PHYSICAL GEOGRAPHY 



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compared with their extent. The largest have an altitude of 1400- 

 1600 metres, and a diameter of 10 15 km. On such dome-shaped 

 lava-cones the angle of inclination at the top of the mountain is 

 only slightly greater than near the base, seldom exceeding 7 -8, 

 and more frequently still less, often only 1 2. The 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 usually 



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 Oda5a- 

 hraun was mainly formed by outpourings from this type of volcano, 

 of which Trolladyngja (1491 metres) is the largest. Another well- 

 known volcano of this description is Skjaldbreid near Thingvellir. 

 Of post-Glacial lava-cones, 16 are known from Iceland. These lava- 

 cones were also very common during the Ice Age. 



The greatest amount of lava which has been poured forth in 



