CHAP. XL 
THE MODERN ERUPTIONS OF ICELAND 
period of eruption. He describes some old craters on the line of the Laki 
fissure, which had been active long before the outbreak of 1783. 1 
When the lava issues from fissures it is in such a condition of plasticity 
that it can be drawn out into threads and spun into ropes. When the slope 
over which it flows is steep it often splits up into blocks on the surface. 
Where the ground is flat the lava spreads out uniformly on all sides, forming 
wide plains as level as a floor. Thus the vast lava-desert of Odadahraun 
covers a plain 3640 square kilometres in area, or, if the small-lava-streams 
north from Vatnajokull be included, 4390 square kilometres. This vast 
flood of lava (about 1700 English square miles in extent) would, according 
to Mr. Thoroddsen, cover Denmark to a depth of 1 6 feet. The whole of 
this enormous discharge has been given forth from more than twenty vents 
situated for the most part on parallel fissures. 
Not less striking is the picture of fissure-eruption to be met with at 
p a pj — the scene of the great lava-floods of 1783. “Conceive now, says 
Mr. Helland, “ these hundreds of craters, or, as they are called by the Ice- 
landers, ‘ borge,’ lying one behind another in a long row ; every one of them 
having sent out two or more streams of lava, now to the one side, now to 
the other. Understand further that these streams merge into each other, so 
as to flow wholly round the cones and form fields of lava miles m width, 
which, like vast frozen floods, flow down to the country districts, and you 
may form some idea of this remarkable region. 
* The basaltic lavas have issued in a comparatively liquid state, form 
thin sheets and reach to great distances. The western stream from the 
Laki eruption of 1783 flowed for upwards of 40 miles; a prehistoric lava 
from Trolladyngja in Odadahraun flowed for more than 60 miles. 
In the course of time the successive streams of lava poured out upon 
one of these wide volcanic plains gradually increase the height of the ground, 
while preserving its generally level aspect. The loose slag-cones of earlier 
eruptions are effaced or swallowed up, as one lava-stream follows another. 
Eventually, when, by the operation of running water or by fissure and sub- 
sidence, transverse sections are cut through these lava-sheets, the obseivei 
can generally notice only horizontal beds of lava piled one above another, 
including the dykes connected with them and intercalated masses of loose 
slaw, that remain as relics of the old craters. 
° In some places the lava has gradually built up enormous, domes, like 
those of Hawaii, having a gentle inclination in every direction as may 
be seen especially in the district between Eloderne Skjalfanafljot and 
J okulsti. Most of the large volcanic piles of North Iceland are of this 
loonb/l in t.wn Orr-nms one 80 1, i 
1 Up. tit. p. 26. 
2 Op. til. p. 24. Mr. Helland allows an 
allows an average thickness of 30 metres for the mass of lava 
80 kilometres (nearly 60 miles), the other 45 kilometres (about 
