228 THORODDSEN 



the island; one of the most active volcanoes on Reykjanes is called 

 Trolladyngja. The best-known volcano near Myvatn is Leirhnukur, 

 but besides this, many smaller crater-rows and separate craters 

 occur; the volcanoes near Myvatn were particularly active during 

 the years 1724 30. In the centre of OdaSahraun rises the volcanic 

 mountain-group Dyngjufjoll with the crater-valley of Askja; it is 

 one of the largest volcanoes of Iceland. The crater-valley, which is 

 surrounded by circular mountain- walls, covers an area of about 

 55 square km. In the south-eastern corner of it there is a deep 

 volcanic depression w r ith a lake; at the edge of the latter a new 

 crater opened on March 29, 1875, and discharged an enormous 

 quantity of pumice over the eastern part of Iceland and, as men- 

 tioned above, the dust was carried as far as to Scandinavia. The 

 greatest eruption which has taken place during historic times in 

 Iceland was the eruption of the above-mentioned crater- rows of Laki 

 in 1783. The lava which poured forth filled valleys, altered the 

 course of rivers and destroyed several farmsteads, fertile meadows 



/ 



and extensive pastures. North of Iceland submarine volcanic erup- 

 tions have occasionally taken place. 



Almost all the volcanoes of Iceland are associated with fissures 

 in the tuff and breccia areas of the palagonite formation. In the 

 southern part of Iceland all the mountain-ridges, valleys and rivers 

 exhibit a decided dependence on tectonic lines of deeply situated 

 fracture from SW. to NE. Open fissures in the surface, all the 

 numerous crater-rows, and the lines joining the volcanoes, have a 

 similar direction. Moreover, hot springs both alkaline springs 

 and sulphur springs - are arranged along the same lines. In North 

 Iceland, on the other hand, the tectonic lines and the fissures and 

 volcanoes, have generally a direction from S. to N. Both these 

 directions probably are combined in a curving band of fracture- 

 lines which lies across the island. In the basalt plateaus of the 

 west coast there are several cauldron-like fissures and concentric 

 fractures, and along the southernmost of these depressions, which 

 extend over both the tuff and breccia areas around Faxafloi, the 

 volcanoes and hot springs are arranged in a semicircle. 



Earthquakes are very frequent in Iceland not only in con- 

 nection with volcanic eruptions, but also apart from them ; in the 

 latter case they are chiefly confined to three districts with well-marked 

 natural boundaries. All the greater earthquake shocks are tectonic 



