PHYSICAL GEOGRAPHY 



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large ones, have another variety of surface lava-sheets which 

 may sometimes be level, but are more frequently broken and cracked 

 in numerous directions; in Iceland they are known as hellnhraun, 

 in the Sandwich Islands as Pahoehoe. Upon the smooth surface, 

 numerous tangled and twisted lava-ropes may be seen, bent in long 

 curves following the undulating surface of the viscous lava. Some- 

 times this lava is compact, and without great irregularities of sur- 

 face, but more frequently, by cooling, the surface has subsided and 



Fig. 8. Sheet-lava near Frambruni. Slope of Trolladyiigja on Odadahraun. 



(Phot. Heinrich Erkes.) 



broken into large pieces, forming a number of hills, ridges, embank- 

 ments and cauldron-shaped depressions, giving to it the aspect of 

 a rough sea with high waves. Sometimes the surface of this sheet- 

 lava is arranged in knots, curls and folds, all as smooth as hard- 

 ened pitch. Beneath the lava-sheets there are often empty spaces, 

 like drain-pipes and tunnels, and sometimes large caverns. Both 

 forms of lava are sometimes found combined in the same stream. 

 Long clefts often occur in the lava-streams, and sometimes enormous 

 cracks, w r hich are due to the subsidence of the substratum. Of lava- 

 clefts of this description Almannagja near Thingvellir is the most 

 famous. In Odaftahraun there are also lava-clefts, 10 15 km. in 



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