[Footnote 124: Only one instance is known where in Staffordshire any igneous rock has been intruded into rocks younger than the Coal-measures (Allport, _Quart. Journ. Geol. Soc._ vol. xxx. p. 551; Sheet 72 S. W. of the Geological Survey, and Horizontal Sections, Sheet 57). It forms a dyke which has been traced near Norton Bridge, Swinnerton and Butterton, running for 8 miles in a N.N.W. direction, and rising through Permian, Bunter and Keuper strata. It is a highly basic olivine-basalt, and is unquestionably a dyke. Mr. J. Kirkby, who has recently mapped and described it (_Trans. North Staffordshire Naturalists' Field-Club_, xxviii. (1894), p. 129), suggests that it may be connected with the igneous rocks of the South Staffordshire coal-field. But of this idea there is no evidence. The last point to which the dyke has been traced is some five-and-twenty miles from the nearest known portion of the dolerites of the coal-field. I have little doubt that this dyke is really an outlying member of the great system of Tertiary dykes described in Book VIII. of the present work.] No trace of vents has been met with in the Coal-measures of the Midland district or among the surrounding older rocks, nor any proof that the abundant sills and veins were connected with the eruption of volcanic materials at the surface. Nevertheless, from the analogy of the structure of these intrusive sheets to that of the sills in such volcanic districts as the southern half of Scotland, we may well believe that they were connected here and there with eruptive vents, and thus that besides the northern and southern districts of Permian volcanoes, there rose a central group among the lagoons of the heart of England. Though no vestige of any such group has been detected, we must remember that a large portion of the Midlands is overspread with Permian and Triassic deposits, and that much more igneous rock may be concealed than appears at the surface. Possibly there may be buried under these younger sheets of red sandstone and marl, lavas and tuffs with their connected vents, such as may be seen where the Permian volcanic series has been laid bare by denudation in Ayrshire and Devonshire. In this respect it would be interesting to make a thorough examination of the Permian breccias of the district, with the view of discovering whether, though the volcanic rocks _in situ_ may still lie covered up, fragments of them may not be found in these deposits. [Illustration: TO ACCOMPANY SIR ARCHIBALD GEIKIE'S "ANCIENT VOLCANOES OF BRITAIN" Map V. MAP OF THE PERMIAN VOLCANIC DISTRICTS OF SCOTLAND _The Edinburgh Geographical Institute_ Copyright J. G. Bartholomew.] BOOK VIII THE VOLCANOES OF TERTIARY TIME CHAPTER XXXIII