120 H. MØLHOLM HANSEN 
of 31, the H percentage is 53, the A percentage 44, the A 3 percen- 
tage 24, and the E (3—2—1) percentage 35. 
The Geranium belt represents the area where the characteristics 
peculiar to geiri are most striking. If we pass from the marginal 
zone through the Geranium belt to the bottom vegetation, or from 
a snow patch with a southern exposure to one with a northern 
exposure, the southern contingent is largest in the Geranium belt 
or the snow patch with the southern exposure, while it again de- 
creases in the bottom vegetation or on the northern slope. This 
is especially due to a decrease in the southernmost E subgroups. 
Looking for the cause of this change we find it in the unusu- 
ally long period during which the formations in question are covered 
with snow. In the case of the southern types of plants, which 
obtain the most favourable life-conditions in the Geranium belt (that 
is to say, the greatest possible protection from the winter cold and 
the most favourable temperature in the period of vegetation), the 
long-lasting snow-covering causes the vegetation period to become 
too short for these plants. 
The number and density of species which attain their highest 
values, 36 and 15 respectively, in the mo, have the values 32 and 
13 in the Geranium belt. This diminution is continued in the 
bottom vegetation, so that here the number of species is only 21 
and the density 10. 
If these changes are continued, with the successive diminution 
of the number of species and the quantity of southern species, as 
a final result we may anticipate to find, at still greater altitudes 
above the sea, the Salix herbacea and Sibbaldia formations described 
by Helgi Jonsson and at still higher levels the Anthelia-vegetation. 
At the level at which Arnarvatnsheidi is situated, in the lower 
zone of the mountain region, we have thus the following types of 
vegetation. 
I. Melar, conditioned by a slight snow-covering and a compara- 
tively strong desiccation of the soil. 
II. The Betula nana mo, where the snow-covering is deeper 
and the moisture of the soil greater: while the melar vegetation is 
peculiar to the denudation area of solifluction, the Betula nana mo 
is peculiar to the accumulation area. 
