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50
I. PHYSICAL GEOGRAPHY.
Greenstones appear in practically all areas of the Swedish Archæan. Among
the greenstones a special group of rocks petrographically related to
hypersthene-diabases is distinguished under the name of hyperites; they occur as a series
of stocks, sheets and dikes, intercalated in the gneisses of the large south-western
gneiss region all along its border towards the more variegated Archæan regions
of south-eastern and central Sweden. Several of the Swedish greenstones furnish
a fine material for tombstones and other monumental objects; in the trade the
name "black granite" is collectively used for such rocks. Superior black granite
is obtained from some of the deposits within the hyperite zone just mentioned,
but chiefly from certain other diposits of more typical diabase rocks, usually
distinguished as "bronzite-diabases", which form a system of dikes running along
the eastern side of the hyperite belt.
AlgOttkian Groups. In several districts there occur non-fossiliferous
sedimentary deposits, in some places of great thickness, resting discordantly
on the Archæan rocks but at the same time older than the oldest Cambrian
strata. Some of them (the Jotnian Group) have an obvious clastic structure
and consist mainly of red sandstones together with conglomerates and
slates; others (the Seve Group) to a large extent have a crystalline
structure and consist of sparagmites, quartzites and crystalline schists,
in some localities also of limestone.
The Jotnian sandstone occurs most extensively in north-west Dalarne (Dala
Sandstone), constituting a formation, about 800 m thick, of flat, horizontal strata
with two diabase beds intercalated, 50 and 80 m in thickness respectively. Other
areas of the same formation are found in the north-eastern part of Värmland,
in Ångermanland and Gästrikland. Presumably the Almesåkra Series in
Småland is of about the same age. To the Jotnian may also be reckoned the
Dais-land Series in Dalsland, a folded formation consisting of conglomerates, quartzites,
slates and greenstone-schists, nearly 1 900 m in thickness.
The Seve Group forms the youngest division of the Algonkian strata of Sweden
and is widely distributed within the Swedish fjeld-regions, forming there a zone
that has a width in places of 100 km; it extends with only a few interruptions
the whole length of the alpine territory along the Norwegian boundary-line, from
Dalarne to the mountains surrounding the western end of Lake Torne Träsk in
the most northerly part of Lappland.
The Cambrian and Silurian Deposits. Compared with the other
formations that are represented in Sweden, the Cambrian and Silurian
deposits stratigraphically form a connected whole, and hence it is now
usual to group them together as one geological system, for which
the name Silurian has been adopted. The Silurian System in a wider
sense, therefore, includes both Cambrian (Cambrium) and Silurian,
the latter subdivided into Lower Silurian (Ordovician) and Upper
Silurian (Gotlandian). These deposits probably originally covered the greater
part of Sweden, but now there only remain a few scattered remnants,
which owing to some cause or other have been protected from destruction.
During the last epoch of the Precambrian times a considerable part of Sweden
appears to have been land; the Cambrian, however, commenced with a
depression or subsidence of such an extension, that only some few isolated parts of
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