It is possible, however, that the "trappean breccia" may be of the same nature as the "uncompressed balls of basalt bedded in a mass of decomposed basalt or basaltic 'wacke' or clay"[121]—that is, a decayed contact layer of the eruptive rock. But if it be regarded as the fragmental accompaniment of a lava-stream, it can hardly belong to the Coal-measures. If the dolerite had been a lava of that age, it ought to be found lying conformably on the Coal-measures. But this it does not appear to do. Making every allowance for the way in which an advancing current of lava might plough up soft sediment on the bottom of the sea or of a lake, we can hardly thus account for the very uneven surface of Coal-measures on which the sheet of igneous rock rests. If the Rowley rock be looked upon as a lava which flowed out at the surface, it must, I think, be assigned to a time subsequent to that of the Coal-measures, when these strata had been upraised and had suffered some amount of denudation. I confess, however, that the petrographical characters of the rock, the alteration of the coals which have been worked underneath it, and the abundant veins of "white rock" which there traverse the seams, induce me to regard this rock as forming no exception to the general rule in the Midlands, but as having been intruded as a sill, now laid bare by denudation. Its fresher condition may arise from its thickness, or from some other circumstance which has not been ascertained.
We have now to consider the probable geological date of the various intrusions of basic igneous material which can be traced over so wide an area in the centre of England. In discussing the subject, Jukes pointed out that in the surrounding district "no igneous rocks of any kind are found in any formation newer than the Coal-measures."[122] This statement is, with the exception of one locality, undoubtedly true.[123] But on any view there must have been a long interval of time between the formation of the highest strata of the South Staffordshire coal-field and that of the lowest Permian deposits of the district. It is quite conceivable, though at present incapable of proof, that the extravasation of eruptive material took place after the close of the Carboniferous period and during the earlier part of the Permian period.
[123] See note on next page.
Jukes further shows that "at whatever period these igneous rocks were produced, they were all existent before the production of the faults and dislocations that have traversed the Coal-measures, and before any great denudation had been effected on the country." This argument may be readily granted. But, so far as we know, many, if not most, of the faults traverse also the surrounding Permian and Triassic rocks, so that igneous masses protruded during those periods would be affected by the same dislocations.
When we consider the history of Palæozoic time in this country, and especially the proof, obtainable everywhere else in Britain, that volcanic energy became quiescent during the accumulation of the Coal-measures, we may well demand better evidence than has hitherto been forthcoming that any portion of the dolerites of the Midlands is of Carboniferous age. It is important to notice that though the dolerite sills and veins are so abundant in the South Staffordshire coal-field, coming even in many places up to the present surface of the ground, no single case has been observed where they rise into the Permian rocks that overlie the Coal-measures unconformably. It is difficult to believe that, had these intrusions taken place after the deposition of the younger formation, they should not be found penetrating it.[124] It seems almost certain that they must be of an age intermediate between the Coal-measures of South Staffordshire and the surrounding breccias and sandstones of the Permian series. And as there is clear evidence of contemporaneous volcanic action in the lowest part of the Permian system to the north in Scotland and to the south in Devonshire, the inference seems not unreasonable that these intrusive basalts of the Midlands are most probably of Permian age.
[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.