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36 I. PHYSICAL GEOGRAPHY OF SWEDEN.
from this that forests are of great importance as regulators of lakes, rivers, and
water-sources. This has also been directly proved by the observations and
calculations of 0. Appelberg, whose researches on this question may be summed up
as follows: cultivated land promotes inundations during rainy periods and want
of water during droughts; meadow ground gives protection from inundations but
increases want of water during droughts; wood land always increases the
quantity of water, but yields only half as much as cultivated land during a rainy
period, whereas during a drought it still renders enough to efficaciously prevent
want of water; lakes act as very efficacious regulators of the water level, by
storing up the abundance of rain falling during a wet period and then
delivering it slowly during a subsequent drought.
3. GEOLOGY.
Sweden, geologically viewed, is a very old country. By far the
largest part of its rocks were formed during the earliest geological
times — the so-called Primitive or Archaean age — and during the next
or Precambrian, anterior to the appearance of organic life.
Furthermore, almost the whole part of the area now called Sweden has been
raised above the sea during the immeasurably long ages which have
passed away since the Silurian strata were deposited. The only
exception to this condition is the southernmost part of the kingdom, the
province of Skåne, which was covered by the sea also during parts of the
comparatively låte Triassic, Jurassic, and Cretaceous periods.
Its continental character during the above-named long ages Sweden
had in common with Norway, Finland, and adjoining parts of Russia.
Within these lands the rocks are almost entirely of Archæan and
Precambrian age, while in most other European countries younger
formations, hundreds and thousands of meters in thickness, prove them
to have been submerged during the larger part of the time when
Sweden was dry land.
The geological formations found in Sweden, and the approximate
percentage of the land occupied by each formation, are as follows: The
Primitive or Archæan system 77-95 per cent.; the different groups of the
Precambrian 9"13 per cent.; the Cambrian and Silurian, unaltered 643
per cent., highly metamorphosed 5-58 per ct.; the Triassic Oo4 per ct.;
the Rhæto-Liassic 016 per ct.; the Cretaceous (Hi per ct. To be
mentioned are also the young beds of gravel, sand and clay etc., or
the Quaternary deposits, which generally form a more or less continuous
surfacecovering on the rock-basement.
Archæan. The rocks included in this system are: different kinds of
gneisses, granulitic gneisses, »hälleflintas», granites, porphyries, gabbros,
diorites, hyperites, and others.
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