PHYSICAL GEOGRAPHY 243 



quite destitute of covering; on the other hand, on lava-streams near 

 Myvatn (above 300 metres above sea-level) lichens are very frequent. 

 There, many species of phanerogams have also made their appear- 

 ance, and both there and in Kelduhverfi the lava is often found to 

 be covered by a thick carpet of moss. On Reykjanes scores of 

 square km. of lava are entirely hidden beneath a soft greyish carpet 

 of Grimmias. 



Between Jokulsa a Fjollum and Jokulsa a Bru, and between 

 Kaldakvist and Skafta, more recent tuffs appear upon the surface 

 of very considerable tracts of the plateau, not covered by ice-striated 

 dolerite-lavas or bv modern, basaltic lavas. Where this tuff does 



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not appear as bare peaks or steep mountain-chains, the surface is 

 covered by lava-gravel disintegrated from the breccia, or else it is 

 covered by blow r n sand, w r hich is widely distributed, not only on 

 the interior plateau but also in the lowlands. Blown sand is of 

 varied quality and origin. It may be coarse or finer; it is some- 

 times so fine that it penetrates everywhere. During violent storms 

 in sandy districts the fine dust is carried to the most remote quar- 

 ters of the island and is deposited as a fine layer all over the sur- 

 face: it even falls on vessels in mid-Atlantic. But naturally most 

 dust falls in districts bordering on the tuff-belt or situated in it, as 

 the dust chiefly originates from the tuff. The atmosphere in distant 

 regions is often yellowish-brown because of the fine dust suspended 

 in the air, and this dust-cloud is known in Iceland as "mistur." 

 This tuff-dust has played a very important part in the formation 

 of the Icelandic soil and subsoil, and it can be demonstrated almost 

 everywhere. In the blo\vn-sand districts proper, and in the neigh- 

 bouring regions, where also larger particles of stone are put into 

 motion, wind-blown sand has a great mechanical influence and is 

 a mighty geological factor; its denudating effect upon the tuff moun- 

 tains has been very great. Harder rocks are also affected by the 

 sand ; dolerites acquire a dull polish with irregular depressions, 

 striations and furrows, while basalt is likewise sand-polished though 

 to a less extent. Tuff and breccia mountains are always more highly 

 disintegrated on the windward side, and the isolated fragments of 

 basalt embedded in the breccia project further from the ground- 

 mass on that side than to leeward. When the wind is stormy, great 

 masses of blown sand are constantly driven through the narrow 

 valleys, which occur between the numerous sharp tuff-ridges east 

 of Tungna, so that no vegetation can thrive there; only here and 



