STUDIES ON THE VEGETATION OF ICELAND 47 
The Mo Vegetation. Figs. 6—7, table 11 A—B. 
The mo is the type of vegetation which occupies the largest 
area of Lyngdalsheidi as well as in the rest of the Icelandic low- 
lands. The term mo as used here includes all such for- 
mations as are normally covered with snow in the win- 
ter, whose degree of moisture is exclusively determined 
by the precipitation, not by the ground water, the soil 
of which is not in motion, uncultivated, and not covered 
with forest or copsewood. Thus defined, mo comprises the 
following of H. Jonsson’s formations: heath, heather mo, grass mo, 
dwarf willow (in part), and grass-field (in part). 
The soil of the mo is always more or less covered with knolls 
as shown in fig. 6. According to the inclination of the surface some 
differences appear which do not, however, seem to be of great im- 
portance in their bearing on the vegetation. Where the surface is 
level or slightly inclined, the knolls are almost polygonal, half a 
metre high and broad, and separated from each other by narrow 
furrows. Where the soil is more inclined, the knolls grow smaller 
and arrange themselves in longitudinal rows parallel to the edge of 
the slope. The form of the knoll undergoes a change, not only in 
that it becomes more elongated, but also because it begins to move 
downwards. This occurs by a displacement of the material of the 
knoll itself, apparent by its bulging in the middle of the more or 
less vertical side facing the valley, and becoming flattened on the 
upper side. Sometimes the upper side is bare, devoid of vegetation. 
If all the knolls become flat and bare on the upper side, and out- 
wardly delimited by a vegetation curve, we get typical solifluction, 
which is especially well developed on melar in the highland tracts. 
If the slope becomes still steeper, the solifluction will assume 
the character of a landslip. Then it is no longer the single knolls 
but the substratum that slips, and in consequence the vegetable 
covering may be preserved intact. Such landslips were observed in 
the highest stage of development in the highlands and the north 
country. 
Since the mo forms the bulk of the vegetation as a feature of 
the landscape, it is obvious that forms transitional between the mo 
and the other types of vegetation must occur. In table 11, Nos. 1—5 
represent the typical mo, No.6 is a transitional form between mo 
and mosathembur, No. 7a transitional form between mo and jadar, 
