If the horizons of the sills furnished any reliable clue to their age, it might be inferred that the rocks were all intruded during the Carboniferous period, and as most of them lie beneath the upper stratigraphical limit of the puy-eruptions, the further deduction might be drawn that they are connected with these eruptions. I have little doubt that in a general sense both conclusions are well-founded. But that there are exceptions to the generalization must be frankly conceded. On close examination it will be observed that the same intrusive mass sometimes extends from the lower into the upper parts of the Carboniferous groups. Thus, in the west of Linlithgowshire, a large protrusion which lies upon the Upper Limestones, crosses most of the Millstone Grit, and reaches up almost as high as the Coal-measures. Again, in Fife, to the east of Loch Leven, a spur of the great Lomond sill, crossing the Carboniferous limestone, advances southward into the coal-field of Kinglassie, In Stirlingshire and Lanarkshire numerous large dolerite sheets have invaded the Millstone Grit and Coal-measures, including even the upper red sandstones, which form the top of the Carboniferous system in this region. It is thus obvious that if the puy-eruptions in the basin of the Forth ceased towards the close of the deposition of the Carboniferous Limestone series, there must have been a subsequent injection of basic lava on a gigantic scale in central Scotland. I shall recur to this subject in [Chapter xxxi.]
Fig. 173.—Section across the Upper Volcanic Band of north Ayrshire. Length about four miles.
1. Andesite lavas of the Clyde Plateau; 2. Tuffs closing the Plateau volcanic series; 3. Hurlet Limestone; 4. Carboniferous Limestone series with coal-seams; 5. Lower tuff zone of the Upper volcanic band; 6. Basic lavas; 7. Upper tuff zone; 8. Basic sill; 9. Coal-measures.
2. NORTH OF AYRSHIRE
In this part of the country another group of puys and their associated tuffs and lavas may be traced from near Dairy on the west, to near Galston on the east ([Map IV.]). The length of the tract is about sixteen miles, while its breadth varies from about a furlong to nearly a mile and a half. I have had occasion to allude to this marked band of volcanic materials which here intervenes between the Carboniferous Limestone and the Coal-measures, and from its position appears to mark the latest Carboniferous volcanoes. Its component rocks reach a thickness of sometimes 600 feet, and as they dip southwards under the Coal-measures, they may extend for some distance in that direction. They have been met with in borings sunk through the northern part of the Irvine coal-field. Even what of them can be seen at the surface, in spite of the effects of faults and denudation, shows that they form one of the most persistent platforms of volcanic rock among the puy-eruptions of Scotland.
Where best developed this volcanic band has a zone of tuff at the bottom, a central and much thicker zone of bedded basalts, and an upper group of tuffs, on which the Coal-measures rest conformably. A few vents, probably connected with it, are to be seen at the surface between Fenwick and Ardrossan. But others have been buried under the Carboniferous sedimentary rocks, and, as already described, have been discovered in the underground workings for coal and ironstone ([p. 434]). These mining operations have, indeed, revealed the presence of far more volcanic material below ground than would be surmized from what can be seen at the surface. Here and there, thin layers of tuff appear in brook-sections, indicating what might be conjectured to have been trifling discharges of volcanic material. But the prosecution of the ironstone-mining has proved that, at the time when the seam of Black-band Ironstone of that district was accumulated, the floor of the shallow sea or lagoon where this deposition took place was dotted over with cones of tuff, in the hollows between which the ferruginous and other sediments gathered into layers. That seam is in one place thick and of good quality; yet only a short distance off it is found to be so mixed with fine tuff as to be worthless, and even to die out altogether.[478]
[478] See Explanation of Sheet 22, Geol. Surv. of Scotland, pars. 29, 33, 45.
3. LIDDESDALE
A remarkable development of puys lies in that little-visited tract of country which stretches from the valleys of the Teviot and Rule Water south-westwards across the high moorland watershed, and down Liddesdale. Through this district a zone of bedded olivine-basalts and associated tuffs runs in a broken band which, owing to numerous faults and extensive denudation, covers now only a few scattered patches of the site over which it once spread. The geological horizon of this zone lies in the Calciferous Sandstones, many hundred feet above the position of the top of the plateau-lavas ([Map IV.]).
So great an amount of material has been here removed by denudation that not only has the volcanic zone been bared away, but the vents which supplied its materials have been revealed in the most remarkable manner over an area some twenty miles long and eight miles broad. Upwards of forty necks of agglomerate may be seen in this district, rising through the Silurian, Old Red Sandstone, and lowest Carboniferous rocks. It fills the geologist with wonder to meet with those stumps of old volcanoes far to the west among the Silurian lowlands, sometimes fully ten miles away from the nearest relic of the bedded lavas connected with them.[479] That these vents, though they rose through ground which at the time of their activity was covered with the volcanic series of the plateaux, do not belong to that series, but are of younger date, has been proved in several cases by Mr. Peach. He has found that among the blocks composing their agglomerates, pieces of the sandstones, fossiliferous limestones and shales of the Cement-stone group, overlying the plateau-lavas, are to be recognized. These vents were therefore drilled through some part at least of the Calciferous Sandstones, which are thus shown to have extended across the tract dotted with vents. After the volcanic activity ceased, fragments of these strata tumbled down from the sides into the funnels. Denudation has since stripped off the Calciferous Sandstones, but the pieces detached from them, and sealed up at a lower level in the agglomerates, still remain. Mr. Peach's observations indicate to how considerable an extent sagging of the walls of these orifices took place, with the precipitation not merely of blocks, but of enormous masses of rock, into the volcanic chimneys. In one instance, between Tudhope Hill and Anton Heights, a long neck, or perhaps group of necks, which crosses the watershed, shows a mass of the red sandstone many acres in extent, and large enough to be inserted on the one-inch map, which has fallen into the vent ([Fig. 175]).