n South Wales,
where they have been carefully worked out, chiefly by Dr Henry
Hicks. In this region, in the neighbourhood of the promontory
of St David's, the Cambrian Rocks are largely developed, resting
upon an ancient ridge of Pre-Cambrian (Laurentian?) strata, and
overlaid by the lowest beds of the Lower Silurian. The subjoined
sketch-section (fig. 27) exhibits in a general manner the succession
of strata in this locality.

From this section it will be seen that the Cambrian Rocks in
Wales are divided in the first place into a lower and an upper
group. The _Lower Cambrian_ is constituted at the base by a great
series of grits, sandstones, conglomerates, and slates, which
are known as the "Longmynd group," from their vast development
in the Longmynd Hills in Shropshire, and which attain in North
Wales a thickness of 8000 feet or more. The Longmynd beds are
succeeded by the so-called "Menevian group," a series of sandstones,
flags, and grits, about 600 feet in thickness, and containing
a considerable number of fossils. The _Upper Cambrian_ series
consists in its lower portion of nearly 5000 feet of strata,
principally shaly and slaty, which are known as the "Lingula
Flags," from the great abundance in them of a shell referable
to the genus _Lingula_. 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 R