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Composition.—The rocks of the Silurian system, as it is exhibited in the country first surveyed by Murchison (the whole Welsh border and large tracts in South Wales) may be said to contain types of the usual sedimentary aggregates—argillaceous, arenaceous, calcareous; nor is there any very clear or exact definition by which they can be discriminated in the cabinet, as a mass or individually, though in the field they are easily and accurately traceable, for limited ranges of country. Compared with the older strata, the argillaceous rocks, in general less indurated, less complicated by divisional planes, and only locally endowed with cleavage, retain their original lamination: the arenaceous rocks deviate from the character of grauwacke, toward ordinary sandstone and conglomerate; the calcareous rocks are not usually so crystalline as in gneiss, nor of so earthy a substance as many of the later secondary limestones, but have a concretionary sub-crystalline texture.

Examined in detail, however, considerable variations appear among the different members of the Silurian system: some of the argillaceous beds are black, others of a liver or grey colour: some arenaceous beds fine grained, and argillaceous (Ludlow) were aptly named by Mr. Murchison "mudstone": others are like common hard gritstone (in the Caradoc): some appear to be principally composed of volcanic ashes, or the disintegrated particles of trap rocks, and are called "volcanic sandstones" (Malvern hills, the Caradoc, &c.). Some of the limestones (Llandeilo) are broad, flaggy, and argillaceous, as usual in shale; others (Aymestry, Wenlock) are purer, more concretionary, and more analogous to the calcareous rocks of the carboniferous system above.

Structure.—In general the accumulation of these rocks appears to have been regular and tranquil; the