62 
THE PERMIAN VOLCANOES 
BOOK VII 
while in the Nithsdale and Annandale ground the detached and much 
denuded areas show a still thinner development. 
(2) Vents . — One of the most interesting features in this south-western 
district of Scotland is the admirable way in which the volcanic vents of 
Permian time have been preserved. Their connection with the lavas and 
tuffs can there be so clearly traced that they serve as a guide in the inter- 
pretation of other groups of vents in districts where no such connection now 
remains. In Ayrshire, the lower part of the Permian volcanic band is 
pierced by several small necks of agglomerate. There cannot, I think, be 
any doubt that these necks mark the positions of some of the vents from 
which the later eruptions took place. Immediately beyond them necks of 
precisely similar character rise through the upper division of the Coal- 
Fig. 203. — The Green Hill, Waterside, Dalmellington, from the south ; a tuff-neck of Permian age. 
measures. There can be as little hesitation in placing these also among 
the Permian vents. And thus step by step we are led away from the 
central lavas, through groups of necks preserving still the same features, 
external and internal, and rising indifferently through rocks of any 
geological age from the Coal-measures backward. Thus, although if we 
began the investigation at the outer limits of the chain of necks, we might 
well hesitate as to their age, yet, when we can fix their geological position 
in one central area, we are, I think, justified in classing, as parts of one 
geologically synchronous series, all the connected groups that retain the 
same general characteristics. It is to denudation that we owe their having 
been laid bare to view ; but at the same time, denudation has removed the 
sheet of ejected materials which may have originally connected most of 
these vents together. 
In this regard, it is most instructive to follow the vents south-eastwards 
from the Ayrshire basin into Nitlisdale for a distance of some eighteen miles. 
If we traced them down that valley to Sanquhar, without meeting with any 
vestige of superficial outflows to mark their stratigraphical position, we 
might possibly hesitate whether the age of those which are so far removed 
from the evidence that would fix it should not be left in doubt. But if we 
continued our traverse only a few hundred yards farther, we should find 
