The Botany of Iceland - 01.12.1914, Side 27

PHYSICAL GEOGRAPHY
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lies 84 metres below tlie level of the sea. Lagoon-lakes, situated
close to the coast, occur also, especially in North Iceland; the
largest of them are Hóp, Höfðavatn and Miklavatn. According to
the condition of the bars and tlie outlets the quality of the water
of the lakes may often change quickly; sometimes thejf contain
fresh water, sometimes brackish water, sometimes salt water. Tlie
fauna in these lakes is also subject to periodical changes. Some-
times marine animals immigrate through the outlets; at other times
they disappear, and a freshwater fauna becomes dominant.
Fishing in the Iceland lakes is of great economic importance
to the inhabitants. In the larger rivers and in many smaller streams,
salmon is caught, and now the fishing is often rented out to Eng-
lish sportsmen, especially in south-west Iceland. Many rivers, and
most of the lakes, abound in trout and salmon-trout (Salmo alpinus
and S. trutta) which play a great röle as a means of support of the
inhabitants. The quantity of living organisms varies greatly according
to circumstances; there appears to be most life in the shallow lakes,
especially in those with lava-bottoms, in which warm or luke-warm
springs are also sometimes found at the lake-bottom, around which
plant- and animal-life collect. Some Phytoplankton (Diatoms) occurs
in the lakes in South Iceland, but on the plateau and in North Ice-
land only Zooplankton (Daphnias) is found, and this is abundant.
In Thingvallavatn, Chara and Nitella grow at a depth of 15—30
metres; in Mvvatn there is a quantity of Nostoc which is found
thrown up in great masses along the sliores. In tlie lakes, there
occur in addition, several species of Limnœa and Pisidium, and
Lepidurus is frequently found in great quantities; there is, also, an
abundance of gnat-Iarvæ and other larvæ which serve as food for
the trout.
Geology. Iceland is almost entirely built up of volcanic rocks,
none of which appears however to be older than Tertiary times.
The foundation of the island is a depressed and broken basalt-
plateau similar to the other Tertiary basalt-plateaus on both sides
of the Atlantic Ocean, in East Greenland, the Færöes, Antrim in
Ireland and on the islands of Mull and Skye. It is therefore as-
sumed that in Tertiary times a continuous liasalt-land extended
across the Atlantic Ocean, that it subsided in Mid-Tertiarv times
and that Iceland and the Færöes are tlie remains of this land. In
Greenland and on Skye the basalt rests on .lurassic strata, in Mull