6. The water-sill, called also tuft, a very porous light-coloured sandstone, of a soft texture from the loose aggregation of the small grains which compose it.
The beds of sandstone are thickest towards the lower part of the series. Thus at Hely field the freestones are 7 fathoms thick, the most considerable of the grits at Shieldon about 11 fathoms, the great hazle at Dufton 10 fathoms, and three of the freestones in the section below that of Dufton 18, 26, and 30 fathoms respectively.
The limestone beds are the most characteristic of this formation, and are the most important to the miner. Of these there are 21 beds in the preceding sections of which the aggregate thickness is about 96 fathoms, that of the whole series being, as I have already mentioned, about 458 fathoms.
The most remarkable are 1, the great limestone, the 3d in the series, from 10 to 11 fathoms thick, consisting of three strata divided by indurated clay. The stone is a brownish black or dark bluish grey encrinal marble in which bivalve shells are imbedded. It bassets at Frosterley in Weardale, where large quantities of it are quarried for agricultural uses and building cement, or for ornamental purposes. It burns to a lime of a mild nature, highly valued as a manure, and contains according to Sir H. Davy 96 per cent. of carbonate of lime.
2. The scar limestone, the 7th in the series, 5 fathoms thick, resembling the great limestone both in its colour and organic remains, and like it divided into three strata. This rock crops out in the little river Nent, and forms the barrier at the cascade called Nent force.
The aqueduct level, carried on by the Commissioners of Greenwich Hospital, begins near Aldstone, and is driven at its commencement immediately below this bed of limestone: it is now two miles long from north to south.
