Fig. 120.—Section across part of the Clyde Plateau to the west of Bowling (reduced from Sheet 6 of the Horizontal Sections of the Geological Survey of Scotland).
1. "Ballagan Beds"; 2. White sandstone; 3. Tuffs, 600 feet thick, with a thin sheet of andesite; 4. Andesite sheets, 500 feet; 5. Stratified tuffs with thin coals, shales, fireclays and plant-remains, 500 or 600 feet; 6 6. A series of andesite-lavas, about 1500 feet thick, enclosing a thin coal-seam at *; 7. Stratified tuffs, 200 feet; 8. Shales with plants and coaly seams, 150 feet; 9. Base of another andesite series, which must be some hundreds of feet thick; 10 and 11. Necks of agglomerate.
Upper Limits and Original Areas and Slopes of the Plateaux.—Where the highest members of the volcanic series can be seen passing conformably under the overlying Carboniferous strata they are frequently found to be mainly composed of fine tuffs, the last feeble efforts of the plateau-volcanoes having consisted in the discharge of showers of ashes. These materials were mingled with a gradually increasing proportion of ordinary mechanical sediment, which finally overspread and buried the volcanic tracts of ground, as these slowly sank in the general subsidence of the region. The characteristic corals, crinoids and shells of the Carboniferous Limestone begin to appear in these ashy sediments. There is thus an insensible passage from volcanic detritus into fossiliferous shales and limestones. Examples of this gradation may be seen in many natural sections along the flanks of the Ayrshire plateau from above Kilbirnie to Strathavon.
It is still possible to fix in some quarters the limits beyond which neither the lavas nor the tuffs extended, and thus partially to map out the original areas of the plateaux. For example, in certain directions the Carboniferous formations can be followed continuously downward below the Main Limestone, without the intervention of any volcanic material, or with only a slight intermixture of fine volcanic lapilli, such as might have been carried by a strong wind from some neighbouring active vents. By this kind of evidence and by the proved thinning-out of the materials of the plateau, we can demonstrate that in the north of Ayrshire the southern limits of the great volcanic bank did not pass beyond a line drawn from near Ardrossan to Galston. We can show, too, that the lavas of the Campsie Fells ended off about a mile beyond Stirling before they reached the line of the Ochil heights, and that the coulées which flowed from the Solway vents did not quite join with those from the Berwickshire volcanoes.
Fig. 121.—Diagram illustrating the thinning away southwards of the lavas of the Clyde Plateau between Largs and Ardrossan. Length about 10 miles.
1. Upper Old Red Sandstone; 2. Sandstones, shales, etc., with "Ballagan Beds"; 3. Tuffs; 4. Andesite lavas; 5. Carboniferous Limestone series.
Moreover, evidence enough remains to enable us to form a tolerably clear conception of the original average slopes of the surface of some of the plateaux. Thus in the great escarpment above Largs and the high ground eastward to Kilbirnie the volcanic series, as already stated, must be at least 1500 feet thick. This thick mass of lavas and tuffs thins away southwards and probably disappears a short distance south from Ardrossan in a space of about ten miles ([Fig. 121]). The original southward slope of the plateau would thus appear to have been about 1 in 35. Again, the northward slope of the same plateau may be estimated from observations in the Campsie Fells. We have seen that above Kilsyth the total depth of the volcanic sheets is about 1000 feet, while to the westward it is much thicker. From the top of the Meikle Bin (1870 feet) above Kilsyth north-eastwards to Causewayhead, where the whole volcanic series has died out, is a distance of 12 miles, so that the slope of the surface of erupted materials on this side was about 1 in 63 ([Fig. 122]).
Judging from the sections exposed along the faces of the escarpments, we may infer that the volcanic sheets had a tolerably uniform surface which sloped gently away from the chief vents, but with local inequalities according to the irregularities of the lava-streams that were heaped up round the vents and flowed outward in different directions and to various distances from them. At the beginning, these flat volcanic domes were certainly subaqueous. While they were being formed, continuous subsidence appears to have been in progress. But the great thickness of the volcanic accumulations, as in the Kilpatrick and Renfrewshire areas, and the paucity of ordinary sedimentary strata among them, make it not improbable that at least their higher parts rose above the water. Where this was the case there may have been considerable degradation of the lava-banks before these were reduced or were by subsidence submerged beneath the water-level. Evidence of this waste is probably to be recognized in the bands of conglomerate, occasionally of considerable thickness, which, particularly in some parts of Ayrshire, intervene between the top of the volcanic group and the Hurlet Limestone. As I shall have occasion to point out further on, there seems to be some amount of evidence in favour of the view that a considerable interval of time elapsed between the close of the plateau-eruptions and the date of that widespread depression which led to the deposition of the Hurlet Limestone over the whole of Central Scotland. If such an interval did occur it would include a prolonged abrasion of any projecting parts of the plateaux, and the production and deposition of volcanic conglomerate.
Fig. 122.—Diagram illustrating the thinning away eastwards of the lavas of the Clyde Plateau in the Fintry Hills. Length about 12 miles.
1. Upper Old Red Sandstone; 2. White sandstone, blue shales and cement-stones ("Ballagan Beds"); 3. Andesite sheet, about 100 feet thick; 4. Tuffs (250 feet), with an included band of ashy sandstone containing plant-remains; 5. Andesite lavas; 6. Carboniferous Limestone series, which to the east lies immediately on the Upper Old Red Sandstone.