CHAP. XL 
THE MODERN VOLCANOES OF ICELAND 
261 
lias been extensively eroded, and has been trenched by many long valleys 
and fjords The composition of the basalts remains remarkably uniform 
over the island. The lava sheets are often decomposing, amygdaloidal, 
and filled with zeolites ; while higher in the series compact basalts abound, 
the uppermost fine-grained sheets being especially constant in structure and 
composition. Numerous dykes traverse the plateau, and some of them cut 
even its highest members. The parallel with the geological structure of the 
Inner Hebrides is continued in Iceland by the appearance of intrusive masses 
of gabbro and granophyre, which represent the deeper parts of the Tertiary 
volcanic series, while the basalts were poured out at the surface. Thus, at 
Papafjord, the gabbro rises into mountainous peaks and, like the similar 
rock in Mull and Skye, is intersected by dykes of a coarse-grained granitoid 
liparite or granophyre. Large dykes and ramifying veins of the same acid 
material, often with a thoroughly granitic aspect, extend into the basalts . 1 
A long series of eruptions has taken place in Iceland since the Glacial 
Period. There were likewise pre-glacial eruptions. The glaciated lava- 
streams are found underneath the modern lavas. So far indeed as is 
known, no evidence exists of any important cessation of subterranean 
activity there since Tertiary time . 3 The existing volcanic phenomena may 
with probability be regarded as the survival of those which were so widely 
manifested over the Icelandic area and the north-west of Europe in the 
older Tertiary ages. A careful study of them may therefore be expected to 
throw light on the history of the Tertiary basaltic plateaux ; while, on the 
other hand, the thorough dissection of these plateaux by the denuding 
agencies will not improbably be found to explain some parts of the sub- 
terranean mechanism of the modern Icelandic volcanoes. 
I11 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. Tlioroddsen, who has examined so large a part of the 
island . 3 The account given by Mr. A. Helland ol the Laki craters has like- 
wise been of much service to me . 4 Among other recent observers I may 
cite Dr. Tempest Anderson,® 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. 1 ’ 
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 
1 Mr. Tlioroddsen, Dansk. Oeografisk Tidsskrift, vol. xiii. 
2 See Dr. Johnston-Lavis, Scottish Geographical Magazine, 1895, p. 442. 
3 See in particular his paper on the volcanoes of north-east Iceland (Bihang till. k. Svensk. 
Vet. Akad. HancV. xiv. ii. No. 5, 1888) and that on Snaefell and Faxebugt in the south-west of 
the island (op. eit. xvii. ii. No. 2, 1891) ; also papers in Dansk. Gcografislc Tidsslcrift, vols. xii. 
xiii. (1893-95) ; Verhand. Gesellsch. Erdkunde. m Berlin, 1894-95. 
4 “ Lakis Kratere og Lavastromme, Uni v ersitsetsprogram , Christiania, 1885. See Mr. 
Thoroddsen’s remarks on this paper, Verhand. Gesell. Erdkunde, 1894, p. 289. 
5 Brit. Assoc. Rep. 1894, p. 650. 
6 Dr. Johnston-Lavis, Scottish Geographical Magazine, September 1895. 
