Vol. 52.] BASALT-PLATEAUX OF NORTH-WESTERN EUROPE. 399 



rally notice only horizontal beds of lava piled one above another, 

 including the dykes connected with them and intercalated masses of 

 loose slag, that remain as relics of the old craters. 



In some places the lava has gradually built up upon its parent 

 fissure an enormous dome, having a gentle inclination in every 

 direction, as may be seen especially in the district between Floderne 

 Skjalfanafljot and Jokulsa. Most of the large volcanic piles of 

 Korth Iceland are of this nature. The highest of them are 1209 

 and 1491 metres high by from 6 to 15 kilometres in diameter. 

 The elliptical crater of the highest of these eminences measures 1100 

 by 380 metres. 1 



There is still another feature of the Icelandic volcanic regions 

 which may be cited as an interesting parallel to the sequence of 

 eruptive discharges among the Inner Hebrides. While the main 

 mass of the lavas is more or less basic, many of them being true 

 basalts, they have been at different times pierced by intrusions 

 and outflows of much more acid liparites, and even of granophyre. 

 Examples of these rocks of post-glacial age have recently been 

 traced on the ground by Thoroddsen, 2 and their petrographical 

 characters have been studied by Backstrom. 3 The wide distribution 

 of such rocks all over the island, their occurrence in isolated bosses 

 among the more basic lavas, and their remarkable internal structures 

 have been noted by several observers. 4 



It will thus be seen how entirely the modern volcanic eruptions 

 of Iceland agree with the phenomena presented by our Tertiary 

 basalt-plateaux. It is to the Icelandic type of fissure-eruptions, and 

 not to great central composite cones like Vesuvius or Etna, that wo 

 must look for the modern analogies that will best serve as commen- 

 tary and explanation for the latest chapter in the long volcanic 

 history of the British Isles. 



IX. The Faults of the Plateaux. 



There can be no doubt that considerable alterations of level have 

 taken place over the volcanic areas of North-western Europe since 

 the eruptions that produced the basalt-plateaux. The mere fact 

 that in many places the lower members of these terrestrial lavas 

 have been submerged under the sea may be taken to prove a subsi- 

 dence there since older Tertiary time. Along the western coast of 

 Skyo this depression is well shown by the almost entire conceal- 

 ment of the bottom of the plateau under the Atlantic. In the Faroe 

 Isles the subsidence has advanced still further, for not a trace of the 



1 Thoroddsen, Bihang till Svensk. Vet. Akad. Handl. vol. xiv. pt. ii. no. 5, 

 pp. 19, 23. 



2 Geol. Foren. Stockholm Forhandl. vol. xiii. (1891) p. 609; Bihang Svensk. 

 Vet. Akad. Handl. vol. xvii. (1891) pt. ii. p. 21 ; Dausk. Geogr. Tidsskr. vol. xiii. 

 (1895). He has also found peaks of gabbro and boulders of the same rock 

 brought down from the Vatnajokull. The gabbro rests upon basalt, seems to 

 be associated with granophyres, and is cut by dykes of liparite. It is regarded 

 by Mr. Thoroddsen as belonging to the older Tertiary series and to occur pro- 

 bably in the same way as the gabbro of Mull (op. cit. p. 35). 



3 G-eol. Foren. Stockholm Forhandl. vol. xiii. (1891) p. 637. 



4 See in particular C. W. Schmidt, Zeitschr. Deutsch. geol. Geeellsch. 

 vol. xxxvii. (1885) p. 737. 



