142 H. MØLHOLM HANSEN 
Under the treatment of the formations in the various localities 
examined, the causes of the differences in vegetation have been more 
precisely stated. At Bjork in the south country the difference be- 
tween the Elyna mo and the Arctostaphylos mo was caused, amongst 
other things, by a difference in the depth of the snow-covering. 
The Elyna mo has a relatively thin covering of snow, that of the 
Arctostaphylos mo is somewhat deeper. However, the difference is 
not considerable enough to cause the appearance of two different 
types of vegegation. At Nordtunga in the south-west country mo 
and forest-ground are covered by snow of different depths; while 
the mo has the snow-covering normally occurring in that part of 
the country, the forest-ground is covered by a considerable layer of 
snow throughout the winter. At Lækjamôt in the north country, 
the mo has likewise the normal snow-covering, whereas the melar 
is bare of snow. All these localities are lowland localities situated 
at c. 50—100 m above sea-level. If we pass from the lowlands to 
the highlands, the difference in the scale of snow-covering will be- 
come increasingly evident. At Lyngdalur in the south country 
(c. 250 m above sea-level), three types of vegetation are easily 
distinguished. The difference between these three types, mosathembur 
and melar with little or no snow-covering, mo with a normal snow- 
covering, and geiri with a deep and constant snow-covering, is due 
to the difference in the snow-covering. In some places a transitional 
form between mo and mosathembur had developed, with an inter- 
mediate depth of snow. 
At Thrasaborgir (c. 400 m above sea-level) the three types 
mosathembur, mo and geiri were likewise devoloped. 
On Arnarvatnsheidi near Ulfsvatn at an altitude of c. 500 m 
above sea-level, the scale of snow-covering was further differentiated. 
The 5 types of vegetation, melar, Betula-nana mo, the knolly mo, 
the sides of the snow patches, and the bottoms and north sides of 
the snow patches, represent 5 different degrees of snow-covering, 
where the first type has the slightest snow-covering of the shortest 
duration, the last, the deepest snow-covering of the longest duration, 
Table 29 gives the distribution of the species in the scales of 
snow-covering of the 6 localities mentioned above. Within each 
locality the formation most devoid of snow is given first, furthest 
to the left, while the formation with the deepest snow-covering is 
put last, furthest to the right. Bjork a is the Elyna mo, Bjork b 
