10. The _Magnesian Limestone_, from 100 to 500 feet in thickness, forms a stripe of generally poor thin soil from Durham to Nottingham, capable of improvement as arable land by high farming, but bearing naturally a poor pasture, intermingled with sometimes magnificent furze. 11. The _Coal Measures_, from 300 to 3000 feet thick, consist of beds of sandstones and dark-blue shales (hard clays), intermingled (_interstratified_) with beds of coal. Where the sands come to the surface, the soil is thin, poor, hungry, sometimes almost worthless. The shales, on the other hand, produce stiff, wet, almost unmanageable clays;—not unworkable, yet expensive to work, and requiring draining, lime, skill, capital, and a zeal for improvement, to be applied to them, before they can be made to yield the remunerating crops of corn they are capable of producing. 12. To the _Millstone Grits_ of 600 feet or upwards in thickness the same remarks apply. They are often only a repetition of the sandstones and shales of the coal measures, forming in many cases soils still more worthless. When the sandstones prevail, large tracts lie naked, or bear a thin and stunted heath; where the shales abound, the naturally difficult soils of the coal shales again recur. These rocks are generally found on the outskirts of our coal-fields. 13. The _Mountain Limestone_, 800 to 1000 feet thick, is a hard blue limestone rock, separated here and there into distinct beds by layers of sandstones, of sandy slates, or of blue shales like those of the coal measures. The soil upon the limestone is generally thin, but produces a naturally sweet herbage. When the limestone and clay (shale) adjoin each other, arable land occurs, which is naturally productive of oats, yet, when the climate is favourable, capable of being converted into good wheat land. In the north of England a considerable tract of country is covered by these rocks, but in Ireland they form nearly the whole of the interior of the island. 14. The _Old Red Sandstone_ varies in thickness from 500 to 10,000 feet. It possesses many of the valuable agricultural qualities of the _new red_, consisting, like it, of red sandstones and marls, which crumble down into rich red soils. Such are the soils of Brecknock, Hereford, and part of Monmouth; of part of Berwick and Roxburgh; of Haddington and Lanark; of southern Perth; of either shore of the Moray Firth; and of the county of Sutherland. In Ireland, also, these rocks abound in Tyrone, Fermanagh, and Monaghan; in Waterford, in Mayo, and in Tipperary. In all these places, the soils they form are generally the best in their several neighbourhoods, though here and there,—where the sandstones are harder, more siliceous and impervious to water,—tracts, sometimes extensive, of heath and bog occur. III.—PRIMARY STRATA.