╭ _Yoredale group_ of shales and grits passing │ down into dark shales and limestones. │ _Thick (Scaur or Main) limestone_ in the │ south and centre of England and Ireland, │ passing northwards into sandstones, │ shales and coals with limestones. Carboniferous ┤ _Lower limestone shale_ of the south and limestone series │ centre of England. The Calciferous │ sandstone group of Scotland (marine, │ estuarine, and terrestrial organisms) │ probably represents the Scaur limestone │ and lower limestone shale, and graduates │ downwards insensibly into the Upper ╰ Old Red Sandstone. The thick beds of mountain limestone, with their characteristic marine fossil shells and corals play an important part in English scenery. In Derbyshire, West Yorkshire, and other places, the limestone crags and hills are made up of the raised floor of a comparatively deep Carboniferous sea, which covered a considerable portion of the British Isles at the beginning of this epoch. [Sidenote: CARBONIFEROUS ROCKS.] The accumulation of the calcareous skeletons of marine animals, with masses of coral, veritable shell-banks of extinct oyster-like lamellibranchs, built up during the lapse of a long period of time, formed widespread deposits of calcareous sediments. These were eventually succeeded by less pure calcareous deposits, the sea became shallower, and land detritus found its way over an area formerly occupied by the clear waters of an open sea. The shallowing process was gradually continued, and the sea was by some means converted into a more confined fresh-water or brackish area, in which were laid down many hundred feet of coarse sandy sediments derived from the waste of granitic highlands. Finally the conditions became less constant; the continuous deposition of sandy detritus being interrupted by the more or less complete filling up of the area of sedimentation, and the formation of a land surface which supported a luxuriant vegetation, of which the débris was subsequently converted into beds of coal. By further subsidence the land was again submerged, and the forest-covered area became overspread with sands and muds.