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In several parts of the world the Hypozoic Strata of gneiss and mica schist are succeeded immediately, or almost immediately, by the Silurian strata, as they were defined in 1836 by sir R. Murchison (Scandinavia, Bohemia, &c.), but in others, (Wales, Cumberland) the Llandeilo rocks, which were assumed as the original base of the Silurians, rest upon a great thickness of arenaceous and argillaceous deposits, among which clay slate is very abundant, and in which organic life is rare. Professor Sedgwick regarded these from 1832 to 1836 as the Cumbrian and lower part of his Cambrian systems; and it not having then been ascertained that the limestone of Bala, which both he and sir R. Murchison appear to have regarded as far below the Silurian strata, was really the same formation as the limestone of Llandeilo, a partial discordance of nomenclature arose, which is not yet well settled, and which cannot be entirely settled without a full discussion of first principles. Whether we have yet sufficiently investigated all the bearings of the case may be doubted; but these things at least are clear. If by the term "system" we mean to collect large groups of mineral deposits, with certain general physical relations, and certain general relations to life, then both in Wales and in Cumberland such a system intervenes between the characteristic Silurians of Murchison and the true hypozoic strata. But if, following views which have lately become prevalent, we fix our attention exclusively on the successive great groups of life, and regard the whole "upper and lower" Silurian series of Murchison as such a group, then certainly there is no other older group known to palæontology, and we may, in the words of an earlier edition of this treatise (1837), "consistently view the organic remains of the clay-slate and Silurian periods as belonging to one long succession of creative energy the first, if our views as to the origin of the gneiss and mica schist be correct,