THULEAN BASALTS 781 



vonian continent of Eria, of whose area the Thulean region forms but a 

 part, distinguished especially by its basaltic flows. 



The general origin, character, extent, and structure of the Thulean 

 region are well and concisely described by Harker, 17 whom we may quote 

 here: 



"These events characterized by a regional extension may be summarized as, 

 first, the outpouring of enormous quantities of lava ; second, the intrusion of 

 innumerable sills ; and, third, the injection of a great system of dikes. The 

 lavas, which can be assigned to an Eocene age, did not emanate from great 

 central volcanoes, but, as Sir Archibald Geikie has shown, welled up through 

 innumerable fissures. The individual flows were not large, but collectively 

 they covered a very extensive region. Indeed, for any evidence to the con- 

 trary, a continuous lava field may have stretched from Antrim to beyond 

 Franz Josef Land, a distance of 2,000 miles. . . . Petrographically, these 

 extrusions and intrusions are characterized by an overwhelming preponder- 

 ance of basic types, with an alkaline tendency which is sometimes rather 

 latent than expressed, but sometimes strongly marked." 



ICELAND w 



Although basalts make up by far the greater proportion of the lavas 

 of Iceland, they have been less studied petrographically than the much 

 less abundant rhyolites and toscanites. The Iceland basalts are some- 

 what indefinitely referred by the authors cited above to three general 

 periods of eruption : the Tertiary plateau basalts, which attain a total 

 thickness of over 3,000 meters, according to Geikie and Hawkes; later 

 pre-Glacial, Glacial, and post-Glacial flows; and the modern basaltic 

 eruptions. While the first seem to have been predominately, if not ex-i 

 clusively, fissure eruptions, like those of the Deccan and Oregon regions, 

 the flows of the second and third periods were frequently accompanied 

 by the formation of low "shield" volcanoes, composed mostly of lava with 

 very little ash, and which are compared by Eeck with Kilauea and other 

 Hawaiian volcanoes. None of the basaltic volcanoes of Iceland, even the 

 most recent, appear to be truly strato-volcanoes of the Vesuvius or Etna 

 type, although some minor explosion craters, hornitos, and other such 

 minor phenomena are to be found. 



The Icelandic basalts of all three periods show both porphyritic and 

 non-porphyritic habits, and in both of these there are represented olivine- 



17 A. Harker : Some aspects of igneous action in Britain. Quart. Jour. Geol. Soc, 

 vol. 73, 1918, pp. xci-xciii. 



ls For some descriptions of the basalts and basaltic volcanoes of Iceland, see A. Geikie : 

 Ancient volcanoes of Great Britain, vol. 2, chapter xl, 1897 ; Thoroddsen : Island. 

 Peterm. Mitth., Erg. Heft no. 152, 1905 ; H. Reck : Isltindische Masseneruptionen, Geol. 

 Pal. Abh. von Koken, vol. 9, Heft 2, 1910; L. Hawkes: Geol. Mag., 1910, p. :\sr> ; A. 

 Holmes : Min. Mag., vol. 18, 1918, p. 190. 



