226 THORODDSEN 



Iceland, issued from volcanic fissures and crater-rows; these are 

 not volcanic mountains, but rows of low craters established along 

 the direct line of a fissure, more frequently upon level land. Of 

 this kind of eruptive vent, 87 are know r n from Iceland at present. 

 Each of the craters in such a chain occurs independently and is 

 built up of scoriae and lava. They are usually low 7 , rarely exceeding 

 a height of 100 150 metres, while the majority of them are even 

 considerably lo\ver; they are often very irregularly formed and com- 

 posed of several rings. Some crater-ro\vs are very long; they often 

 attain a length of 5 10 km., and some are longer still, as for in- 

 stance, the Laki crate r-row of 1783, which has a length of 30 km., 

 and contains about a hundred separate craters of various sizes. 

 Some crater-ro\vs are so small that they resemble tov-volcanoes. 



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In some places the lava has w r elled up out of fissures in large 

 streams without any visible craters. The largest of these fissures is 

 Eldgja, north of Myrdalsjokull ; it has a length of 30 km., and has 

 poured out lava-streams sufficient to cover an area of about 700 

 square km. In some places "explosive craters" occur, cauldron- 

 shaped depressions produced by a single volcanic explosion. The 

 best known of these craters is Viti, on the side of Mount Krafla, 

 north of Mvvatn. It w r as formed by a sudden outburst on May 17, 

 1724. For a long time afterwards there w^as a large, boiling slough 

 at the bottom of the crater, but this is now converted into a greenish- 

 coloured quiescent lake. The majority of the volcanoes in Iceland 

 are basalt volcanoes, and have poured out streams of basaltic lava, 

 and ejected basaltic slag and ashes. Only in the neighbourhood of 

 Torfajokull liparitic volcanoes occur of post-Glacial origin, and of 

 peculiar aspect wiiich have poured out lavas rich in silica. The 

 interior of the lava-mass is grey or reddish brown, but the surface 

 of it is jet-black, as it consists of obsidian, covered here and there 

 by light, almost w'hite, pumice. The largest liparitic lava-streams 

 are those called Hrafntinnuhraun and Domadalshraun. In some 

 places streams of liparitic blocks in a half-melted condition have 

 flowed out from craters in the mountain-sides, and several volcanoes 

 have ejected liparitic pumice, as for instance, the volcano of Askja, 

 1875. Many volcanoes in Iceland are buried beneath the sno\v and 



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the glaciers, and as mentioned above, when they break out into 

 eruption , large masses of ice are melted and the glaciers burst, 

 which causes the neighbouring level lands to be inundated by 

 enormous floods of water, with huge fragments of ice tossing on 



