[284] Mr. Thoroddsen, Dansk. Geograf. Tidsskrift, vol. xiii.
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 pumiceous 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.[285]
[285] Mr. Thoroddsen, op. cit.
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,[286] and their petrographical characters have been studied by Mr. Bäckström.[287] 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.[288] 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 acid eruptions are known to have occurred in modern times, some of the liparites are crusted with sulphur and from the connected fissures steam still rises.
[286] Geol. Fören. Stockholm Förhandl. xiii. (1891), p. 609; Bihang. Svensk. Vet. Akad. Handl. xvii. ii. p. 21 (1891); Dansk. Geograf. Tidsskrift, xiii. (1895).
[287] Geol. Fören. Stockholm Förhandl. xiii. (1891), p. 637.
[288] See in particular C. W. Schmidt, Zeitsch. Deutsch. Geol. Gesellsch. xxxvii. (1885), p. 737.
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, therefore, to the Icelandic type of fissure-eruptions, and not to great central composite cones like Vesuvius or Etna that we must look for the modern analogies that will best serve as commentary and explanation for the latest chapter in the long volcanic history of the British Isles.[289]
[289] In his memoir of 1874, Professor Judd announced his conclusion that there were formerly five great volcanoes amongst the Western Isles, and that the lavas of the plateaux had issued from these. He subsequently reiterated this view (Quart. Journ. Geol. Soc. xlv., 1890, p. 187), and ridiculed the explanation of fissure-eruptions. The evidence adduced by me in a paper published in 1896 (same journal, vol. lii. p. 331) and reprinted with additions in this chapter, will, I trust, be regarded by geologists as having finally settled this question.
As a further but more ancient illustration of the type of volcanic action which appears to have been prevalent during the formation of the Tertiary volcanic plateaux of Britain, I may again refer to the vast basalt-fields of Western America. The basalt of Idaho stretches out as an apparently limitless plain. Along its northern boundary, this sea of black lava runs up the valleys and round the promontories of the older trachytic hills with almost the flatness of a sheet of water. It has been deeply trenched, however, by the streams that wind across it, and especially by the Snake River, which has cut out a gorge some 700 feet deep, on the walls of which the successive beds of basalt lie horizontally one upon another, winding along the curving face of the precipice exactly as those of Antrim and the Inner Hebrides do along their sea-worn escarpments. Here and there, a low cinder-cone on the surface of the plain marks the site of a late outflow. One is struck, however, with the singular absence of tuffs and volcanic conglomerates. The basalts appear to have flowed out stream after stream with few fragmentary discharges.