3. The Millstone Grit, which underlies the Mountain Limestone, can scarcely be ranked as a distinct formation, for it differs but little from the Coal Measures, excepting in its feeble development of coal. It is composed of sandstones and shales similar to those in the Coal Measures, and like them, too, it has no limestones or calcareous beds, and but few and doubtful indications of any marine conditions. In this group we include all the beds, from the top of the highest limestone, with marine fossils, to the base of the Brockwell Coal, the lowest workable seam in the Coal Measures. The thickness in Northumberland is about 500 feet; and this corresponds pretty nearly with Forster's section of the formation southward of the Stublick Dike. The characteristic beds are thick gritty sandstones, which sometimes have supplied millstones, and hence the name Millstone Grit. Such beds are made up chiefly of rounded pebbles of quartz and felspar bound together by a siliceous cement : some pebbles in the rock are as large nuts, and some few an inch in diameter. At Warkworth this rock, in some parts, is bound by a calcareous cement, and here and there appear grains of protoxide of iron and garnets. In borings made at Shortridge, two coal seams, each 6 inches thick, were passed through. The proportion of siliceous rocks to the argillaceous in this group is about six to four. Sigillaria, Stigmaria, Favularia, and other Carboniferous plants, occur in the sandstones; and at Berling Carr there are tracks and casts of annelids in slaty sandstones.
These beds in Northumberland range south-south westwards in a narrow zone, from 2 to about 5 miles wide, in the same direction as the Mountain Limestone, on which they rest conformably, from near the mouth of the Aln to the Tyne; but beyond the Tyne they are, through the influence of the Stublick Dike, deflected westward, parallel with that dike, to the borders of the county. Some of the high fell lands of Northumberland south of the Stublick Dike and of the western parts of Durham, are capped by Millstone Grit. North of the Tyne it reaches an altitude of only 460 feet ; but in this southern district the Grey Millstone appears on the- mountains between Wolsingham and Stanhope. The Grindstone Sill, another characteristic bed, is
