(2) _Vents._--One of the most interesting features in this south-western district of Scotland is the admirable way in which the volcanic vents of Permian time have been preserved. Their connection with the lavas and tuffs can there be so clearly traced that they serve as a guide in the interpretation of other groups of vents in districts where no such connection now remains. In Ayrshire, the lower part of the Permian volcanic band is pierced by several small necks of agglomerate. There cannot, I think, be any doubt that these necks mark the positions of some of the vents from which the later eruptions took place. Immediately beyond them necks of precisely similar character rise through the upper division of the Coal-measures. There can be as little hesitation in placing these also among the Permian vents. And thus step by step we are led away from the central lavas, through groups of necks preserving still the same features, external and internal, and rising indifferently through rocks of any geological age from the Coal-measures backward. Thus, although if we began the investigation at the outer limits of the chain of necks, we might well hesitate as to their age, yet, when we can fix their geological position in one central area, we are, I think, justified in classing, as parts of one geologically synchronous series, all the connected groups that retain the same general characteristics. It is to denudation that we owe their having been laid bare to view; but at the same time, denudation has removed the sheet of ejected materials which may have originally connected most of these vents together. In this regard, it is most instructive to follow the vents south-eastwards from the Ayrshire basin into Nithsdale for a distance of some eighteen miles. If we traced them down that valley to Sanquhar, without meeting with any vestige of superficial outflows to mark their stratigraphical position, we might possibly hesitate whether the age of those which are so far removed from the evidence that would fix it should not be left in doubt. But if we continued our traverse only a few hundred yards farther, we should find some fragmentary outliers of the Permian lavas capping the Upper Coal-measures; and if we merely crossed from the Nith into the tributary valley of the Carron Water, we should see preserved in that deep hollow a great series of Permian lavas, tuffs and agglomerates. It is only by a happy accident that here and there these superficial volcanic accumulations have not been swept away. There was probably never any great thickness of them, but they no doubt covered most, if not all, of the district within which the vents are found.