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52
I. PHYSICAL GEOGRAPHY.
Iii the Jämtland Silurian area all the three main divisions of the System are
represented. The Silurian along the base of the mountain range in Lappland
consists of quartzites, sandstones and of slates, all of them practically without
fossils.
The Silurian of Dalarne lacks the Cambrian strata; on the Archæan rocks
there rests directly a calcareous conglomerate (Obolus conglomerate); on the top
of that there comes first Orthoceras limestone and then the other Silurian strata,
of which there may be mentioned the Leptæna limestone and the Orsa
Sandstone, strata that do not occur in the other Silurian areas.
The Silurian of Närke and Östergötland presents mainly the same sequence
of strata as that of Västergötland, but recent disintegration is more noticeable.
In the Skåne Silurian the great thickness of the Upper Silurian shales is very
remarkable, according to calculation as much as 1 000 m. These shales, more
especially light-grey, marly clay-shales (Colonus shales or Cardiola shales), are
the ones that most frequently lie exposed in the Skåne Silurian area. The
youngest of the Upper Silurian deposits here are a series of sandstone,
limestone and marl shales similar to those of Gottland.
Lake Viioern
Section of Mt. Kinnekulle.
From West to East. Scale of height five time^as great as that of length.
In the island of üland the exposed rocks are almost exclusively orthoceras
limestone; owing, however, to the strata having a slight slope to the east, the
underlying thick layers of alum and other Cambrian shales only crop out along
a portion of the west coast of the island. The underlying Cambrian sandstone
is in its turn visible on the opposite Småland shore, across Kalmar Sound.
Gottland consists entirely of Upper Silurian strata, Gotlandian, viz. marl-shales,
sandstone (Bursvik sandstone) and oolite (lower division), on the one hand, and
limestones that have been formed by the accumulation of fossil remains, on the
other (upper division); to a large extent they are remnants of ancient coral-reefs.
Silurian with westerly fades is found furthest south in a district in the
westernmost part of Jämtland, and occurs further north, running in an almost
unbroken zone of some 20 to 50 kilometers in breadth from Frostviken, south, to
Lake Torne Träsk, north, skirting the Norwegian frontier.
Contrary to the state of things in the normally developed Silurian areas,
where the original horizontal position of the strata has been preserved almost
intact, the western Silurian strata, as well as those of the Seve Group, have been
uplifted, bent, and crumpled in the folding processes that produced the
Scandinavian mountain range. The sections here often exhibit sequences that are
very difficult to interpret. Törnebohm explained the abnormal sequences in the
fjeld-regions by an "overthrust" theory, to the effect that while the folding of
the mountain range was proceeding, Archæan and Algonkian on a large scale
were overthrusted over the Silurian strata. The limit of this abnormal
overlapping is marked on the map, p. 47, by a thick black line.
After gradually rising above the surface of the sea that covered it during
the Silurian Period, Sweden retained its character of continent throughout
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