Vol. 52.] BASALT-PLATEAUX OF NORTH-WESTERN EUROPE. 397 



has not been continuously opened to the surface. An interesting 

 example of such intermittent chasms is supplied by the great rent 

 which gave forth the enormous volume of lava in 1783. The moun- 

 tain of Laki, composed of palagonite-tuflf, stands on the line of the 

 dislocation, but has not been entirely ruptured. The fissure has 

 closed 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. 1 



Some fissures have remained mere open chasms without any 

 discharge of volcanic material; others have served as passages for 

 the escape of lava and the ejection of loose slags and cinders. 2 



In some instances, according to Mr. Thoroddsen, lava wells out 

 from the whole length of a fissure without giving rise to the forma- 

 tion of cones, the molten material issuing either from one or from 

 both sides, sometimes flowing out tranquilly, but more usually 

 giving rise to long ramparts of: slags and blocks of lava piled up on 

 either side. In the great majority of cases, however, a row of cones 

 is formed along the line of the open fissure. Thus, on the Laki 

 fissure, which runs for about 20 miles in a north-easterly direction, 

 the cones amount to some hundreds in number. Hekla itself appears 

 to have been built up along a main fissure, with parallel subsidiary 

 rents on which rows of cones have been formed. 3 



The cones consist generally of slags, cinders, and blocks of lava. 

 According to Mr. Helland's observations, along the marvellous line 

 of the Laki fissure they are on the whole not quite circular but 

 oblong, their major axis coinciding with the line of the chasm on 

 which they have been piled up. In many places they are exceed- 

 ingly irregular in form, changes in the direction of outflow of lava 

 or of escape of steam having caused the cones partially to efface 

 each other. 



As regards their size, the cones present a wide range. Some of 

 them are only a few yards in diameter, others several hundred 

 yards. Generally they are comparatively low mounds. On the 

 Laki fissure some are only a couple of yards high ; the majority are 

 much less than 50 yards in height, and hardly one is as much as 

 100 yards. 4 And yet these little monticules, as Mr. Helland 

 remarks, represent the pipes from which milliards of cubic metres 

 of lava have issued. While other European volcanoes form con- 

 spicuous features in the landscape, the Icelandic volcanoes of the 

 Laki district, from which the vastest floods of lava have issued in 

 modern times, are so low that they might escape notice unless they 

 were actually sought for. 5 



As they have generally arisen along lines of fissure, the cones are 

 for the most part ranged in rows. The hundreds of cones that 



1 A. Helland, ' Lakis Kratere og Lavastromme,' p. 25. 



2 Mr. Thoroddsen has observed that in the Reykjanes peninsula, in the 8.W. 

 of Iceland, by the subsidence of one side of a fissure, a row of four craters lias 

 been cut through, leaving their segments perched upon the upper side (' Globus,' 

 vol. lxix. no. 5). 



3 Johnston-Lavis, op. tit. p. 457. 



4 Mr. Thoroddsen, however, states that there are about 100 between 20 and 

 100 metres in height. 5 A. Helland, op. cit. p. 27. 



