Though only a few scattered patches of the Permian bedded lavas and tuffs have been preserved, enough is left to indicate that the vents were active only in the early part of the period represented by the Scottish Permian red sandstones, for it is entirely in the lower part of these strata that volcanic rocks occur. The eruptions gradually ceased, and the sheets of ejected material, probably also the volcanic cones, were buried under at least several hundred feet of red sandstone. Whether or not any portion of the erupted material was for a time built up above the level of the water, there seems to be no question that the vents were, on the whole, subaqueous. [Illustration: Fig. 206.--Section of sills traversing the Permian volcanic series. River Ayr, Ballochmyle. _a_, Coal-measures; _b_ _b_, Basic lavas; _c_ _c_, Brick-red sandstones with tuff; _d_, Red tuff and volcanic breccia; _e_ _e_, Dolerite sills.] 3. _Sills._--The phenomena of sills and dykes are less clearly developed among the Permian volcanic rocks of the Ayrshire basin than among those of older formations. In the section exposed in the course of the River Ayr at Howford Bridge, a coarsely crystalline dolerite which extends for nearly 300 yards up the stream, cuts the Permian lavas, of which it encloses patches as well as pieces of sandstone. At the contact, the rock becomes fine-grained (Fig. 206). Through the coarsely crystalline material run long parallel "segregation veins" of a paler, more acid substance, as among the Carboniferous sills. Similar rocks are well seen in the Dippol Burn near Auchinleck House. Passing outward into the Coal-measures, we encounter a much larger display of similar intrusive sheets. The best district for the study of these sills lies around Dalmellington. The Coal-measures are there traversed by many intrusions, which have produced great destruction among the coal-seams. Some of the rocks are extremely basic, including a beautiful picrite like that of Inchcolm (Letham Hill, near Waterside). The age of these sills must be later than the Coal-measures into which they have been injected. Some of them are obviously connected with the agglomerate-necks, and the whole or the greater number should thus probably be assigned to the Permian period.[94] The phenomena of intrusion presented by these rocks reproduce the appearances already described in connection with the basic intrusive sheets of Carboniferous age. [Footnote 94: Explanation of Sheet 14, Geol. Surv. Scotland, p. 22.] 2. Basin of the Firth of Forth The other district of Southern Scotland, where traces of volcanic action later in age than the Coal-measures may be observed, lies in the basin of the Firth of Forth (Map V.). They include no bedded lavas, and only at one locality do any relics of a covering of stratified tuffs overspread the Carboniferous formations. The evidence for the old volcanoes consists almost entirely of necks of tuff, which mark the position of vents of eruption.