Fig. 136.—Trachytic sills, Knockvadie, Kilpatrick Hills.
1. Upper Old Red Sandstone; 2. "Ballagan Beds"; 3. Tuffs; 4. Lavas of the Plateau; 5. Agglomerate of necks; 6. Trachyte sills; 7. Dolerite dyke (? Tertiary).
4. Close of the Plateau-eruptions
The relative geological date when the eruptions of each plateau ceased can fortunately be determined with much more precision than the time of their beginning. The Hurlet Limestone, so well known as the lowest thick calcareous seam in the Carboniferous Limestone series, of which it is generally taken as the base, can be identified over the whole of Central Scotland, and thus forms an excellent stratigraphical horizon, from which the upward termination of the volcanic sheets underneath it can be measured.
When the volcanic episode of the plateau-eruptions came to an end, such banks or cones as rose above the level of the shallow sea which then overspread Central Scotland were brought beneath the water, as I have already remarked, either by prolonged denudation or more probably in large part by the continued subsidence of the region. The downward movement may possibly for a time have been accelerated, especially in some districts. Thus the Hurlet Limestone, though usually not more than five or six feet thick, increases locally to a much greater thickness. At Petersfield, near Bathgate, for example, it is between 70 and 80 feet in depth, while at Beith, in North Ayrshire, it increases to 100 feet ([Fig. 137]), which is the thickest mass of Carboniferous Limestone known to exist in Scotland. At both of these localities the limestone lies upon a series of volcanic rocks, and we may perhaps infer that the subsidence advanced there somewhat more rapidly or to a greater extent, so as to form hollows in which the limestone could gather to an abnormal depth. The water would appear to have become for a time tolerably free from mechanical sediment. The limestone is hence comparatively pure, and is extensively quarried all over the country for industrial purposes. It is a crinoidal rock, abounding in many species of corals, brachiopods, lamellibranchs, and gasteropods, with trilobites, cephalopods, and fishes.
Fig. 137.—Section across the edge of the Clyde plateau, south-east of Beith.
1. Plateau-lavas; 2. Tuffs and volcanic conglomerates; 3. Hurlet Limestone; 4. Coal-bearing strata above the limestone; 5. Dolerite dyke.
A variable thickness of strata intervenes between the top of the volcanic series and the Main Limestone. Sometimes these deposits consist in large measure of a mixture of ordinary sandy and muddy material with the washed-down tuff of the cones, and probably with volcanic dust and lapilli thrown out by the latest eruptions. Thus along the flank of the hills from Barrhead to Strathavon, yellow and green ashy sandstones, grits and conglomerates are succeeded by ordinary sandstones, black shales and ironstones, while here and there true volcanic tuff and conglomerate make their appearance.[441] Further west, in the Kilbirnie district, the limestone lies directly on the tuffs that rest upon the andesites (Figs. [137], [138]).
[441] Explanation of Sheet 22, Geol. Surv. Scotland, p. 12.
