LICHKNOLOGY OF ICELAND 175 



cording to Thoroddsen, oi' 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 essentially 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 ani- 

 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 role is played by the frozen ground, owing to the fact 



