No indication of any sills has been met with among the Devonshire Permian rocks. None of the lavas which I have seen have the internal characters of true sills, while in the field their association with the sandstones and breccias in no observed case points to intrusion.
Though much remains to be done in this region before an adequate account can be given of the interesting series of eruptions which concludes the long volcanic history of the South-west of England, enough is known to indicate the general character of the phenomena. The eruptions were on even a feebler scale than those of the Permian period in Scotland, but they seem to have resembled them in their general character. Small puy-like vents were opened, from which dark scoriaceous lavas and showers of gravelly tuff and stones were discharged over the floor of the inland sea or lake-basin in which the red sandstones and breccias were accumulated. These outflows and explosions took place too, as in Scotland, towards the beginning of the deposition of the red strata, and entirely ceased long before that deposition came to an end. In each area the eruptions mark the close of Palæozoic volcanic activity in Britain. The varied and recurrent volcanic episodes which distinguished each successive geological period from the Archæan onwards now definitely terminate, not to be resumed until after the passing of the whole of the vast cycle of Mesozoic ages.
2. ERUPTIVE ROCKS IN THE MIDLAND COAL-FIELDS
Between the thick and thoroughly marine development of the Carboniferous Limestone in Derbyshire and in South Wales, there lies the region, already referred to, wherein both the Carboniferous Limestone and Millstone Grit die out against what must have been a ridge of land or group of islands that stretched in a general east and west direction from the high grounds of Wales through Shropshire, Staffordshire and Leicestershire. On the slopes of this ridge the limestone is gradually overlapped by the Millstone Grit, and both are in turn overlapped by the Coal-measures, which are then found lying immediately on the more ancient rocks of the region—Cambrian or pre-Cambrian, Silurian and Old Red Sandstone. The gradual subsidence that led to the deposit of several thousand feet of Carboniferous strata over the regions to north and south, before the beginning of the Coal-measure period, does not seem to have sensibly affected the persistence of this old terrestrial surface, which probably lay on an axis of upward movement, so that, amidst the surrounding depression, its position above water was on the whole maintained. But there are indications that the inequality of movement in this part of the earth's crust was of much older date than the Carboniferous period. The Old Red Sandstone is conformably continuous below the base of the Carboniferous system, and in Wales is estimated to be some 10,000 feet thick. No break has yet been detected in this vast accumulation of sedimentary material, though it is highly probable that some such unconformability must exist in it as that between the Scottish Lower Old Red Sandstone, which passes down into the Upper Silurian shales, and Upper Old Red Sandstone, which graduates upward into the base of the Carboniferous formations. But even if such a break should be discovered, it will not account for the position of the Coal-measures on Cambrian or even perhaps older rocks. It is hardly conceivable that, had these rocks been covered with a full development of Old Red Sandstone, they could have been stripped of it by denudation before the deposition of the Coal-measures. It seems much more probable that the discrepancy in the terrestrial movements had commenced in Old Red Sandstone time, and that these ridges of ancient Palæozoic rocks never sank below the waters in which the vast thickness of red sandstones, marls and conglomerates was laid down.[112]
[112] See a discussion of this subject in Jukes' Preface to his South Staffordshire Coal-field.
But apart from the question of its antiquity, this tract of persistent land has a special interest in the history of volcanic action in Britain, for it was the scene of some remarkable protrusions of eruptive material which took place after a part, and possibly after the whole, of the Coal-measures were accumulated. The date of these protrusions cannot be fixed with greater precision; but there can be no doubt that they belong to one of the later volcanic periods in the geological history of Britain, and the account of them is therefore included in the present Chapter of this work.
In the English Midlands south of Stafford, over a tract of country about 700 square miles in extent, stretching from Birmingham on the east, across the vale of the Severn, to the uplands of Shropshire on the west, the Coal-measures, partly isolated into outliers by denudation and partly separated by overlying younger formations, are pierced by masses of intrusive igneous rocks. Many of these masses have long been familiar to geologists. Those, for example, of the Clee Hills of Shropshire, and the Rowley, Barrow and Pouk Hills of Staffordshire and Worcestershire, have been frequently described, their relations to the surrounding strata have been minutely sought out, their composition has been chemically determined, and their microscopic structure has been investigated. But they have been studied rather as individual masses of local importance. No attempt has yet been made to ascertain how far they are capable of being grouped together as one connected series, linked with each other in chemical and mineralogical characters, and containing a definite record in the volcanic history of the country. This is a task which, it is to be hoped, some competent inquirer will before long undertake.
In the meantime it is only possible to review here the already published information, and to gather from it what may at present be surmised to have been the history of these later eruptions of the Midlands.
The areas where the igneous rocks now to be described are exhibited may be conveniently placed in the following five groups:—1st, Titterstone Clee Hill; 2nd, Brown Clee Hill; 3rd, The Forest of Wyre Coal-field; 4th, The Coalbrookdale Coal-field; and 5th, The South Staffordshire Coal-field.
1. The Titterstone Clee Hill forms a ridge about seven miles long and a mile and a quarter broad, running in a north-easterly direction over the Old Red Sandstone uplands of the south of Shropshire. The ground rises gradually towards the south-west, until it reaches there a height of 1754 feet ([Fig. 232]). On the north-western side of the ridge, the last vanishing representative of the Carboniferous Limestone can be seen to be overlapped the Millstone Grit, which, as it is traced towards the south-west, is in turn overlapped by the Coal-measures, and these, about 400 feet thick, then rest immediately on the Old Red Sandstone. Two sheets of columnar olivine-dolerite, possibly originally connected, lie as cakes on the summit and eastern slope of the ridge, and cover in all a space of about a square mile and a half. The larger sheet, which varies from 60 to 180 feet in thickness, overlies the Coal-measures, and the coals of the Cornbrook coal-field have been worked underneath it. The smaller mass, which may be 300 feet in thickness, forms the summit of the ridge. On its eastern side it reposes on Coal-measures, which are there much disturbed; but on the west side, where it forms a bold capping to the escarpment, it is underlain at once by the Old Red Sandstone. There cannot be any doubt that these masses of eruptive material are sills, which have been injected into the Carboniferous strata, and partly between these strata and the Old Red Sandstone. One or more dykes of eruptive rock have been met with in mining, and the coal on approaching them undergoes alteration.[113]