The Botany of Iceland - 01.12.1914, Page 37

PHYSICAL GEOGRAPHY
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large ones, liave another variety of surface — lava-sheets —■ which
may sometimes be level, but are rnore frequently broken and cracked
in numerous directions; in Iceland they are known as helluhraun,
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 Trölladyngja on Odáðahraun.
(Pliot. 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-streains, and sometimes enormous
cracks, which are due to the subsidence of the substratum. Of lava-
clefts of this description Almannagjá near Thingvellir is the most
famous. In Odáðahraun there are also lava-clefts, 10—15 km. in