156 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



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 indi- 

 viduals 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 depres- 

 sions. 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 Mthsdale into the long narrow glen of the Carron 

 Water, we find its floor and sides still covered with sheets of lava 

 and tufl". And so we may travel onward from the vale of the Nith 

 into that of the Capel Water, and thence into the Water of Ae and 

 across into the great strath of Annandale, and detect, if not actual 

 vents, at least beds of lava and tuff and layers of volcanic detritus 

 that were ejected from them. 



All along these valleys, which were already valleys in Carboni- 

 ferous 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 my- 

 self, have failed to find in most cases any evidence of such disloca- 

 tions among the rocks that form the surface of the country, while it 

 is sometimes possible to prove that they really do not exist there. 



Though only a few scattered patches of the Scottish Permian 

 bedded lav^s and tuffs have been preserved, enough is left to indicate 



