[268] See Dr. Johnston-Lavis, Scottish Geographical Magazine, 1895, p. 442.
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.[269] The account given by Mr. A. Holland of the Laki craters has likewise been of much service to me.[270] Among other recent observers I may cite Dr. Tempest Anderson,[271] who has made himself familiar with extensive tracts of Iceland. He was accompanied one year by Dr. Johnston-Lavis, who has published a narrative of the journey.[272]
[269] See In particular his paper on the volcanoes of north-east Iceland (Bihang till. k. Svensk. Vet. Akad. Handl. xiv. ii. No. 5, 1888) and that on Snaefell and Faxebugt in the south-west of the island (op. cit. xvii. ii. No. 2, 1891); also papers in Dansk. Geografisk Tidsskrift, vols. xii. xiii. (1893-95); Verhand. Gesellsch. Erdkunde zu Berlin, 1894-95.
[270] "Lakis Kratere og Lavaströmme, Universitætsprogram," Christiania, 1885. See Mr. Thoroddsen's remarks on this paper, Verhand. Gesell. Erdkunde, 1894, p. 289.
[271] Brit. Assoc. Rep. 1894, p. 650.
[272] Dr. Johnston-Lavis, Scottish Geographical Magazine, September 1895.
It is a mistake to suppose that the Icelandic volcanoes are generally built on the plan of such mountains as Vesuvius or Etna. Mr. Thoroddsen can evidently hardly repress his impatience to find 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, plays a very subordinate part in Iceland.
Fig. 292.—Fissure (gjá) in a lava-field, Iceland. (From a photograph by Dr. Tempest Anderson.)
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 such fissures appear to be specially marked, one in southern Iceland running from south-west to north-east, the other, in the north part of the island, stretching from south to north.[273] 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 outbreaks of Odádahraun and Myvatn have almost all issued from fissures of that nature.