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 hyphæ 
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: — 
Crustaceous-lichen-associations grow 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 
