PHYSICAL GEOGRAPHY 249 



is situated upon the same lava-stream, hut as the level above the 

 sea is somewhat higher, the water most frequent!}' penetrates into 

 the lava, and the soil is sandy, hard and dry and covered with a 

 good and vigorous growth of grass. The sedges disappear or retreat 

 to small patches where the local conditions allow the accumulation 

 of a greater amount of moisture. The thicker soil-layers upon the 

 lava-streams, usually originate from tuff-dust (mohella), which has 

 been carried thither and has gradually filled up all the depressions, 

 and from glacial clay deposited by rivers. Several lava-streams with 

 a thinner layer of soil support coppice woods - - e. g. on Thingvalla- 

 hraun, and Hvitarhraun - - and heather, as on Reykjaheidi near Keldu- 

 hverfi, and others. In the lowlands the quantity of the plant-growth 

 upon the lava-streams is closely connected with their age, and by 

 the end of a century, a number of species has already settled down 

 on a lava-stream, as may easily be seen upon the Skafta-lavas of 

 1783 and the Leirhnuk-lavas of 1724 30; on the other hand, the 

 lava-streams of Sveinagja, which date from 1875, are still very poor 

 in phanerogams. 



In the most thickly inhabited districts the substratum of the 

 soil generally consists of older and more recent glacial and alluvial 

 formations, very often in connection with "mohella," volcanic ashes 

 and lava-gravel. In the lowlands, which were covered by the sea 

 at the close of the Glacial period, marine sand and clay layers are 

 most frequently found immediately upon the basal rock. The clay, 

 which was [deposited by the glacial rivers of the Ice Age, often 

 occurs in layers of considerable thickness ; it is most frequently 

 bluish-grey in colour and turns blackish-blue on being wetted; it is 

 very tough and dense, and can often become rather hard. The clay 

 contains a very insignificant amount of carbonate of lime, usually 

 only 0.1 0.2 per cent, while the Danish Yoldia-clay, according to 

 Johnstrup, contains 5 15 per cent of carbonate of lime. Along 

 the rivers the banks of clay may often be traced for several kilo- 

 metres without any disturbance being observed in the position of 

 the layers, which is extremely regular and nearly always horizontal. 

 The thickness of the clay-formations varies greatly; in the most 

 highly situated parts of the lowland area and in the valleys, it is 

 sometimes as much as 20 30 metres, further down from 5 to 15 

 metres. The thickness diminishes the nearer the coast is approached ; 

 but it may vary greatly. The clay occurs not only where rivers cut 

 through, but also as a substratum below r morasses. Marine sand 



