PHYSICAL GEOGRAPHY 209 



insula, and Skogaibss and Seljalandsfoss below Eyjafjallajokull. These 

 cascades have a height of about 100 metres and more. The largest 

 rivers, only, have been mentioned above, but in addition to these, 

 hundreds of streams of greater or lesser volume occur, often with 

 beautiful waterfalls and cascades in connection with picturesque 

 clefts and rocks. 



As mentioned above, the torrential and changeable glacier-rivers 

 have a destructive influence upon the cultivation and vegetation of 

 the plains. The greensward is torn off and large areas are covered 

 by gravel, therefore the level country south of Vatnajokull is in 

 several places turned into a desert almost destitute of vegetation 

 except where special natural conditions afford a shelter from the 

 destructive effect of the rivers. Where the action of the glacier-rivers 

 is suddenly arrested by any natural phenomenon the level country 

 again becomes quickly covered with plants. As an example may be 

 mentioned the fact that Hverfisfljot, in the year 1783, was forced 

 out of ils bed by a great lava-stream, and a considerable stretch 

 of land Brunasandur - which had previously been irrigated by 

 cold and torrential river-branches was freed from these, only a few 

 clear streams of filtered glacier-water with a slight current issuing 

 from the edge of the lava-streams and flowing down the level 

 country; so that where in 1783 there w r as a gravelly and sandy 

 flat without plant-life and without means of sustenance for human 

 beings there is now a parish with seven farmsteads and abundance 

 of meadows and pasture-lands for the sheep and cattle of the in- 

 habitants. In itself the glacier-water is not inimical to vegetation; 

 it is only the torrential current, the changeableness of the water- 

 courses, and the lo\v temperature of the water which have a de- 

 structive effect upon plant-growth; where these factors are not active, 

 the glacier-water, with its contents of fine clay, is on the contrary 

 a fertilizer; therefore in the neighbourhood of the mouths of the 

 largest glacier-rivers, where there is only a slight current and the 

 water has become warm on the way, fertile tracts of meadows are 

 often found where the glacier-water is profitably utilized for irriga- 

 tion. Water from rivers such as Thjorsa and Hvita has, by analysis, 

 been proved to contain an unusually large quantity of alkali and 

 phosphoric acid. 



Lakes. There are many lakes in Iceland, but the majority of 

 them are of small size. The largest lakes Thingvallavatn and 



