Subdivisions:—
UPPER SILURIAN.
| 1. | Ludlow rocks;—slightly micaceous grey-coloured sandstones. Blue and grey argillaceous limestones. Dark-coloured shales and flagstones, with concretions of earthy limestone, containing marine shells, Orthocerata, Spirifers, and Trilobites. Fishes. |
| 2. | Wenlock, or Dudley limestone;—sub-crystalline blue and grey limestone, abounding in Trilobites, Crinoidea, Polyparia, Spirifers, Orthocerata, &c. |
| 3. | Wenlock shale;—dark grey argillaceous shale, with nodules of sandstone. |
LOWER SILURIAN.
| 1. | Caradoc sandstone;—shelly limestones, and finely laminated, slightly micaceous, greenish sandstones. Corals, Shells, Trilobites. |
| 2. | Llandeilo flags and limestones. Freestone, conglomerates, grits, and limestones. Dark-coloured flags. Beds of schist with abundance of Trilobites and shells. The lowermost beds are full of small bivalves, termed Lingulæ. |
Obs.—The Silurian System, (so named by Sir R. Murchison, from the Silures, the ancient Britons who inhabited those parts of our island in which the geological relations of these strata were first recognised by him,) occupies the border counties of England and Wales, and spreads over a vast area of both North and South Wales, intervening between the Carboniferous series and the Cambrian or ancient slate-rocks of that country.[23] The strata are entirely of marine origin, and many of the beds (as the well-known Dudley or Wenlock limestone) are composed of shells, corals, crinoideans, and remains of that remarkable family of crustaceans termed Trilobites, cemented together by carbonate of lime. A few remains of Fishes occur: Reptiles are unknown. No vegetable relics, excepting Fuci, have been found in Britain below the Devonian or Old Red formation.[24]
[23] "The Silurian System, founded on Geological Researches in the Border Counties of England and Wales." In two parts, royal 4to., with map, sections, &c., by Sir R. I. Murchison, G.C. St.S. &c. In studying the beautiful map which accompanies the work, it must be borne in mind that ten years have elapsed since Sir R. Murchison abrogated the boundary line that separates the Cambrian and Silurian rocks in this chart, from the conviction that those deposits constitute but one natural system (see Wond. p. 803). For an account of the Silurian rocks of other countries, see "Geology of Russia," by the same Author. A summary of the characters of the Silurian System, by Sir R. Murchison, is given in Geolog. Journal, vol. viii. pp. 173-183.
[24] "The Silurian System" contains excellent figures of all the organic remains known at the period of its publication.
The Cambrian Formation. This term is applied to a largely developed series of unfossiliferous slate-rocks and conglomerates, many thousand yards in thickness.