The Botany of Iceland - 01.12.1914, Qupperneq 46

230
THORODDSEN
spouting, (2) intermittently spouting springs, (3) alternately spouting
springs, (4) constantly boiling springs which do not spout, (5) springs
with a high teinperature and a quiet surface or which boil quietly
in tlie middle. All the boiling springs deposit siliceous sinter. The
most famous hot spring is Geysir, in the vicinity of Haukadalur in
South Iceland, in the centre of a group of other boiling springs.
This group of springs was mentioned for the first time in 1294 and
has often undergone alteration bv earthquakes, especially in 1630
and 1789. Gej7sir’s eruptions now take place very irregularlv and
inany days may intervene between them. At the end of the 18th
and the beginning of the 19th century, Geysir’s activity was at its
maximum, and it could throw up fountains of water to a height of
50—60 metres, whereas now the water rarely rises above 30 metres.
The neighbouring hot spring Strokkur, began its activity in 1789
and ceased during the earthquakes of 1896. At first Strokkur threw
up higher fountains of water than did Geysir, not only boiling water
and steam, but also cold water. Other large groups of hot springs
are found near Reykir in Ölfus, in Reykholtsdalur, on Hveravellir
on the plateau NE. of Langjökull, and in many other places.
Sulphur springs occur in abundance in the volcanic districts,
but not outside the palagonite-formation; also alkaline springs are
common in the basalt districts. The alkaline springs are found upon
the level land, in valleys and upon mountain-sides where there is
much underground water, but the sulphur springs commonly occur
upon mountain ridges and other dry localities where the water has
an outlet through tlie underlying lava, etc. Several of the solfataras
deposit a considerable amount of sulphur in small heaps where
the sulphurous vapours rise from the soil. Sulphur from Iceland
had a commercial importance even in the 13th century, and the
trade in sulphur was especially Iucrative in the 16th century. Since
that time the export of it has gradually decreased and now has
entirely ceased. The sulphurous vapours which rise through the
clefts and cracks in the earth have a great effect upon the neigh-
bouring rocks, which are transformed and decomposed in various
ways — coloured clays, gypsum, iron-alum (Halotrichit), etc. being
formed. The mountains which have been penetrated by the hot
sulphurous vapours are easily recognizable at a long distance, owing
to their naked and discoloured appearance; they are alwavs light-
red, yellow and white in colour and are entirely destitute of plant-