The Botany of Iceland - 01.12.1914, Side 80

264
THOKODDSEN
them to be as large as those at Ulfsvatn, viz. in East Iceland on
the plateau south-east of Snæfell, at a height of 690 metres above
sea-level. This was in 1894, but the inclemency of the weatlier pre-
vented me from making a halt at the place. Here, also, several of
the knolls had burst and discharged a large quantity of clay-soil.
This form of knoll occurs especially at the edge of the bogs where'
the foot of the knoll stands in water during the melting of the
snow: where the supply of water is too abundant these knolls do
not occur. It is a fact peculiar to all knolls, both in the lowlands
and on the plateau, that they consist of móhella-soil and humus
intermixed with clay; this is true also of the “rudemarks” with the
exception that more gravel and stones are found in them than in
the plant-covered ground. As mentioned above, tliere is no direct
connection between the mud-flow phenomena and the “rudemarks”
and knolls, buf it may happen that a flat witli either of these sur-
face-forms is afterwards affected by mud-flows; this is tlien a se-
condary phenomenon. Where subsoil-ice is absent, as for instance
in the neighbourhood of the hot springs whicli are scattered in
hundreds all over the countrjr, I do not remember to have seen
“rudemarks” or “thufur” proper. Upon the plateau itself “rude-
marks” are rather rare, but, on the other hand, various mud-flow
phenomena are common; thus, stones are often seen arranged in
bands on tlie slopes, and in various other ways which, however, I
regret to say, have not as yet been more closely investigated. On
basaltic plateaus covered with coarse basalt-gravel and blocks of rock,
patches of clay are now and then seen which appear, in the
middle, to have thrust up from below and spread out slightly on
all sides.
Hesides these knolls there are many otlier similar eminences
in the gravel-covered ground which are called knolls, but are due
to quite different causes and have a different origin. Sometimes
nodules on the greensward are due to the form of the substratum,
as where a thin layer ol’ soil occurs upon lava, or upon a stony
and rugged bottom. In blown sand, knolls are formed around roots
of Elymus arenarius, Halianthus peploides and other plants. Where
the ground-water stands very high, as in many extensive boggy
tracts, so that the entire surface freezes into an ice-cake, only small
knolls of organic material are seen, formed of moss and Carices.
On hard grassland, on gravelly flats and similar places, small