LICHENOLOGY OF ICELAND 219 



3. ROCK-LICHEN ASSOCIATION. 



By far the greater part of the rocky substratum of Iceland 

 consists of basalt, but recent lava and liparite occur also, the latter, 

 however, in a small quantity only. All these three kinds of rocks 

 are fine-grained volcanic rocks. Considered from a chemical point 

 of view, lipartite differs distinctly from the other two, in that it is 

 of the same mineralogical composition as granite, and is conse- 

 quently rich in silica. 



How the lichens penetrate into these substrata with their hyphse 

 has not been investigated even in the case of a single species. 



The same applies also to the Icelandic tuff - - cemented volcanic 

 ashes of a similar chemical composition as lava, but of quite dif- 

 ferent physical qualities. 



We shall now consider more fully the individual substrata and 

 their vegetation. 



a. Basalt. 



On this kind of rock there occur, as on many others, lichen- 

 vegetations which vary greatly. They may be classified according 

 to different principles exactly as is the case with vegetations on 

 loose soil. I consider it best - - as in the case of earth-vegetations - 

 to take the plants themselves as a guide in the classification, and 

 shall therefore treat the associations in three main groups, viz. 

 associations of crustaceous, foliaceous and fruticose lichens respec- 

 tively; under the last group there are two essentially different sec- 

 tions, viz. erect and pendulous lichens. 



With regard to these associations it may be said in general 

 that: 



Crusta ceo us-lichen -associations grow T on rocks of all 

 possible angles of declivity on horizontal surfaces, on vertical 

 or sloping rock-faces, and on roofs of caves. 



Foliaceous lichens grow in a similar manner to crustaceous 

 lichens on horizontal surfaces, on vertical or sloping rock-faces, and 

 in caves. 



Erect fruticose lichens are found only on horizontal and 

 on gently inclined surfaces, because they are as a rule very slightly 

 attached to the substratum, in fact, they are generally attached to 

 other plants which in their turn are anchored to the substratum, 

 they are not themselves immediately attached to the rock-substratum. 

 They are absent from vertical rock-faces and from the roofs of caves 



