The volcanic intercalations which diversify the Lower Carboniferous formations of Southern Scotland extend but a short way across the English Border, and although, over the moors and hills of the north of Cumberland and Northumberland, the Carboniferous sandstones, limestones and shales are well exposed, they present no continuation of either the plateau or puy-eruptions which play so prominent a part in the geology of Roxburghshire and Dumfriesshire. This deficiency is all the more noticeable seeing that the Carboniferous system is exposed down to its very base, in the deep dales of the North of England. Had any truly interstratified volcanic material existed in the system there, it could hardly fail to have been detected. But while contemporaneous volcanic rocks are absent, the northern English counties contain many intrusive masses of dolerite, diabase, andesite or other eruptive rocks, which may be found traversing all the subdivisions of the Carboniferous system. These eruptive materials have taken two forms: in some cases they rise as Dykes, in others they appear as Sills. Dykes.--With regard to the dykes, some are probably much later than the Carboniferous period, and consequently will be more appropriately considered in Chapters xxxiv. and xxxv. The great Cleveland dyke, for example, which runs across the Carboniferous, Permian, Triassic and Jurassic formations, is probably referable to the Older Tertiary volcanic period. One dyke known as the Hett Dyke, has been plausibly claimed as possibly of Carboniferous age. It runs in a W.S.W. direction from the Magnesian Limestone escarpment at Quarrington Hill, a few miles to the east of Durham, through the great Coal-field, across the Millstone Grit and Carboniferous Limestone, disappearing near Middleton in Teesdale. Its total length is thus about 23 miles. It varies in breadth from about 6 to about 15 feet, and appears to increase in dimensions as it goes westward.[1] [Footnote 1: Sedgwick, _Trans. Geol. Soc._ 2nd series, iii. part 1 (1826-28), p. 63; _Trans. Cambridge Phil. Soc._ ii. (1822), p. 21. Sir J. Lowthian Bell, _Proc. Roy. Soc._ xxiii. (1875), p. 543.] The age of this dyke cannot at present be satisfactorily fixed. It must be later than the Coal-measures through which it rises. Sedgwick long ago pointed out that though it reaches the escarpment of the Magnesian Limestone, it does not cut it; yet it is found in coal-mining to traverse the Coal-measures underlying the Limestone. He was accordingly inclined to believe it to be of older date than the Magnesian Limestone. At its western extremity it approaches close to the Great Whin Sill of Teesdale, though no absolute connection between the two has been established. Mr. Teall, however, has called attention to the similarity between the microscopic structure of the rock forming the Hett Dyke and that of the mass of the Whin Sill, and he is strongly inclined to regard them as belonging to the same period of intrusion.[2]