CHAP. XXXI 
AYRSHIRE , NITHSDALE AND ANNANDALE 
65 
finders and sheets of “ white trap,” or highly altered basalt, were found to run 
out from the neck into the surrounding strata . 1 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 the coal-fields, opportunities are afforded of study mg 
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 connection 
is traceable between the positions of the vents and such lines of dislocation 
as can be detected 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 Dalmellington. 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 this south-western district of Scotland upwards of sixty 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 Nith. The area over which they are distributed 
measures roughly about forty miles from north-west to south-east, and at 
its greatest breadth twenty miles from south-west to north-east. Within 
this tract the vents are scattered somewhat sporadically in groups, some- 
times numbering twenty nfcks in a space of sixteen square miles, as m 
the remarkable district ol Dalmellington. 
In considering their distribution we cannot but be impressed by the 
striking manner in which these necks keep to the valleys and low ground a. 
I have already alluded to this characteristic, as shown by the volcanoes ot 
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 in- 
dividuals 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 Carbon- 
iferous 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 m 
these depressions. They ascend the valley of Muirkirk, and may be met 
with even at its very head, near the base of the Hagsliaw 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 
outlyhm fragments of bedded lava show that these vents flooded the bottom 
of that°valley with molten rock. Turning out of Nithsdale into the long 
i Explanation of Sheet 23, Geol. Surv. Scotland, p. 39. 
VOL. II 
