Fig. 198.—Volcanic Breccia invading and enclosing Carboniferous Slate, White Bull Head.
Further examination, however,reveals that this seemingly regular sequence is entirely deceptive. At various points the breccia abruptly truncates the sandstones, and involves large pieces of them, as shown in [Fig. 198] A. At other places, the lower side of the breccia, or what would be its base if it were a regular bed, cuts out the strata and sends veins into them (B). And the same structure is visible, on its upper side, or what would be its top (C).
It is clear that these highly-inclined bands of breccia are not contemporaneous with the deposition of the Carboniferous Slate, but have been introduced into their position at some time subsequent not only to the deposition, but to the disturbance and elevation of the strata. The peninsula of White Bull Head is crossed by several other similar bands. On Black Bull Head, also, together with abundant felsitic and doleritic intrusions, a similar breccia or agglomerate is to be seen. In some parts it is compact in texture with spheroidal flinty lumps, and weathers somewhat like a nodular felsite. This variety ends off rather abruptly to the north, but swells out southward, and then runs out into a high, narrow headland, in which it contains asbestos, as well as rounded crystals of hornblende. It has here disrupted the shales and sandstones, and near the junction is largely composed of fragments of them, the strata themselves being jumbled, bent, and broken up.
The only semblance of a neck-like mass of this volcanic fragmental material occurs on White Bull Head, where one of the bands expands about the centre of the ridge, and is there full of large blocks of grey sandstone. The breccia appears to have filled fissures which have been opened between the bedding planes of the highly tilted strata, giving rise to long narrow dyke-like intercalations. We have seen that among the Carboniferous volcanic phenomena such dyke-like masses of agglomerate occasionally present themselves in the vents both of the plateaux and the puys.
In one or two places I noticed what may be traces of cleavage in the breccia. The rock is not one that would yield easily to the rearrangements required for the production of this structure, and the doubtful cleavage may be deceptive. If we are justified in regarding the introduction of this volcanic material as having necessarily taken place after the tilting of the strata, we may not unreasonably infer further that the eruptions could only have been effected at no great distance from the surface. But the Carboniferous Slate in which these agglomerates lie is the lowest member of the Carboniferous system. As there is no known unconformability throughout this system in the south of Ireland, the whole of the rest of the pile of Carboniferous strata, amounting to a depth of several thousand feet, once probably extended over this region. It must, therefore, have been not only after the plication, but after extensive denudation of the formations that the fissures were filled with agglomerate. These geological changes no doubt occupied a vast period of time. While, therefore, no positive evidence has yet been gathered to fix the age of these volcanic eruptions of the south-west of Ireland, it is tolerably clear that they cannot be assigned to the Carboniferous period, but must belong to some later volcanic epoch. They may be of Permian age, perhaps even as late as the Tertiary volcanic series.
BOOK VII
THE PERMIAN VOLCANOES
CHAPTER XXXI
THE PERMIAN VOLCANOES OF SCOTLAND
Geographical Changes at the Close of the Carboniferous Period—Land- and Inland-Seas of Permian time—General Characteristics and Nature of the Materials erupted—Structure of the several Volcanic Districts: 1. Ayrshire, Nithsdale, Annandale; 2. Basin of the Firth of Forth.
The close of the Carboniferous portion of the geological record in Britain is marked by another of those great gaps which so seriously affect the continuity of geological history. No transitional formation, such as in other countries marks the gradation from the Carboniferous into the succeeding period, has been definitely recognized in this country. The highest Carboniferous strata are here separated from all younger deposits by an unconformability, indicating the lapse of vast periods of time whereof, within the British area, no chronicle has been preserved.
When we pass from the Carboniferous system to that which comes next to it in order of time, we soon become sensible that great changes in geography, betokening an immense interval, took place between them. The prolonged subsidence during which the Coal-measures were accumulated, not only carried down below sea-level all the tracts over which the Carboniferous system was deposited, but possibly submerged the last of the islets, which, like those of Charnwood Forest, had survived so many geological changes. Eventually, however, and after what may have been a vast period of quiescence, underground movements began anew, and the tracts of Coal-measures were unequally ridged up into land. The topography thus produced appears to have resulted in the formation of a series of inland seas somewhat like those of the Old Red Sandstone, but probably less in area and in depth. In these basins the water seems to have been on the whole unfavourable to life, for the red sand and mud deposited in them are generally unfossiliferous, though, when the conditions became more suitable, calcareous or dolomitic sediment accumulated on the bottom, to form what is now known as the "Magnesian Limestone," and muddy sediment was deposited which is now the "Marl Slate." In these less ferruginous strata, betokening a less noxious condition of water, various marine organisms are met with.[87]