266 
THE TERTIARY VOLCANOES 
BOOK VIII 
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 
Large conical volcanoes of the Yesuvian type built up of alternating 
lavas and tuffs are not common in Iceland, but some occur and rise 
into lofty glacier-covered mountains, such as Orsefajokull (6241 feet), 
Eyjafjallajokull (5432), and Snaefellsjbkull (4577). Hekla (4961) also 
is similarly composed of sheets of lava and tuffs, but has not been built 
as a cone. It forms an oblong ridge which has been fissured in the direc- 
tion of its length and bears a row of craters along the fissure. 2 
Explosion-craters likewise occur among the modern volcanic phenomena 
of Iceland. One of these was formed by a violent explosion at Askja on 
29th March 1875. It has a diameter of only about 280 feet, yet so 
great was the vigour of the outburst that pumieeous stones were spread 
over an area of more than 100 Danish (468 English) square miles, and 
the dust was carried as far as Norway and Sweden. Nine years later Mr. 
Thoroddsen found the bottom of this crater filled with bluish-green boiling 
mud, which will probably in the end become a sheet of still water. The 
borders of these Icelandic explosion-craters seem to be very little higher 
than the ground around them. Most of the ejected material is expelled 
with such force and to such a distance that only a small fraction of it 
falls down around the orifice of eruption. 3 
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 lavas are as a rule more or less 
basic — many of them being true basalts — they have been at different times 
pierced by much more acid liparites and obsidians. Examples of these 
rocks of post -Glacial age have recently been traced on the ground by 
Mr. Thoroddsen, 4 * and their petrographical characters have been studied by 
Mr. Backstrom. 6 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. 6 The 
liparites and obsidians are contrasted with the basalt by the colours and 
forms of their streams. Some of them are so black as to look like heaps 
of coal, though their surfaces pass into grey pumice. They have flowed 
out in a much less liquid condition than the basalts, and have consequently 
formed short, thick and irregular sheets. The liparites and basalts appear 
to have been nearly contemporaneous. They certainly belong to the same 
volcanic cycle and their vents lie close to each other. Though none of the 
1 Mr. Thoroddsen, op. tit. xiv. ii. No. 5, pp. 10, 23. 
- Mr. Thoroddsen, Dansk. Geogrctf. Tidsskrift, vol. xiii. 
1 Mr. Thoroddsen, op. tit. 
4 Qeol. Fiiren. Stockholm Forhandl. xiii. (1891), p. 609 ; Bihang. Svensk. Vet. Akad. Hand/.. 
xvii. ii. p. 21 (1891) ; Dansk. Geograf. Tidsskrift , xiii. (1895). 
s Qeol. Fiiren. Stockholm Forhandl. xiii. (1891), p. 637. 
8 See in particular C. W. Schmidt, Zcitsch. Deutsch. Geol. Gesellsch. xxxvii. (1885), 
p. 737. 
