236 



THOHODDSEN 



and knolls separated by sandy areas and labyrinths of branched 1 

 clefts and small valleys. Tuff mountains, owing to the loose nature 

 of the rock, are relatively poor in water as the latter often dis- 

 appears into the ground, and does not appear until at a distance 

 from the mountain. This is especially the case with the tuff-moun- 

 tains on the plateau where, during summer, not a single stream or 

 spring is met with for long distances, but only large, deep, dry 

 river-beds and water -courses filled with coarse gravel and large 



Fig. 14. Remaining portion of a "mohella" upon a wind-eroded gravelly ilat (orfoka). 



boulders. These river-beds are due to the melting of the snow in 

 spring or during periods of thaw in winter when, for a short time,, 

 they are all filled with torrential floods of water. 



The surface of the breccia mountains is usually concealed by 

 loose, angular and porous fragments of lava which have been dis- 

 integrated from the breccia; the nature of the rock is often seen 

 only in clefts and in a few 7 prominent protuberances and projecting 

 rocks. The power of resistance of the rock against the action of 

 water and wind differs however greatly, because tuff and breccia 

 are of all possible degrees of hardness, although a loose texture is 

 the most common. Tuff is easily disintegrated, and water and wind 



