(2.) WALSALL AND BARR.--This is the typical area for the well known Barr Trilobite _Illenus Barriensis_. The quarries of the Woolhope Limestone which afford it are now disused, but a good section of the fossiliferous shales above is displayed in the railway cutting between Aldridge and Walsall. The overlying Wenlock or Dudley Limestones are mined at the town of Walsall itself, but good fossils are now comparatively rare. (3.) DUDLEY AND THE WREN’S NEST.--By far the most notable and interesting of the Silurian exposures are those of the neighbourhood of Dudley. In the three exposures of Dudley Castle, the Wren’s Nest, and Sedgley Hill, the Silurian limestone rises up in steep dome-like forms. This limestone, which is that of the Wenlock of Siluria, is here composed of two calcareous bands--the higher about 28 feet in thickness, and the lower about 42 feet--separated from each other by an intermediate zone of about 90 feet of gray shales. The limestone has been worked for centuries as a flux for the ironstones of the coalfield. The hills have been mined to a great depth, and all the best limestone rock extracted. The intervening and enveloping shales have been allowed to remain, and the present structure of the hills is that of a central dome surrounded by two enveloping shells, separated from each other by two more or less empty spaces. Where the dip of the rock is high, and these excavated parts are exposed, they form deep moat-like hollows, bounded by walls of shale. Where the dip is low, and the overhanging rocks are supported by the vast pillars left by the workmen, these excavations form magnificent caverns of peculiar weirdness and beauty. In the heart of the hill at greater depths they form damp gloomy chasms of enormous extent, which can only be seen to perfection when lit up by artificial light. The Dudley limestone bands and the surrounding calcareous shales have long been famous for the abundance and beauty of their included fossils. Many of the type species of Murchison’s Silurian System came from this locality; but since the superficial limestones have been worked out, good specimens are exceedingly rare. An excellent collection of the fossils of these beds is laid out in the Dudley Museum, and another in the Geological Museum of the Mason College. Many good collections are in the possession of private individuals in Dudley and elsewhere. The best localities for fossils at present are the shaly slopes on the flanks of the Wren’s Nest, where the usual Wenlock Brachiopods and Corals are abundant, but the beautiful Trilobites of the formation are but rarely met with. The Wenlock limestone occurs both in the Dudley Castle Hill and in the Wren’s Nest. The Aymestry limestone is only met with in a single locality on the flanks of Sedgley Hill, where it yields occasional specimens of its characteristic fossil, the _Pentamerus Knightii_. CARBONIFEROUS ROCKS.