174 



01. Al (iAI.I.OI. 



Pertusaria xanthostonui. 



I 'nvnlaria sophodes v. orbata. 



exiim. 



Lccidca vernalis. 



erythrophaea. 



Bercngeriana. 



Tornoensis. 



fusccscens. 



Nvlanderi. 



Lecidea entcrolcuca (with var. 



Lauren I. 



Gyalecta Hcckhuusii. 

 Biiellia myriocarpa. 

 Arlhonia proximella. 

 Sagedia analepla. 



i>risea. 



kentrospora. 

 Pvrenula micula 



If we compare the birches of Iceland and Denmark with re- 

 ference to their lichen-vegetation, a characteristic difference will he 

 seen as regards the species. In Denmark Eoernia Prnnaslri. K. fur- 

 I'liracea, (A'lrarid oldiica. Usnea barbata and lidinalind [dsliaiald form 

 the dominant feature of the vegetation. In Iceland they do not ap- 

 pear to he of any importance, or are quite absent. The number of 

 the species is greater in Iceland, yet I cannot depend upon this not 

 bring due to insufficient investigation of the birches of Denmark. 



How the matter stands as regards "mass-occurrence" and "fre- 

 quency-number" in Iceland and Denmark, I am not prepared to 

 say, because, as I have already mentioned, I myself have not seen 

 lichen-bearing birches in Iceland. 



2. THE EARTH-LICHEN ASSOCIATIONS. 



In the previous pages we have made a survey of the general 

 biology of the earth lichens. Here we shall consider more closely 

 the special Icelandic conditions, viz. the characteristic qualities of 

 the Icelandic soil, and, finally, the lichen vegetation found in the 

 plant-associations. 



In a preceding part of this work Professor Thoroddsen has 

 given an exhaustive description of the Icelandic soil, and of its 

 geological and agricultural qualities. To this I refer the reader, and 

 it will suffice here merely to point out such features of it as are 

 of importance to the lichen-vegetation. 



As stated by Thoroddsen, the Icelandic soil consists entirely 

 of a finely divided mass, derived from the fundamental rock of the 

 island, or of the same chemical and minera logical composition as 

 the latter. In other words it is the Basalt, in grains of every 

 possible size, ranging from enormous blocks of rock to particles as 

 fine as dust, which constitutes the soil available to the lichens all 

 over Iceland. The liparite which occurs here and there is, ac- 



