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 Rocks are developed, and their general equivalency: 

TABULAR VIEW OF THE CAMBRIAN FORMATION.

Britain.    |      Europe.      | America.
|                     |
a.  Tremadoc Slates. | a.  Primordial zone  | a.  Potsdam
|                     | of Bohemia.         |   Sandstone.
| b.  Lingula Flags.   | b.  Paradoxides      | b.  Acadian
Upper  <                      | Schists, Olenus     |   group of New
Cambrian. |                     | Schists, and        |   Brunswick.
|                     | Dictyonema schists  |
\                    | of Sweden.          |
|                     |
a.  Longmynd Beds.   | a.  Fucoidal         |   Huronian
|                     | Sandstone of Sweden |  Formation?
| b.  Llanberis Slates.| b. Eophyton       |
|                     | Sandstone of Sweden.|
Lower  <  c.  Harlech Grits.   |                     |
Cambrian. | d. Oldhamia       |                     |