ANNIVEESARY ADDKESS OP THE PKESlDESTr. 1 55 



forced up the wall of the funnel, while here and there sills run out- 

 ward from the necks into the surrounding Coal Measures. Again, 

 in one of the necks in the Muirkirk Coal-field which was pierced by 

 a mine driven through it from side to side, fingers and sheets of 

 ' white trap,' that is, some highly altered basalt, were found to run 

 out from the neck into the surrounding strata/ Dark heavy basalt 

 or some still more basic rock has here and there filled up a vent. 

 As so many of the necks rise through coal-fields, opportunities 

 are afforded of studying the effects of volcanic action upon the coal- 

 seams, which for some distance from them have been destroyed. 



Another feature which can be recognized from the information 

 obtained in mining operations is that, in the great majority of 

 instances, no connexion can be traced between the positions of the 

 vents and such lines of dislocation as can be traced at the surface or 

 in the underground workings. Some vents, indeed, have evidently 

 had their positions determined by lines of fault, as, for instance, 

 that of the Green Hill below Dalmellicgton. Yet in the same 

 neighbourhood a number of other examples may be found where the 

 volcanic funnels seem to have avoided faults, though these exist 

 close to them. 



In regard to the distribution of the vents in Scotland, two distinct 

 aggregations of them may be recognized. The first and most 

 important of these embraces the Ayrshire, Nithsdale, and Annandale 

 districts in the west ; the second lies in the basin of the Firth of 

 Forth, far to the east. 



In the western district upwards of 60 distinct vents have been 

 mapped in the course of the Geological Survey. They run from the 

 north of Ayrshire to the foot of the Southern Uplands, and descend 

 for some distance the vale of the Mth. The area over which 

 they are distributed measures roughly about 40 miles from north- 

 west to south-east, and at its greatest breadth 20 miles from south- 

 west to north-east. But within this tract the vents are scattered 

 somewhat sporadically in groups, sometimes numbering as many as 

 twenty necks in a space of sixteen square miles, as in the remark- 

 able district of Dalmellington. 



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 Eed Sandstone and Carboniferous 

 periods. But it is displayed by the Permian volcanoes in a still 

 ^ Explanation of Sheet 23, Geol. Surv. Scotland, p. 39. 



