As regards their geographical distribution, the Cambrian
Rocks have been recognised in many parts of the world, but there
is some question as to the precise limits of the formation, and
we may consider that their most typical area is in 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_.
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