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42
I. PHYSICAL GEOGRAPHY OF SWEDEN.
turned and compressed by foldings to a mountain-range, which, though
not forming now any very prominent topographical feature, yet very
likely reached a considerable altitude before the upper portions of it,
in the course of time, was removed by denudation.
A series of conglomerates, sandstones, and slates named the
Visingsö-group, which now forms the bottom of lake Vettern, its islands, and
lower shores, is by some geologists regarded as being of Precambrian
age; by others it is supposed to belong to the Triassic.
A great part of our alpine range consists of a series of
mica-schists, quartzites etc. (the Seve-group), which are remarkably crumpled,
inverted, metamorphosed, and dislocated, and supposed to be of
Precambrian age.
During Precambrian and younger periods, repeated eruptions of
diabase took place. The molten röck spread, in the form of more or
less thick lava flows, on and between the strata of the sandstone spoken
of (for instance in the district of the Dala-sandstone) or filled up
fissures in the rock-basement, thus forming more or less vertical dykes.
The rocks of the Precambrian territories are in many places quarried
and made use of; thus, for instance, the Dala sandstone and Gefle
sandstone for building purposes, mill- and grind-stones, and the clayslate
of the Dalsland group for roofing.
Cambrian and Silurian deposits, consisting of sandstone,
alum-slate, various kinds of limestone, and clay-slate, are met with in several
of the provinces of Sweden. The Silurian district of Jemtland is the
greatest in area. It extends widely around lake Storsjön, and is
continued northwards by a broad belt of Silurian rocks extending through
Lappland and skirting the alpine highlands almost parallel with the
boundary between Sweden and Norway. Going south, another district
of the same character is found in southern Dalarne, encircling the
granite region between lakes Siljan, Orsasjön, and Oresjön. Deposits of
Cambrian sandstone and alumslate and of Silurian limestones and
clay-slates also occur west and south-west of lake Hjelmaren in Nerike, in
the district between Motala, Vadstena, Skeninge, and lake Roxen in
Östergötland, in Kinnekulle near lake Venern, in mount Billingen and
the neighbourhood of Sköfde and Falköping in Vestergötland, in
Halle-and Hunneberg — mountains east of Venersborg — in southern and
south-eastern Skåne, in Gotland and Öland, and on the mainland along
the western shore of Kalmar Sund, opposite the last-named island.
All these Cambrian-Silurian districts are only remnants of series of
strata — formerly more widely extended — deposited by the sea, which
during Cambrian and Silurian times covered the greater part of Sweden
and large areas of the adjoining countries.
The sandstone rests immediately on the Archæan rocks and is,
together with the black or dark slate (alum-slate) on top of it, considered
as belonging to the Cambrian system. The overlying grey clay-slates,
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