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GEOLOGY.
51
the present-day Scandinavian Peninsula emerged above the surface of the sea.
In the sea-water an abundant animal life developed (acalephs, trilobites,
ortho-ceratites, graptolites, snails, crinoids and corals), and there were then deposited
the oldest fossiliferous strata, viz. those of the Silurian System. The rocks of
this system consist of sandstone with accompanying conglomerate and
alum-shales etc. (Cambrium), lime-stones, clay-shales, marl-shales and sandstone
(Ordo-vician and Gotlandian). All of these are considered normal in development, and
are called Silurian of easterly or normal fades, in opposition to the schists etc.
of crystalline development that are met with in the western parts of the
fjeld-regions (and which are called westerly fades).
Of the Silurian areas with normal fades the Jämtland one is the largest.
In Central and Southern Sweden there are the following Silurian areas: one in
Dalarne, one in Närke, one in Östergötland and one in Västergötland embracing
the region between Skövde and Falköping and including also Billingen,
Mösseberg. Kinnekulle, and the twin hills Halleberg and Hunneberg; furthermore the
Silurian area in Skåne should be mentioned, and the fact that the whole of
Öland, the Småland coast facing that island and the whole of Gottland are also
Silurian.
Mt. Kinnekulle, Lake Vänern, as seen from Lidköping.
The strata of the Silurian System at Kinnekulle are perhaps the most
typical and perfectly developed; moreover they are the most easily accessible
and lend themselves therefore aptly for study. The following are the principal
strata., enumerated from below upwards: On the Archæan, consisting of a. gneiss,
there lie, in order, Cambrian: b. Sandstone, grey or yellowish, about 34 m in
thickness; c. Alum-shales (black clay-shales containing layers and concretions
of bituminous Limestone), 22 m. — Ordovidan, Lower Silurian: d. Ceratopyge
limestone and Lower Graptolite shales, 4 m; e. Orthoceras limestone,
grey and red, 5.3 m; f. Chasmops limestone (grey limestone and shales)
10 m; g. Trinucleus shales (red., green and black clay-shales) 28 m, with a
covering layer, 5 m, of Brachiopod shales (impure limestone and calcareous
shales, grey and green-grey). — Gotlandian, Upper Silurian: h. Upper
Graptolite shales (dark clay-shales). The summit of the hill consists of diabase,
forming a bed 30 m in thickness above the Upper Graptolite shales.
The other Västgöta Hills also have a covering of diabase, which owing to its
hardness has protected the Silurian strata beneath from destruction. The Silurian
areas in Dalarne, Närke, Östergötland, and Skåne, on the other hand, have been
preserved down to the present time by dislocations at or after the close of the
Silurian Period, when they were thrown down to a level beneath that of their
surroundings, whereby they were rendered less exposed to those agents that
wear away and erode exposed rocks.
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