Northumberland and Durham. 65 



6. The watcr-si/I, called also /////, 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 1 1 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. 



Vol. iv. i 



