PHYSICAL GEOGRAPHY 237 



carry the finer palagonite-dust down into the valleys or to distant 

 quarters of the island where it is deposited and retained by the 

 vegetation, frequently forming thick layers (mohella). The heavier 

 lava-fragments which have thus been deprived of matrix are left 

 behind. On stretches of level ground the lava-gravel, thus loosened, 

 sometimes attains a thickness of several metres. As a general rule 

 the surface of the tuff-mountains is much affected by the action of 

 water and air and along the sides of the fissures the effects may 

 be traced far down. Sometimes these fissures are filled with zeolites, 

 calcite or gypsum; and sometimes the mass which fills the fissures 

 is harder than the surrounding rock so that the surface presents 

 the appearance of a network of raised lines, while the tuff in the 

 intervening spaces has been disintegrated and carried away. On 

 the ridges and peaks of several tuff mountains the surface is, as 

 it were, pock-marked with numerous small irregular hollows, chan- 

 nels and pot-holes which are probably due to the combined action of 

 water and drifting sand. There are often a great number of clefts 

 and fissures in the tuff-mountains which can sometimes be seen 

 from a great distance because of the plant-growth which retreats 

 to them to find shelter from the storm. On the whole the varied 

 forms of surface in the tuff districts greatly influence the details of 

 the distribution of the plants of the place. 



As tuff is far more easily decomposed than basalt, soil is formed 

 more quickly upon tuff-mountains in cases where external factors 

 such as sand-drifts and storms do not interfere. Therefore on the 

 tuff and breccia mountains of southern Iceland there is a thick 

 coating of soil and a luxuriant plant-growth right to the verge of 

 the mountain; this is rarely the case on basalt mountains. Even 

 on steep mountain-sides of tuff and breccia there exists a luxuriant 

 vegetation of various species. This is especially conspicuous in Myr- 

 dalur (south of Myrdalsjokull) where, for instance, the extreme point 

 of Reynisfjall is densely covered with plants. A luxuriant vegetation 

 is also found in Vikurklettar and in several other places. On the 

 whole, as mentioned above, it is characteristic of the lower moun- 

 tains, south of the great Jokulls, to be covered by a comparatively 

 luxuriant plant-growth, while the level country is barren, owing to 

 the destructive action of the glacier-rivers. In the case of the sea- 

 fowl cliffs, where manure is supplied by the sea-fowl, such tuff- 

 mountains are far more densely covered with Cochlearia, Archangelica, 

 etc., than are the basalt mountains. 



16* 



