The Botany of Iceland - 01.12.1914, Side 52

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THORODDSEN
and knolls separated by sandy areas and labyrinths of branched
clefts and small valleys. Tuff mountains, owing to the loose nature
of the rock, are relatively poor in water as the latter often dis-
appears into the ground, and does not appear until at a distance
from the mountain. This is especially the case with the tuff-moun-
tains on the plateau where, during summer, not a single stream or
spring is met with for long distances, but only large, deep, dry
river-beds and water-courses íilled with coarse gravel and large-
Fig. 14. Remaining portion of a "móhella” upon a wind-eroded gravelly llat (örfoka).
boulders. These river-beds are due to the melting of the snow in
spring or during periods of thaw in winter when, for a short time,.
they are all tilled with torrential lloods of water.
The surface of the breccia mountains is usually concealed bjr
loose, angular and porous fragments of lava which have been dis-
integrated from the breccia; the nature of the rock is often seen
only in clefts and in a few prominent protuberances and projecting
rocks. The power of resistance of the rock against the action of
water and wind differs however greatly, because tuff and breccia
are of all possible degrees of hardness, although a loose texture is
the most common. Tuff is easily disintegrated, and water and wind