These are followed by 1000 feet of dark
shales and flaggy sandstones, which are known as the "Tremadoc
slates," from their occurrence near Tremadoc in North Wales;
and these in turn are surmounted, apparently quite conformably,
by the basement beds of the Lower Silurian.
[Illustration: Fig 27. GENERALIZED SECTION OF THE CAMBRIAN ROCKS
IN WALES.]
The above may be regarded as giving a typical series of the Cambrian
Rocks in a typical locality; but strata of Cambrian age are known in
many other regions, of which it is only possible here to allude to
a few of the most important. In Scandinavia occurs a well-developed
series of Cambrian deposits, representing both the lower and
upper parts of the formation. In Bohemia, the Upper Cambrian, in
particular, is largely developed, and constitutes the so-called
"Primordial zone" of Barrande. Lastly, in North America, whilst the
Lower Cambrian is only imperfectly developed, or is represented by
the Huronian, the Upper Cambrian formation has a wide extension,
containing fossils similar in character to the analogous strata
in Europe, and known as the "Potsdam Sandstone." The subjoined
table shows the chief areas where Cambrian Rocks are developed,
and their general equivalency:
TABULAR VIEW OF THE CAMBRIAN FORMATION.
_Britain._ | _Europe._ | _America.
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