Fig. 131.—Section of south end of Dumbuck Hill. East of Dumbarton.
As a rule, the fragmental materials of the plateau-necks are quite unstratified. Their included blocks, distributed irregularly through the mass, have evidently undergone little or no assortment after they fell back into the vents. Occasionally, however, a more or less distinct bedding of the agglomerate or tuff may be observed, the layers having a tendency to dip inward into the centre. One of the most conspicuous examples of this structure is to be found in the hill of Dumbuck, to the east of Dumbarton. This neck, which forms so prominent a feature in the landscape, presents a precipitous face towards the south, and allows the disposition of its component materials to be there seen. The agglomerate consists of a succession of rudely stratified beds of coarser and finer detritus, which on both sides are inclined towards the centre, where a plug of fine-grained olivine-basalt has risen and spread out into a columnar sheet above ([Fig. 131]). In general form this basalt resembles such intrusions as that of Largo Law, to be afterwards described ([Fig. 226]), where what may have been the hollow or bottom of the crater is filled with basalt.
Fig. 132.—Section across the East Lothian plateau to show the relative position of one of the necks.
1. Lower Carboniferous sandstones and shales; 2. Red and green tuffs with a seam of limestone (l); 3. Band of basic sheets at the base of the lavas; 4. Trachytes; 5. Phonolite neck.
(b) Necks of Andesite, Trachyte, Dolerite, Diabase, or other massive Rock.—When the vents have been filled by the uprise of some molten rock, it is generally, as we have seen, of a more acid character than the ordinary lavas of the plateaux. Frequently it consists of some variety of trachyte or andesite, commonly of a dull yellow or grey tint and waxy lustre. Good examples may be seen among the remarkable group of necks on either side of the valley north of the village of Strathblane and in those above Bowling. The three great necks in East Lothian, already alluded to,—Traprain Law (Figs. [132], [133]), North Berwick Law ([Fig. 109]), and the Bass Rock ([Fig. 110])—are masses of phonolite and trachyte, obviously related to the trachytes of the adjacent plateau. A smaller but very perfect instance of a vent similarly filled is to be seen in the same neighbourhood on the shore to the east of North Berwick Law.[438]
[438] See "Geology of East Lothian," Geological Survey Memoir, p. 40.
Examples occur where the funnels of eruption have been finally sealed up by the rise of more basic material, and this has happened even in a district where most of the lava-form necks consist of trachyte or some other intermediate lava. Thus, in the Campsie Fells, several such bosses appear, of which the most conspicuous forms the hill of Dungoil (1396 feet,[ Fig. 128]). Further west, among the Kilpatrick Hills, bosses of this kind are still more numerous. The group of bosses near Ancrum and Jedburgh is mainly made up of olivine-dolerites and olivine-basalts ([Fig. 130]). This more basic composition of itself suggests that these bosses may be connected rather with the puy- than with the plateau-eruptions.
(c) Necks of Composite Character.—In not a few examples, the vents have been filled with agglomerate which has been pierced by a plug or veins of lava-form material. Many illustrations of this composite structure may be observed along the west front of the great escarpments from Fintry to Ardrossan (see Figs. [124], [125], [127] and [128]). In that region the intruded rock is often a dull yellowish or grey trachytic or andesitic material. Olivine-basalt is the chief rock intruded in the vents in the Dumbarton district. Among the Roxburghshire vents, where the injected material is commonly olivine-basalt or dolerite, it occasionally happens, as in Rubers Law, that the uprise of the lava has almost entirely cleared out or concealed the agglomerate, and in some of the bosses, where no agglomerate is now to be seen, the basalt may have taken its place ([Fig. 130]).
The largest and most interesting vents connected with this type of Carboniferous volcano, are those which occur within the limits of the plateaux, where they are still surrounded with lavas and tuffs that probably came out of them. Of these by far the most extensive and remarkable lies among the high moorlands of Renfrewshire between Largs and Lochwinnoch, where the ground rises to more than 1700 feet above the sea (see [Fig. 129]). This area, as already remarked, is unfortunately much obscured with drift and peat, so that the limits of its rocks cannot be so satisfactorily traced as might be desired. I think it probable that several successive vents have here been opened close to each other, but their erupted ashes probably cannot be distinguished. Over a space measuring about four miles in length by two and a half in breadth, the rocks exposed at the surface are fine tuffs, breccias and coarse agglomerates, largely made up of trachytic, andesitic or felsitic material, and pierced by innumerable protrusions of various andesitic, trachytic or felsitic rocks in bosses and veins, as well as also by dykes of a more basic kind, such as dolerites and basalts. Some of the tuffs present a curiously indurated condition; and they are frequently much decayed at the surface.[439] Another large mass of tuff and agglomerate lies a little to the south-west of the main area.
[439] This tract of ground was mapped for the Geological Survey by Mr. R. L. Jack, now in charge of the Geological Survey of Queensland. See Sheet 31, Geological Survey of Scotland.