THE BRYOPHYTA OF ICELAND 



rough, and therefore offers more favourable conditions to the mosses 

 for attaching themselves to it. When a basalt and a tuff surface 

 occur side by side, as is often the case, the difference is very 

 obvious. Mosses grow more scantily on basalt, since it is distinctly 

 seen that the majority of the tufts are situated in, or proceed from, 



Fig. 31. Tuff cleft (Blciksagil) near Barkarslaclr. 



a fissure, while tuff surfaces are often covered with a continuous 

 moss-carpet, composed of many different species. It is also a fact 

 of great importance that the water which flows down from above, 

 and atmospheric humidity, are available to the mosses of basalt 

 surfaces only as long as the water is flowing or is deposited, while 

 the tuff is able to absorb water through its porose surface, and thus 

 has a constant supply of necessary moisture for the vegetation. 

 Owing to the slight power of resistance of the tuff against the ero- 



