C CHAPTER VI. CARBONIFEROUS PERIOD. 1. Stratigraphical Evidence. ARBONIFEROUS rocks occupy large areas in the British Isles and they are known to have a wide subterranean extension (see "Historical Geology," p. 181). They exhibit three principal facies, which may be called (1) the southern or Culm-measure type, (2) the central or Pennine type, (3) the northern or Scottish type. The southern type is found in Devonshire and in the south-west of Ireland; it consists mainly of black and grey shales (cleaved into slates in Ireland), the limestones being very thin and insignificant. The lower part consists of green and grey shales with bands of sandstone (Baggy and Pilton Beds), the middle of black shales and thin limestones (Lower Culm-measures), the upper of hard grey grits with bands of shale (Middle and Upper Culmmeasures). The central type is more varied and consists of a series which is divisible into four groups as follow:— 4. The Coal-measures (shales, sandstones, and coals). 3. The Millstone Grit (sandstones and shales). 2. The Upper or Yoredale Limestones and shales. 1. The Lower Limestones, with shale and sandstone at the base. Nos. 1 and 2 are known as the Lower Carboniferous series, and sometimes attain a thickness of 8,000 feet. Nos. 3 and 4 are the Upper Carboniferous series, and are as much as 12,000 feet thick in some parts of Lancashire and Yorkshire. This type prevails over the larger part of England and Ireland. The northern type is differently divided in Northumberland and Scotland, but the following classification would apply to both areas : Upper. { Lower. The Coal-measures (2,000 to 3,000 feet). A limestone group with shales and coals (1,500 A carbonaceous shale group (800 to 2,500). A sandstone group with basal conglomerates (1,000 to 3,000 feet). Certain areas in the north of Ireland exhibit beds of a similar type. The records of the Carboniferous period being thus more complete than those of earlier times, and the rocks being more fully exposed and more easily classified, we possess more certain grounds on which to reconstruct the geography of the British area, at any rate during the early stages of the period. It is clear that the period was ushered in by the partial submergence of the great continent which included so large a part of Britain in the preceding (Devonian) period. The movement of depression seems to have been very different from the movement which raised that continent; the upheaval was effected by a force which acted rather horizontally than vertically, forcing up the earth's crust by lateral compression into a series of mighty ridges and furrows. The Carboniferous submergence was apparently an even and uniform downward movement gradually bringing the lower portions of the pre-existent land beneath the level of the sea. The only district which presents evidence of differential movement is that of the Old Red Sandstone tract of Shropshire, for though the Upper Old Red exists there, the Lower Carboniferous beds are absent, and the several stages of the Upper Carboniferous rest unconformably on the Old Red Sandstone and Silurian rocks; whence we may infer that though this was low ground at the close of Devonian time, it was not submerged beneath the Lower Carboniferous sea, but was undergoing elevation which kept it above water till late in the Carboniferous period. In dealing with the evidence derivable from the lithological changes in the Lower Carboniferous rocks, I propose to follow a different method from that adopted in previous chapters. Although there can be no doubt that the greater part of England and Ireland was submerged during the formation of the Carboniferous Limestone, I think there are grounds for believing that a large island of irregular shape existed over the area now covered by St. George's Channel, and that it stretched northward into Scotland, and eastward through the centre of Wales and the midland counties of England. Instead therefore of discussing the conclusions to be deduced from a study of the English, Scotch, and Irish rocks respectively, I propose to adopt a more synthetic method, and, taking the existence of this island as a theorem, to state the facts which may be regarded as strong evidence, even if they do not amount to proof of the proposition. It will be convenient to start with Shropshire, as the evidence for the existence of land in that county has just been alluded to; it is corroborated by the rapid northerly thinning of the Carboniferous Limestone series in Gloucester and Monmouth; near Bristol this series has a total thickness of about 2,600 feet, including the lower and upper shales; near Chepstow it is about 1,500 feet thick, and in the Forest of Dean it is only 840 feet, having lost 1,760 feet in a distance of twenty to twenty-five miles; if its attenuation continued at the same rate it must have died out some ten miles north of the Forest of Dean coalfield, i.e. a little to the south of the 52nd parallel of latitude, and as a matter of fact it appears to be absent at Newent. The same northerly thinning occurs throughout the South Wales coal-basin, the Limestone series on its southern border being variously stated as from 1,000 to 2,000 feet thick, while along the northern border it is not more than 500 or 600 feet thick, and near Haverfordwest it appears to die out altogether, and to be overlapped by the Coalmeasures. These facts may be taken as indicating the existence of land a few miles north of the present boundary of the limestone.1 From Pembrokeshire we pass across to the coast of Wexford in Ireland, and as we find very similar conditions existing there, it is exceedingly probable that the coast-line we are following was continuous from Pembroke to Wexford. Carboniferous rocks occur round Wexford Harbour, and extend south-westward in a narrow strip between areas of Cambrian rocks, and are found again on either side of the entrance to Waterford Harbour. Mr. Kinahan states that the dark shales and limestones which are found along the southern margin graduate into red shales, sandstones, and conglomerates along their northern margins, and that "these rocks seem to have accumulated in a narrow bay which shallowed out eastward." 2 That the Cambrian, Ordovician, and Granitic areas of Wexford, Carlow, and Wicklow were land at the time 1 That the thinning of the limestone was in the direction of land, and not away from it, is shown by the simultaneous thinning of the Millstone grit and the overlap of the Coal-measures. We know also that the Carboniferous limestone was often formed in shore-lines. close proximity to when the lower beds of the Carboniferous limestone were formed is shown by the overlap of this limestone on to the granite of Carlow, and it is probable that this land being steep and mountainous was not wholly submerged till the close of the Carboniferous period, and perhaps not even then. This lower limestone continues to border the older rocks through Kildare, but in County Dublin it appears to be overlapped by shales belonging to the Calp and Upper Limestone series. The limestones between Howth, Swords, and Rush are believed to belong to the Lower Limestone, but they show unmistakable evidence of the close neighbourhood of land. The Hill of Howth was clearly an island in this Lower Limestone sea, and part of a shore conglomerate still exists there. Between Rush and Skerries the limestones include thick beds of conglomerate containing pebbles derived from the neighbouring Ordovician rocks; there is some doubt whether these limestones belong to the Lower or Upper division,'1 but there is no doubt that farther northwest, near the Naul, the Upper Limestone rests against the Ordovician shore, and that it is moreover overlapped by the succeeding (Yoredale) shale group. At one place there are boulder beds, consisting of blocks from the Ordovician rocks cemented together by grey Carboniferous Limestone, and Professor Jukes observes: "This is evidently a portion of the very beach or margin of the Carboniferous sea in which the fallen blocks and shingle from the wasting land above were enveloped in the calcareous deposits of the Carboniferous period." 2 The facts described in the memoir referred to seem explicable only on the supposition that the beds were deposited in a bay which had land to the south, east, and north 1 See "Mem. Geol. Survey (Ireland), Expl. of Sheets 102 and 112," pp. 65, 66. 2 Op. cit., pp. 59, 60. |