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 CHAP. VI. them, so that the beds are vertical or even retroverted, and bent in anticlinal and synclinal axes. The Abberley Hills, Woolhope Forest and May Hill, are very interesting groups of Silurian strata.

Mineral Veins.—In the Shelve district of Shropshire, and at Nant y Moen, seven miles north of Llandovery, the lead mines are so related to the axis of irruption of the igneous rocks, as to leave no doubt of the propriety of classing them as an effect of the same volcanic excitement, not perhaps contemporaneous with the irruption of trap, but certainly and strictly associated with it, and dependent upon it. Sulphate of barytes, sulphuret of iron and carbonate of lime, accompany the ores of lead.

There is almost a total absence of proof, in the mineral composition and organic contents of the Silurian strata, of the contemporaneous existence of dry land: for all the early periods at least, the absence of land plants, and the infrequency of conglomerates, seem to justify a doubt whether the sea of that period was subject, in the regions now dried, to any thing of the nature of violent land flood, or great littoral agitation. Yet it is not only probable but proved by some instances in Wales, that the bed of the Silurian sea had been somewhat disturbed before the completion of the system. For, along considerable lengths of the boundary of the Caradoc sandstone, this littoral rock is found to rest unconformedly on previously disturbed rocks of the Llandeilo and earlier groups. There is, however, nothing to contradict the assumption that, till the close of the lower palæozoic period, nearly all the strata of the British Isles and the continent of Europe were covered by the sea in which they were formed: indeed,