of the place.
As tuff is íar more easily decomposed than basalt, soil is formed
more quickly upon tuff-mountains in cases where external factors
such as sand-drifts and storms do not interfere. Therefore on the
tuff and breccia mountains of southern Iceland there is a thick
coating of soil and a luxuriant plant-growth right to the verge of
the mountain; this is rarely the case on basalt mountains. Even
on steep mountain-sides of luff and breccia there exists a luxuriant
vegetation of various species. This is especially conspicuous in Mýr-
dalur (south of Mýrdalsjökull) where, for instance, the extreme point
of Reynisfjall is densely covered with plants. A luxuriant vegetation
is also found in Víkurklettar and in sevérai other places. On the
whole, as mentioned above, it is characteristic of the lower moun-
tains, south of the great Jökulls, to be covered by a comparatively
luxuriant plant-growth, while the level country is barren, owing to
the destructive action of the glacier-rivers. In the case of the sea-
fowl cliffs, where manure is supplied by the sea-fowl, such tuff-
mountains are far more densely covered with Cochlearia, Archangelica,
etc., than are the basalt mountains.
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