Illustrations of the varying structure of these vents are given in the accompanying figures. In [Fig. 127], a section is drawn through the two necks Dumgoyn and Dumfoyn, which have already been shown in outline and in ground-plan. The relation of these two vents to the neighbouring plateau to the right can here be seen. [Fig. 128] gives a section taken through the great vent of the Campsie Hills, with the minor adjacent necks of Dungoil, Bin Bairn, and the Meikle Bin.
The diagram in [Fig. 129] is meant to convey in a general way what appears to be the structure of the central vent of the Renfrewshire plateaux, to be afterwards referred to. But, as already mentioned, the limits of the various rocks are too much obscured to allow an accurate delineation to be given of their areas and relations to each other. The Berwickshire plateau supplies abundant interesting examples of tuff necks which rise through the Old Red Sandstone many miles distant from the edge of the lavas. This structure is shown in [Fig. 130].
Fig. 130.—Section across Southern Berwickshire to show the relation of the volcanic plateau to the vents lying south from it.
1. Upper Silurian strata; 2. Upper Old Red Sandstone; 3. The volcanic plateau; 4. Agglomerate and tuff of the vents; 5. Basalt and dolerite; 6. Lower Carboniferous strata.
Indications may occasionally be observed of an agglomerate vent having been first occupied by one kind of material and then, after being in great measure cleared out by explosions, having been subsequently filled up with another. As an example of this structure I may cite again the double neck of the Knock Hill a little to the north of Largs, of which the outline is shown in [Fig. 23], and the ground-plan in [Fig. 125, B]. This hill rises from the red sandstone slopes that front the great Ayrshire plateau and forms a conspicuous cone the top of which is rather more than 700 feet above the sea. Its summit commands a remarkably extensive and interesting panorama of the scenery of the Clyde, but to the geologist perhaps the most striking feature in the landscape is the range of terraced hills behind, mounting up into the great vents of the Renfrewshire uplands. On these declivities the successive lava-streams that have built up the plateau can be seen piled over each other for a thickness of more than 1000 feet, and presenting their escarpments as parallel lines of brown crag with green slopes between.
The Knock has had its upper part artificially dressed, for lines of trench have been cut out of its rocks by some early race that converted the summit of the hill into a strongly intrenched camp. From the apex of the cone the ground falls rapidly westward into a hollow, beyond which rises a lower rounded ridge of similar materials. It is possible that this western ridge may really form part of the main hill, but the grass-covered ground does not afford sufficient exposures of the rocks to settle this point. From the contours of the surface, it may be inferred that there are two closely adjacent vents, and that the western and lower eminence is the older of the two. This hill or ridge consists partly of a coarse agglomerate, and partly of veins and irregular protrusions of a dark, compact, slightly cellular lava. The stones in the fragmental rock are different olivine-basalts, or other basic lavas, and sandstones. The paste is rough, loose and granular. The sandstone fragments are much indurated and sometimes bleached.
The Knock itself is formed mainly of a remarkably coarse and strikingly volcanic agglomerate. Round the outside, and particularly on the south-east, the rock is finer in texture, compact, and gravelly, or like a mudstone, with few or no imbedded blocks, dull-green to red in colour, and breaking with a clean fracture which shows angular lapilli of various basalts or diabases. At the southern end of the neck, where the surrounding red sandstone can be seen within a few feet of the tuff, the latter is bright red in colour, and contains much debris of red sandstone and marl. Possibly this finer tuff, which is traceable as an irregular band round the outside of the neck, may mark an older infilling of the vent than the agglomerate of the centre; but there is no sharp line to be drawn between the two, though a hollow can sometimes be traced on the surface where they join.
The agglomerate of this locality is one of the most characteristic among the plateau-necks of the Clyde region. Its blocks sometimes measure from two to three feet in diameter. They consist almost wholly of a dark crystalline porphyritic olivine-basalt. These blocks are subangular in form, often with clean-fractured surfaces. Though occasionally slightly cellular, they are never slaggy so far as I could see, nor are any true scoriæ to be noticed among them. The blocks suggest that they were derived from the disruption of an already solidified mass of lava. The agglomerate is entirely without any trace of stratification.
Through this tumultuous accumulation of volcanic debris some irregular veins of olivine-basalt, sometimes glassy in structure, have been injected, and reach nearly to the summit of the hill. This intrusive material resembles generally some of the dark intrusive masses in the Dumbartonshire necks. Like these, it exhibits a tendency to assume a more or less distinctly columnar structure, its columns having the same characteristic wavy sides and irregular curvature. The intrusive rocks in the two eminences of the Knock may be paralleled among the stones in the agglomerate. The neck on its north-eastern side rises steeply from the red sandstones which it pierces, but which, although they are much jointed and broken, are not sensibly indurated. Unfortunately the actual junction of the igneous and sedimentary rocks is concealed under herbage.
