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. All along these valleys, which were already valleys in Carboniferous time, traces of the volcanic activity of this epoch may be detected. But, so far as I am aware, in not a single case has any vent been observed to have been opened on the high surrounding ridges. There has obviously been a determining cause why the volcanic orifices should have kept to the plains and the main valleys with their tributaries, and should have avoided the hills which rise now to heights of 500 to 1000 feet or more above the bottoms of the valleys that traverse them. It might be said that the valleys follow lines of fracture, and that the vents have been opened along these lines. But my colleagues in the Geological Survey, as well as myself, have failed, in most cases, to find any evidence of such dislocations among the rocks that form the surface of the country, while it is sometimes possible to prove that they really do not exist there.