the highest stratum of Allenheads; and beds of this group appear on Kilhope Law at an elevation of 2206 feet, and on the top of Cross Fell, which is 2901 feet above the sea-level.
4. The Coal Measures which overlie the Millstone Grit, occupy, in Northumberland, the triangular area having the Tyne as a base line 14 miles in length; its eastern side is the coast, from the mouth of the Coquet to Tynemouth, and its western side a wavy line from the Coquet mouth to near Wylam on the Tyne. From this base line they extend into the central portion of the county of Durham; and by a fault called the Stublick Dike they are prolonged, in a narrow band up the valley of the Tyne to the extremity of the county of Northumberland, and into Cumberland, a distance of 27 miles. The whole length of this coal field is about 55 miles, having an area of about 700 square miles, marked by undulating ground of moderate elevation rising gently to the west: the highest hills northward of the Tyne are nearly 400 feet, but some in Durham attain to 740 feet above the sea-level.
In these Coal Measures there is almost a repetition of the same characters we find in the Carbonaceous group of the Mountain Limestone, with this exception, that while there were marine relics and a few thin limestones in the earlier period, limestones are altogether absent from the Coal Measures, and there are scarcely any indications of marine conditions. The sandstones, shales, and ironstones are similar in both; but in the more recent period the coals are thicker, richer, and more bituminous. From the variable thickness of the coal-seams in different parts of the field, and from some seams being split into two by the intervention of shales, of greater or less thickness, it is difficult to correlate the seams in distant parts of the field. There appear, however, to be in all fifty-seven seams, having an aggregate thickness of about 80 feet; but as several are thin, only twelve are workable, yielding about 50 feet of good coal. The Three-quarter Coal, which is of poor quality, crops out in the Ouseburn; the High Main, one of the best for domestic use, is 6½ feet thick, and crops out on the declivity of the hill leading to Denton
