LICHENOLOGY OF ICELAND 157 
(silica-lichens), and those from calcareous rocks, (calcareous lichens), 
although the observers disagree somewhat among themselves as 
regards the explanation of this phenomenon. 
Stahlecker has shown that rocks composed of different kinds 
of mineral-grains, are affected by the lichens so that the basic grains 
are the first to be corroded, then the acid. The physical and mine- 
ralogical qualities of the mineral-grains are, on the other hand, of 
no importance. The same author maintains that lichens are able to 
corrode quartz; this is denied by Bachmann. 
On the other hand, how rocks with glassy structure, without 
distinct, separate grains of mineral matter, as for instance obsidian, 
the ground-mass in porphyries, pumice, etc., are affected, is not 
known. 5 
The corrosion must be assumed to take place in part actively 
on the part of the hyphæ, by their excreting acids. But nothing is 
known regarding this point. 
The degree to which the rock is disintegrated is, as I have 
shown (1908, p. 300), of great importance, the freshest, recently-bared 
rock-surfaces being devoid of lichens, while progressive disintegration 
is accompanied by the presence of crustaceous, foliaceous and fruti- 
cose lichens in fixed succession. 
As far as my knowledge and that of other investigators goes, 
I must assume that a floristic difference will be proved to exist in 
the lichen-vegetation found on different kinds of rock, especially 
between that found on calcareous and siliceous rocks — a circum- 
stance which is already partially known. 
It is thus seen that both floristically and biologically the che- 
mical condition of the substratum is the determining factor, whilst 
its physical condition appears to be less important (compare above 
on bark-lichens). But as yet exhaustive lists of lichens from different 
kinds of rock are wanting, and these alone can give a closer in- 
sight into this floristic difference. That species exist which are con- 
fined to one particular substratum, for instance lichens which are 
exclusively “calcareous lichens,” is quite certain, but I do not think 
it has been definitely proved. 
Rock-lichens may be divided into three groups: crustaceous, 
foliaceous and fructicose lichens. 
In the crustaceous lichens two sub-groups may be recognized: 
the epilithic and the endolithic. 
The epilithic crustaceous lichens have a hyphal layer 
