CHAP. XL 
THE MODERN ERUPTIONS OF ICELAND 
263 
of 30 kilometres (more than 18 English miles), and a depth of 130 to 200 
metres (426 to 656 feet). Over its vertical walls lofty waterfalls plunge 
from the crest to the bottom. . , f . 
Occasionally a fissure has not been continuously opened to the surface. 
An interesting example ot such intermittent chasms is snppl.ed by the 
great rent which gave forth the enormous volume of lava in I/ 80 . 1 
mountain of Laki, composed of palagonite tuff, stands on the line 0 us 
dislocation, but has not been entirely ruptured. The fissure lias closec up 
beneath the mountain, a short distance above the bottom of the slope, as 
is shown by the position of a couple of small craters.. , 
Some fissures have remained mere open chasms without any discharge 0 
Fig. 293. — Cones on the great Laki fissure, Iceland. (From a photograph by Dr. 
Tempest Anderson.) 
volcanic material ; others have served as passages for the escape of lava and 
the ejection of loose slags and cinders. ,, f 
In some instances, according to Mr. Thoroddsen, lava wells out fiom the 
whole length of a fissure without giving rise to the formation of cones 
the molten material issuing either from one or from « e 
flowing out tranquilly. Thus from three points on the great Eldgja ch 
lava spread out quietly without giving rise to any craters, though at it 
southern prolongation of the fissure, where it becomes nauouei a 
low slag-cones was formed. The three lava-streams flooded the low groum 
over an area of 693 square kilometres, or 270 English square miles. In 
the great majority of cases, however, the lava as it ascent s in ie ssmc 
gives rise to long Lmparts of slags and blocks of lava piled up on either side 
or to a row of cones along the line of the open chasm. Thus, on the Laki 
1 Mr. A. Helland, op. tit. p. 25. ... ,, 
* Mr Tlioroddsen lias observed that in the Reykjanes peninsula in the south 
by the subsidence of one side of a fissure, a row of four craters has been cut throu 
segments perched upon the upper side. Globus, vo . xix. 1 °- 
-west of Iceland, 
gh, leaving their 
