398 SIK A. GETKIE ON THE TEETIARY [May 1 896, 



mark the line of the Laki fissure present an extraordinary picture 

 of volcanic energy of this type. In other instances the cones occur 

 in groups, though this distribution may have arisen from the 

 irregular uprise of scattered vents along a series of parallel fissures. 

 Thus to the north-east of Laki a series of old cones, entirely sur- 

 rounded by the lavas of 1783, lie in groups, the most northerly of 

 which consists of about 'one hundred exceedingly small craters that 

 have sent out streams of lava towards the north-north-east. 1 



It would appear from Mr. Helland's observations that the same 

 fissure has sometimes been made use of at more than one 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. 2 



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 

 Yatnajokull be included, 4390 square kilometres. This vast flood 

 of lava (about 1700 English square miles in extent) would, accord- 

 ing to Mr. Thoroddsen, cover Denmark to a depth of 16 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 Laki — 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 Icelanders, " 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 in width, which, like 

 vast frozen floods, flow down to the country districts, and you may 

 form some idea of this remarkable region/ 3 



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 opera- 

 tion of running water or by fissure and subsidence, transverse 

 sections are cut through these lava-sheets, the observer can gene- 



1 A. Helland, ' Lakis Kratere og Lavastromme,' p. 25. 2 Op. cit. p. 26. 



3 Op. cit. p. 24. Mr. Helland allows an average thickness of 30 metres for 

 the mass of lava which issued in two streams, one 80 kilometres (nearly 50 

 miles), the other 45 kilometres (about 28 miles) long. He estimates the total 

 volume of lava discharged in the 1783 eruption at 27 milliards of cubic metres, 

 equal to a block 10 kilometres (6 miles 376 yards) long, 5 kilometres (3 miles 

 188 yards) broad, and 540 metres (1771 feet) high ; op. cit. p. 31. Mr. Tho- 

 roddsen remarks that the older estimates of the volume of lava discharged by 

 this eruption have been greatly exaggerated. He puts the area covered by lava 

 at 565 square kilometres, and the contents at 12| cubic kilometres (Verhandl. 

 Gesellsch. Erdk. Berlin, 1894, p. 296). 



