198 OLAF GALLOE 
decimetres (dm?) with intermediate spaces of equal size, viz. about 
1 metre. 
The majority of the heaths, regarded as landscapes, are easily 
recognizable in Iceland by their greenish-brown tone of colour, 
which makes them conspicuous even at a fairly long distance. 
They occur on mountain-sides up to a height of about 400 metres. 
It is stated that on slopes (mountain-sides) the ground and hence 
also the plant-covering is flat, whilst they are knolly and uneven 
on a horizontal substratum. These features agree exactly with my 
observations. 
In the following I shall give some examples of the more fre- 
quent facies of the heath and their lichen-vegetation. 
Type I. Dry heaths on level (not knolly) ground. 
(a) Heaths rich in phanerogams but either poor in or devoid 
of lichens. 
Near Häls parsonage in Fnjoskädalur (North Iceland) I noted 
down that there occur extensive heaths the character plants of 
which are Empeirum, Betula nana and various Glumiflore, mostly 
grasses. Each of these occur in all the sample-areas, 1. e. they have 
the frequency-percentage 100. The ground, which is gently sloping, 
consists of fine, reddish sand, and is covered by a continuous carpet 
of the above-mentioned character-plants and by a few others which 
have a lesser frequency-degree, e. g. Dryas, Silene acaulis, etc. 
Both the open ground and the birch-clusters are de 
void of lichens. 
The reason of this phenomenon merits fuller discussion. As 
mentioned above, we can, on the whole, point out eight essential 
factors which determine the presence or absence of earth-lichens in 
a particular association, viz. the chemical composition of the soil, 
the size of its grains, thermal conditions, water-contents, drifting 
soil, burrowing animals, a layer of decaying leaves, snow-covering, 
and competitive relations with other plants.. Among these eight 
factors we must consider more fully the layer of decaying leaves 
and the competitive relations with neighbours. It is impossible to 
believe that all the other factors mentioned above, could have an 
injurious influence on a lichen-covering on the heath-areas in question. 
But the two powerfully acting factors just mentioned are without 
doubt instrumental in the existing want of lichens. The fact is, 
that dwarf-birches, where they form dense growths, are fairly high 
in growth, cast rather a deep shade, and shed a considerable number 
