Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/289

 against and forming the gorge of the Nettlebridge River in steep mural precipices 50 feet high, and composed of boulders or fragments tons in weight.

No valley to the N.E. of Mells is cut down sufficiently deep through the overlying marls, Lias, and Oolites to expose the breccia ; but its presence at Wick, north of Bath, on the one hand, and at Mells, before named, on the other, clearly proves that the removal of the superincumbent secondary strata ranging from the New lied to the Great Oolite inclusive would reveal it.

The northern part of the Bristol coal-basin, or that portion ranging from Bristol, Holy Trinity, and Wick, on the south, to Cromhall and Tortworth, on the north, possesses on its eastern, northern, and western sides abundant evidence of this once continuous conglomerate within the basin ; but now, owing to the removal of all the overlying newer or mesozoic rocks from the centre of the coal-field, and the exposure of the Upper Coal-measures and Pennant sandstone at Coal-pit Heath, Tate, &c, its presence is only indicated by extensive and continuous patches resting on the high ridge of land occupied by Tytherington, Olveston, and Almondsbury, and on to the tableland around Henbury, Leigh Downs, and Clifton, all on the western side of the north basin. The accompanying diagram (fig. 2) exhibits the characteristic condition of the Breccia, and is a carefully prepared illustration of the well-known mass overhanging the river Avon on its right bank, and about 200 feet above the river ; this, with many others, shows the gradual passage into finer breccia and pebbles, and ultimately into the fine-grained sandstones which cover up the Clifton Downs west of the Observatory, and the table- land to the north. This isolated mass, with other evidence on both banks of the Avon, at and above the same level, clearly determines the age of the gorge of the Avon to be Post-Liassic in time, and shows that the river must have cut its downward or deepening channel through a large amount of superincumbent Secondary rocks ; for from the sea- level, at Portishead and Pill, &c, up to the height of the reptilian conglomerate on Durdham Down, we have a constant succession of these beds resting upon the older rocks of the river, and here and there clinging to the precipitous faces of Carboniferous Limestone or Old Bed Sandstone rocks that constitute the gorge of the tortuous Avon, and finally spreading themselves under the finer sandstone of the Keuper over the great mass of Carboniferous Limestone, &c, that constitutes the heights on the western side of the coal-basin, 300 feet above the sea.

Crossing the channel we find the same phenomena existing between Tidenham near Chepstow, and Pyle near Bridgend, to the west. Its northern range is defined and bounded by the southern outcrop of the Carboniferous Limestone and Coal-measures of the South-Welsh coal-field, where, as in Gloucestershire and Somersetshire, it rests in patches upon the older rocks, and, as on the eastern side of the Severn, is the source of the calamine, lead, and hydrated oxides of iron of the South- Wales area.