Burn; other good coals, such as the Bensham, Beaumont, and Brockley, are worked in collieries west of Newcastle. Several seams are now worked in the neighbourhood of Sunderland, where the overlying Magnesian Limestone has been pierced to reach the Carboniferous beds: one pit there is 1800 feet deep.
Though some of the posts or sandstones are thick, one being 84 feet, yet the proportion of shales is greater in the Coal Measures than in the Mountain Limestone. Assuming the total thickness of these measures to be 2000 feet, then we have 80 feet of coal, 960 feet of sandstone, and 960 feet of shale; and from this large amount of argillaceous matter, the soil of the Coal Measures is more moist and clayey than that over either the Millstone Grit or the Mountain Limestone, and the scenery, too, is of a tamer character, with fewer bold cliffs and high hills. The low portions of the undulating ground are mostly valleys of denudation; for where the thick argillaceous strata have cropped out the soft material has been swept away, and the surface hollowed by denuding agencies, leaving the harder sandstones as low rounded hills.
No undoubted marine organisms appear in these Coal Measures, excepting one Brachiopod (Lingula mytiloides), which was found by Mr. Kirkby in a bed of shale, at Ryhope colliery, 680 feet below the Magnesian Limestone; but this genus seems to have had the capacity of living in brackish water, probably in an estuary, for we find it also in the Mountain Limestone associated with coal seams, along with Anthrocosia and fish remains. Of fishes several species have been found in different zones, such as {{taxon|Gyracantlius formosus and tubereulatus, Megalichys Hibberti, Diplodus gibbosus, Ctenoptychius pectinatus and dentieulatus, and species of Palceoniscus, Platysomus, Ithizodvs, Aniblypterus, Orthacanthus, Leptacanthus, &c. The successful researches of Mr. Kirkby and of Mr. Atthey have extended our knowledge of the Ichthyology of the Coal Measures. From sections of jaws and teeth, prepared by Mr. Craggs, and found in the shales and coal of the Low Main Seam, Professor Owen has recently described eleven new genera of fishes about the size of the minnow, one of which is of the Sauroid type; and associated with them
