216 OLAF GALLOE 
The heaths of Iceland and Denmark, regarded from the point 
of view of a landscape, resemble each other as regards their whole 
physiognomical feature, besides which there is a great similiarity 
as regards the frequency-degree of the lichens (as far as this 
can be decided by a rough estimate). The difference as regards 
mass-occurrence (stated in weight) is not known (but at a rough 
estimate it would not seem to be great; the mass-occurrence is 
greatest perhaps on the Danish heaths). With respect to growth- 
forms the similiarity also appears to be rather great, but it will 
probably, on a closer investigation, be lessened by the fact that more 
crustaceous lichens will be found on the heaths of Iceland, than on 
those of Denmark. The systematic species of the two countries 
differ by no means slightly from each other. 
It may therefore be said generally, that the conception “heath,” 
as we know it from Denmark, does not undergo any great funda- 
mental change through a closer investigation of the Icelandic heaths. 
After having thus dwelt upon the appearance of the lichen- 
vegetation, it now remains for us briefly to point out the conditions 
which the lichens find on the heath and which have a determining 
influence as regards whether they thrive or do not. 
The following are the most essential: — 
(1) Nowhere on the heaths did I observe, that the chemical 
composition of the soil had a detrimental influence on the lichen- 
vegetation — but in other localities, for instance near Solfataras, 
etc., the conditions were very unfavourable to them. 
(2) Thermal conditions and water contents are so closely con- 
nected with each other, that it is usually difficult to separate them. 
Damp, cold soil is generally unfavourable to many lichens (compare 
Bogs), whilst desiccation is not detrimental to them in a climate 
where the precipitation is as great as it is in Iceland. The greatest 
degree of moisture which permits the growth of heath-vegetation 
(i. e. F °/o 100 chamæphytes) is however also favourable to lichens 
(mountain-heaths at higher altitudes). 
(3) Loose, drifting soil frequently bears heath-vegetation, when 
the soil does not drift very greatly. But such heaths are devoid 
of lichens. 
(4) Leaf-fall, which covers the lichens, does not hamper them 
greatly on the heaths; luxuriant dwarf-birch growths and in some 
degree a few other larger species may, however, by this means 
prevent the appearance of lichens. 
