246 Geikie — Permian Yolcanos in Scotland. 



these liillocks rise conspicuously above the neighhouring ground, 

 and when the geologist has familiarized himself with them in 

 one part of the district, his eye soon detects their form in 

 other localities not before visited. They are not outliers of a 

 deposit once covering here a greater space. Though surrounded 

 by Carboniferous sandstones, and shales, they do not lie upon 

 them ; on the contrary, they descend vertically through the coal 

 measures like so many huge pipes, sometimes standing on lines 

 of fault, while the coal near them is altered. They are, in short, 

 true volcanic "necks," each representing a former crater or focus of 

 eruption. In one case, that of Helenton Hill, near Monkton, I 

 found the sides of the neck still partially crusted with a mass of 

 rough scoriaceous melaphyre — the remnant of the slaggy scoria which 

 once coated the walls of the volcanic orifice. These necks vary in 

 size from only a yard or two to 500 or 600 yards in diameter, and 

 though their present conspicuous, blunted, conical outline is due, of 

 course, to much subsequent denudation, one can readily enough 

 imagine them to be cones of tuff, marking the position of volcanos 

 that have only recently become extinct. 



II. That these necks come indiscriminately through the faulted 

 Upper Goal-measures, Carboniferous limestone and lower or calciferous 

 sandstones, may be taken as good evidence that they are later than the 

 Carboniferous period. A very brief examination of the ground is 

 enough to raise a suspicion that they may have been connected with 

 the trap-tuff already noticed. And the more they are studied, 

 the more probable does this connexion become, until we are con- 

 vinced that they can only be the vents through which the ash 

 and trap were ejected. The material of which the necks consist is 

 identical with that forming the gravelly tuff or peperino, save that 

 it is much coarser and unstratified. But this is a distinction which 

 might be expected to exist between the material which consolidated 

 at the actual focus, of eruption, and that which was thrown to some 

 distance and was stratified under water along with ordinary 

 sandy sediment. In each case the paste is dull dirty-red, felspathic, 

 gritty or gravelly, derived from the trituration of the fragments im- 

 bedded in it, which are all pieces of different melaphyres, having the 

 same characters as those which form the sheets in the ring. 



As the bedded volcanic rocks of this part of Ayrshire lie upon 

 Carboniferous strata, and are overlaid with Permian, they must be 

 either of Carboniferous or of Permian age. The mode of occurence 

 of the necks affords a presumption that the whole volcanic series 

 must be later than Carboniferous times, and this inference is fully 

 borne out by the relation of the rocks of the encircling ring to the 

 Permian sandstones of the basin. No feature more speedily arrests 

 the attention of a visitor to the valley of the Ayr than the great 

 contrast between the general aspect of the Permian sandstones and 

 those of the Carboniferous series below. The latter are of dull 

 purplish red or grey colours, thin-bedded, interstratified with endless 

 seams of red purple or grey shales and nodrdar marls. The former, 

 on the other hand, are marked out at once by the strange brilliance 



