BRITISH LIMESTONES. 197 The Carboniferous limestone of Denbighshire and Flintshire is similar in appearance and composition to that of Derbyshire ; its thickness varies from 1000 to 1500 feet, and it is extensively quarried for lime and cement. In the South-west of England it forms an encircling zone around the Somersetshire coal-basin, and rises into the table-land of the Mendip Hills. It also forms a similar zone round the margins of the South Wales and Forest of Dean coal-fields ; and from its tendency to form mural cliffs and terraced escarp ments, it produces, along the valleys of the Wye and the Avon, features at once hold and beautiful.