174 OLAF GALL@E 
Pertusaria xanthostoma. Lecidea enteroleuca (with var. 
Urceolaria sophodes v. orbata. Laureri). 
= exigua. Gyalecta Beckhausii. 
Lecidea vernalis. Buellia myriocarpa. 
— . erythrophza. Arthonia proximella. 
—  Berengeriana. Sagedia analepta. 
— , Tornoénsis. —  grisea. 
==" „lusceseens. —  kentrospora. 
= Nylanderi. Pyrenula micula. 
If we compare the birches of Iceland and Denmark with re- 
ference to their lichen-vegetation, a characteristic difference will be 
seen as regards the species. In Denmark Evernia Prunastri, E. fur- 
furacea, Cetraria glauca, Usnea barbata and Ramalina fastigiata form 
the dominant feature of the vegetation. In Iceland they do not ap- 
pear to be of any importance, or are quite absent. The number of 
the species is greater in Iceland, yet I cannot depend upon this not 
being 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. j 
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. 
Ås 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 mineralogical 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- 
