The Ancient Volcanoes of Great Britain, Volume 2 (of 2)

by Archibald Geikie

In almost all cases, the necks of the Ayrshire region consist of a gravelly tuff or agglomerate, reddish or greenish in colour, made up of blocks of such lavas as form the bedded sheets, together with fragments of the stratified rocks through which the chimneys have been blown out. Thus, in some of the necks, pieces of black shale are abundant, as at Patna. In other cases, there are proofs of the derivation of the stones from much greater depths, as in the Green Hill of Waterside, where fragments of fine greywacke are not infrequent, probably derived from the Silurian formations which lie deep beneath the Carboniferous and Old Red Sandstone series.

The fragmentary material of the necks is generally unstratified, but a rude stratification may sometimes be noticed, the dip being irregularly inward at high angles towards the middle of the vent. This structure, best seen in the vents of the Fife coast, as will be shown in the sequel, may be detected in some of the necks of the Dalmellington district.

Fig. 205.—Ground plans of Permian volcanic vents from the Ayrshire Coal-field. On the scale of six inches to a mile.

1. Neck half a mile north-west from Dalmellington; 2. Neck at Auchengee, four miles north-east from Patna; 3. Neck at head of Drumbowie Burn, five and a half miles due north from Dalmellington; 4. Patna Hill, 853 feet above sea-level (for outline of this hill see the preceding Fig.); 5. Neck on Kiers Hill (1005 feet above the sea), two miles south from Patna, with lava adhering to part of the wall.

Occasionally some form of molten rock has risen in the funnel, and has partially or wholly removed or concealed the agglomerate. This feature is especially noticeable among the necks that pierce the Dalmellington coal-field. Portions of basic lavas traverse the agglomerate or intervene between it and the surrounding strata. These have probably in most cases been forced up the wall of the funnel, while here and there sills run outward from the necks into the surrounding Coal-measures. Sometimes a thin sheet of lava, adhering to the wall of a funnel, may be the remnant of a mass of rock that once filled up the orifice. In one of the necks of the Muirkirk Coal-field, which was pierced by a mine driven through it from side to side, fingers and sheets of "white trap," or highly altered basalt, were found to run out from the neck into the surrounding strata.[93] Dark heavy basalt, or some still more basic rock, has here and there filled up a vent. As so many of the necks rise through the coal-fields, opportunities are afforded of studying the effects of volcanic action upon the coal-seams, which for some distance from them have been destroyed.

[93] Explanation of Sheet 23, Geol. Surv. Scotland, p. 39.

Another feature, which can be recognized from the information obtained in mining operations, is that, in the great majority of instances, no connection is traceable between the positions of the vents and such lines of dislocation as can be detected at the surface or in the underground workings. Some vents, indeed, have evidently had their positions determined by lines of fault, as, for instance, that of the Green Hill below Dalmellington. Yet in the same neighbourhood a number of other examples may be found where the volcanic funnels seem to have avoided faults, though these exist close to them.

In this south-western district of Scotland upwards of sixty distinct vents have been mapped in the course of the Geological Survey. They run from the north of Ayrshire to the foot of the Southern Uplands, and descend for some distance the vale of the Nith. The area over which they are distributed measures roughly about forty miles from north-west to south-east, and at its greatest breadth twenty miles from south-west to north-east. Within this tract the vents are scattered somewhat sporadically in groups, sometimes numbering twenty necks in a space of sixteen square miles, as in the remarkable district of Dalmellington.

In considering their distribution we cannot but be impressed by the striking manner in which these necks keep to the valleys and low grounds. I have already alluded to this characteristic, as shown by the volcanoes of the Old Red Sandstone and Carboniferous periods. But it is displayed by the Permian volcanoes in a still more astonishing way. Beginning at the northern end of the long chain of necks in the West of Scotland, we find a row of them on the plains fronting the volcanic plateau of the Ardrossan, Dunlop and Stewarton Hills. Thence we may follow them, as single individuals or in small groups, across the broad lowland of Ayrshire, southward to the very base of the great chain of the Southern Uplands. There, a cluster of some two dozen of them may be seen rising out of the Carboniferous rocks on the low grounds, but they abruptly cease close to the base of the hills; not one has been detected on the adjacent Silurian heights. Moreover, if we turn into the valleys that lead away from the great Ayrshire plain to the interior, we find necks of the same character in these depressions. They ascend the valley of Muirkirk, and may be met with even at its very head, near the base of the Hagshaw Hills. Again, on the floor of the remarkable transverse valley trenched by the Nith across the Southern Uplands, Permian necks pierce the Coal-measures, while the outlying fragments of bedded lava show that these vents flooded the bottom of that valley with molten rock. Turning out of Nithsdale into the long narrow glen of the Carron Water, we observe its floor and sides to be covered with the sheets of lava and tuff already noticed. And so travelling onward from the vale of the Nith into that of the Capel Water, thence into the Water of Ae and across into the great strath of Annandale, we may detect, if not actual vents, at least the beds of lava and layers of volcanic detritus that were ejected from them.