LICHKNOLOGY 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 iirst 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 Bach ma nn. 



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. 



The corrosion must be assumed to take place in part actively 

 on the part of the hypha3, 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: cruslaceous, 

 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 



