242 THORODDSEN 



where numerous glacier-rivers branch out upon the plateau, there 

 are flats of rolled gravel and clay which are sometimes so steeped 

 in water from the melting glaciers that they cannot be traversed. 



On the interior plateau large areas (about 6500 square km.) are 

 occupied by lava-streams - usually sheet-lava, with intervening tracts 

 of slaggy lava. These lava-deserts are very poor in plant-life, and 

 in the most highly-situated districts, they are almost entirely destitute 

 of vegetation. Water is also very scarce, as the rain-water and the 

 melting snow from the snow-drifts penetrate into the lava and do 

 not reappear until far away as springs. The surface of these deserts 

 consists of a hard, stiffened stony mass without a vestige of soil- 

 covering, the hollows often filled with volcanic ashes and blown 

 sand. The vegetation upon the lava-streams differs greatly on the 

 plateau from what is found in the low-lying district, even if the lava- 

 streams are of the same age; in the latter locality the oldest post- 

 glacial lava-streams are often covered by a thick layer of soil, sup- 

 porting heather-moors and coppice- woods ; while on the plateau^ 

 lavas of the same age are quite bare. From Trolladyngja, a volcana 

 in Odadahraun, an enormous lava-stream (Frambruni), 110 km. in 

 length, has flowed dow r n through Bardardalur to Ullarfoss. Up at 

 the volcano itself the lava, to a height of 1000 metres above sea- 

 level, is entirely destitute of vegetation ; lower down from Dyngju- 

 fjoll to Bardardalur (800200 metres), the vegetation becomes denser 

 each step. The blown sand which has accumulated in the low-lying 

 tracts, is here and there covered with lyme grass, which is soon 

 followed by dwarf willows and heather. Down in Bardardalur (160 

 100 metres) the lava is entirely covered with greensward and 

 river-gravel so that only a few lava-peaks protrude. Near Lunda- 

 brekka a rather thick layer of peat is formed on the surface of the 

 lava; otherwise the large lava- waste of Odadahraun (600 1200 metres) 

 is almost destitute of plant-growth. The few r plants which occur are 

 especially met with where hollows in the low-lying tracts are filled 

 with blown sand; most frequently some tufts of Elymus arenarius 

 can be seen, and here and there a few specimens of Silent maritima, 

 Cerastium alpimim and Armeria maritima. In places where the moun- 

 tain streams from Dyngjufjoll have carried down a little clay, soil- 

 formation has taken place to a small extent and extremely small 

 specimens of Salix herbacea and Pohjyonnm viviparum exist there. 

 Lichens, which occur so frequently on low-lying lava-streams, are 

 very rarely met with on Odadahraun, so the lava-surface is usually 



