The Botany of Iceland - 01.12.1914, Page 41

PHYSICAL GEOGRAPHY
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compared with their extent. The largest have an altilude of 1400—
1600 metres, and a diameter of 10—15 km. On such dome-shaped
lava-cones the angle of inclination at tlie top of the mountain is
only slightly greater than near the base, seldom exceeding 7°—8°,
and more frequently still Iess, oflen only 1°—2°. Tlie summit of
such volcanoes consists of a circular or elliptical mouth or depres-
sion, often of large dimensions, some having a diameter of 1000
metres or more. The walls of the depression (crater) are usualty
Fig. 10. Row of craters.
cleft by concentric fissures, so that the descent from the lip to the
bottom of the crater is, as it were, a series of steps. The sides of
these volcanoes are entirely covered with knotty sheets of lava, and
long tunnels and caves are very common in the sides of the vol-
cano. In some cases the depression is filled with lava to such an ex-
tent, that the only indication of the circumference is a ring of small
lava-pinnacles and lava-ridges. The immense lava-waste of Odáða-
hraun was mainly formed by outpourings from this type of volcano,
of which Trölladyngja (1491 metres) is the largest. Another well-
known volcano of this description is Skjaldhreið near Thingvellir.
Of post-Glacial lava-cones, 16 are lcnown from Iceland. These lava-
cones were also very conimon during the Ice Age.
The greatest atnount of lava which has lieen poured forth in