E. B. Bailey — Iceland, a Stepping -Stone. 471 



Generally a fissure is only used once, but the well-known Lain 

 fissure, with its crater-row dating from 1783, carries upon itself 

 a crater of prehistoric age. 



Most of the volcanic fissures are not accompanied by faulting, 

 though this is a rule to which there are numerous exceptions. 

 Two more examples may be quoted. 



One of a series of graben occurring 25 km. east of My vatn — north 

 of Ask]* a — had for a time fault-scarps 10-20 m. high, 15 km. long, 

 and 400-500 m. apart. During a great earthquake in 1875 lava 

 rising along the western wall, filled this depression. Then a crater 

 row 22 km. long was developed, and poured out 300 c.kra. of lava. 

 The Ogmundarhraun lava in the neighbourhood of Krisuvik — east of 

 Eeykjanes — is also interesting in this connexion. It issued in 1340 

 from two parallel fissures. The southern part of the strip between 

 these fractures has sunk 66 m. since the beginning of the outflow,, 

 and on the western wall overlooking the subsidence are the halves 

 of four bisected craters of which the corresponding halves have been 

 carried downwards. 



A major fault has not always located the volcanoes of its 

 neighbourhood. These are often situated on parallel fractures, 

 especially on the upthrow side of the great dislocation. I have ventured 

 to italicize the foregoing statement, as it is so reminiscent of the 

 distribution of the intrusion which rose along the fault boundary of 

 the cauldron subsidence of Glen Coe. The Icelandic examples 

 chosen by Thoroddsen to illustrate this tendency are as follows : — 



For 50 km. a fracture reaches from Krisuvik (east of Keykjanes) 

 to Hengill, where the northern side has sunk 200-300 m. Parallel 

 with it on the upthrow side is a wonderfully continuous system of 

 crater-rows borne on subsidiary fractures. A similar case occurs on 

 the south side of the Snaefellsness Peninsula overlooking Paxa Plow. 

 Here too the craters are mostly on the upthrow side. 



At the same time Thoroddsen does not wish to emphasize this 

 point. In the Odadahraun, craters mostly occur on the margins of 

 horsts. In Myvatn, farther north, the marginal position recurs, 

 though other crater-rows are found upon the mountain horsts. 

 What is clear about the distribution of contemporary vulcanicity in 

 Iceland, viewed as a whole rather than in detail, is that it 

 characterizes a great field of subsidence — to which in the present 

 summary the name New Iceland has been assigned. 



Choked eruption-fissures and the ruins of major volcanoes persist 

 from Tertiary and Glacial times, but crater-rows of any but post- 

 Glacial age have been dismantled. The following statistics refer 

 solely to post-Glacial occurrences of various types of volcanoes. 

 Thoroddsen has counted 87 major eruption-fissures with their 

 attendant strings of craters; 6 strato-volcanoes (Vesuvius type); 

 16 lava-domes (Kilauea type) ; 13 explosion craters and crater- 

 groups (Puy type); 2 sub-Glacial outbursts. Of these 130 post- 

 Glacial volcanoes 25-30 have been active during historic times. 



The accumulation of material that goes to make up a great volcano, 

 whether of the Vesuvius or Kilauea type, often renders the original 

 dependence of the volcano upon a fissure a matter of hypothesis 



