We come next in the descending order to the most ancient of the primary fossiliferous rocks, that series which comprises the greater part of the strata formerly called "transition" by Werner, for reasons explained in Chap. VIII., pp. [91] and [92.] Geologists have also applied to these older strata the general name of "grauwacké," by which the German miners designate a particular variety of sandstone, usually an aggregate of small fragments of quartz, flinty slate (or Lydian stone), and clay-slate cemented together by argillaceous matter. Far too much importance has been attached to this kind of rock, as if it belonged to a certain epoch in the earth's history, whereas a similar sandstone or grit is found sometimes in the Old Red, and in the Millstone Grit of the Coal, and sometimes in certain Cretaceous and even Eocene formations in the Alps.

The name of Silurian was first proposed by Sir Roderick Murchison, for a series of fossiliferous strata lying below the Old Red Sandstone, and occupying that part of Wales and some contiguous counties of England, which once constituted the kingdom of the Silures, a tribe of ancient Britons. The strata have been divided into Upper and Lower Silurian, and these again in the region alluded to admit of several well-marked subdivisions, all of them explained in the following table.

UPPER SILURIAN ROCKS.
Prevailing Lithological characters. Thickness in Feet. Organic Remains.
1. Ludlow
formation		 Tilestones. Finely laminated reddish and green sandstones and shales. 800? Marine mollusca of almost every order, the Brachiopoda most abundant. Serpula, Corals, Sauroid fish, Fuci.
Upper Ludlow. Micaceous grey sandstone. 2000
Aymestry limestone. Argillaceous limestone.
Lower Ludlow. Shale, with concretions of limestone.
2. Wenlock formation. Wenlock limestone. Concretionary limestone. 1800 Marine mollusca of various orders as before, Crustaceans of the Trilobite family.
Oldest bones of fish yet known.
Wenlock shale. Argillaceous shale.
LOWER SILURIAN ROCKS.
3. Caradoc formation. Caradoc sandstones. Flags of shelly limestone and sandstone, thick bedded white freestone. 2500 Crinoidea, Corals, Mollusca, chiefly Brachiopoda, Trilobites.
4. Llandeilo
formation.		 Llandeilo flags. Dark coloured calcareous flags. 1200 Mollusca, Trilobites.

UPPER SILURIAN ROCKS.

Ludlow formation.—This member of the Upper Silurian group, as will be seen by the above table, is of great thickness, and subdivided into four parts,—the Tilestone, the Upper and Lower Ludlow, and the intervening Aymestry limestone. Each of these may be distinguished near the town of Ludlow, and at other places in Shropshire and Herefordshire, by peculiar organic remains.

1. Tilestones.—This uppermost division was originally classed by Sir R. Murchison with the Old Red Sandstone, because they decompose into a red soil throughout the Silurian region. At the same time he regarded the tilestones as a transition group forming a passage from Silurian to Old Red. It is now ascertained that the fossils agree in great part specifically, and in general character entirely, with those of the succeeding formation.

2. Upper Ludlow.—The next division, called the Upper Ludlow, consists of grey calcareous sandstone, decomposing into soft mud, and contains, among other shells, the Lingula cornea, which is common to it and the lowest, or tilestone beds of the Old Red. But the Orthis orbicularis is peculiar to the Upper Ludlow, and very common; and the lowest or mudstone beds, are loaded for a thickness of 30 feet with Terebratula navicula ([fig. 410.]), in vast numbers. Among the cephalopodous mollusca occur the genera Bellerophon and Orthoceras, and among the crustacea the Homalonotus ([fig. 418.] [p. 354.]). A coral called Favosites polymorpha, Goldf. ([fig. 401.] [p. 346.]) is found both in this subdivision and in the Devonian system.

Fig. 409.