LICHENOLOGY OF ICELAND 175 
cording to Thoroddsen, of no importance as far as soil-formation 
is concerned. 
Consequently, whether the soil is of the one or the other geo- 
logical origin — glacial soil, or soil deposited in water, or deposited 
by wind, (aeolian deposits) — its chemical or mineralogical com- 
position is essentiaily the same in all cases. 
The circumstances which are of importance, regarding the soil 
as a lichen-substratum are therefore essentially the following: — 
(1) The chemical composition (mineral earth or earth rich in 
humus), (2) the size of the grains, (3) thermal conditions, 
(4) the water-contents, (5) drifting soil, (6) burrowing ant 
mals, (7) leaf-fall, (8) and the snow-covering. 
To these must be added, what is perhaps the most important, 
(9) competitive relations with other plants. 
(1) The chemical composition of the loose soil is, as a 
whole, somewhat different in Iceland from that in Denmark, as was 
first pointed out by P. Feilberg (see Thoroddsen, in vol. I, 
p. 252, of the present work). With regard to the amount of nutri- 
tion present, the difference is doubtless of very little consequence 
as regards lichens. On the other hand, it is indirectly a highly im- 
portant fact, that the great amount of iron-salts and humus cha- 
racteristic of the soil of Iceland, conditions a plant-growth which, 
taken as a whole, is very widely different from that of Denmark, 
and causes a competition among the plant-species which is highly 
conducive to the wide distribution of lichens all over Iceland. 
(2) The size of the grains (fineness, respective coarseness) of 
the soil is, as mentioned above, hardly of any direct importance, 
but no doubt of indirect importance by being the means of bringing 
about various conditions of heat and moisture in the finer and 
coarser kinds of soil. 
(3) The thermal conditions are far more unfavourable in 
Iceland than in Denmark, far greater tracts of ground being frozen, 
during a greater part of the year. As long as the upper soil-layers 
are frozen, the plant-covering also will frequently be thoroughly 
chilled, and the lichens will therefore lie dormant. On the other 
hand, it hardly has a direct influence upon the lichens if the 
ground is frozen farther down, as they are attached to the 
ground only very superficially, frequently only a few millimetre at 
the uppermost part near the surface. Quite another and far greater 
but indirect rôle is played by the frozen ground, owing to the fact 
