PHYSICAL GEOGRAPHY 



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and S. annuum. As the environment of the ridges differs greatly 

 sometimes bogs, sometimes dry grassland, sometimes heather 

 the vegetation on the ridges also differs somewhat in the different 

 districts owing to immigration from these plant-associations. Mosses 



Fig. 30. Drycts octopetala (Vallanes; June 26. 1909). 

 (Phot. A. Hesselbo.) 



are few in number, but there is often an abundance of crustaceous 

 lichens, chiefly various species of Lecideas and Lecanoras, which often 

 impart a strongly variegated appearance to the rocky boulders. 



River gravel has one characteristic plant all throughout Ice- 

 land, viz. Chamcenerium latifolium , the splendid, purple flowers of 

 which occur in large patches upon gravel-tongues between branching 

 rivers, and can be seen from a distance. Besides plants common 

 to gravelly flats several willows often occur here, viz. Salix glauca, 

 S. lanata and S. phy lid folia, and also Saxifragacece , Galium verum, 

 G. silvestris, and others. Clayey flats with a denser vegetation and 

 a soil rich in humus frequently occur at the outer edges of gravelly 



