396 SIR A. GEIKIE ON THE TERTIARY [May T896, 



will not improbably be found to explain some parts of the subter- 

 ranean mechanism of the Icelandic volcanoes. 



In calling attention to some of the more obvious analogies which 

 may be traced between the modern and the ancient volcanoes, I am 

 more particularly indebted to the excellent memoirs of the resident 

 Icelandic geologist Mr. Th. Thoroddsen, who has examined so large 

 a part of the island. 1 The account given by A. Helland of the Laki 

 craters has likewise been of much service to me. 2 Among other 

 recent observers I may cite Dr. Tempest Anderson, 3 who has made 

 himself familiar with extensive tracts of Iceland, and Dr. Johnston- 

 Lavis, who has published the narrative of a journey in company 

 with him. 4 



It is a mistake to suppose that the Icelandic volcanoes are 

 generally built on the plan of such mountains as Vesuvius or Etna. 

 Evidently Mr. Thoroddsen can hardly repress his impatience on find- 

 ing these two Italian cones cited in almost every handbook of geology 

 as types of modern volcanoes and their operations. The regular 

 volcanic cone composed of alternations of lavas and tuffs hardly 

 occurs in Iceland at all. The fundamental feature in the Icelandic 

 eruptions is the production of fissures which reach the surface and 

 discharge streams of lava from many points. 



Two systems of fissures appear to be specially marked, one running 

 from south-west to north-east, the other from south to north. 5 

 Hekla and Laki belong to the former. The dislocations have often 

 followed the boundaries of the ' horsts ' or solid blocks of country 

 which have withstood terrestrial displacement. The vast outbursts 

 of Odadahraun and Myvatn have almost all issued from fissures of 

 that nature. 



The violent eruption of 1875 in Askja found its exit at the 

 intersection of two lines of fissure. Many large fissures were opened 

 on the surface in a nearly north-and-south direction, which could be 

 followed for 80 kilometres. Some of them became the theatre of 

 intense volcanic activity. 6 



Many lines of fissure are traceable at the surface as clefts or 

 8 gjas,' that run nearly straight for long distances, with a width 

 of 1 to 3 yards, and of unknown depth. 7 Occasionally a fissure 



1 See in particular his paper on the volcanoes of N.E. Iceland (Bihang till k. 

 Svensk. Vet. Akad. Handl. vol. xiv. pt. ii. no. 5, 1888) and that on Snaefell and 

 Faxebugt in the south-west of the island (pp. cit. vol. xvii. pt. ii. no. 2, 1891) ; 

 also papers in the Dansk. Geografisk. Tidsskrift, vols, xii., xiii. (1893-95), and 

 in the Verhandl. Gesellsch. Erdkunde zu Berlin, 1894 & 1895. 



2 ' Lakis Kratere og Lavastromme,' Universitsetsprogram, Christiania, 1885. 

 See Mr. Thorodd sen's remarks on this paper, Verhandl. Gesellsch. Erdk. 

 Berlin, 1894, p. 289. 



3 Brit. Assoc. Eep. 1894 (Oxford meeting), p. 650. 



4 Johnston-Lavis, Scot. Geogr. Mag. 1895. 



5 In the Snaefell promontory they run nearly east and west, Thoroddsen, 

 Bihang Svensk. Vet. Akad. Handl. vol. xvii. pt. ii. no. 2, p. 91. 



6 Thoroddsen, op. cit. vol. xiv. pt. ii. no. 5, p. 63. 



7 On the various modes of origin of these chasms, see Tempest Anderson, 

 Brit. Assoc. Rep. 1894, p. 650. Mr. Thoroddsen describes a fissure in the 

 south of Iceland running N.E. for 30 kilometres, with a depth of 130 to 200 

 metres. It has discharged three great lava-streams, covering a total area of 

 693 square kilometres. 



