ANNIVERSARY ADDRESS OF THE PRESIDENT. .I53 



the Permian sandstones of the Lochmaben basin of Annandale, for 

 breccias largely made up of pieces of the bedded lavas are found 

 close to the northern edge of the basin on the west side of the 

 E-iver Annan. To this remarkable adherence of the lavas and 

 tuffs to the bottom of the Permian valleys I shall afterwards more 

 specially refer. 



The thickness of the whole volcanic group cannot be very accu- 

 rately determined. It reaches a maximum in the Ayrshire basin, 

 where, at its greatest, it probably. does not exceed 500 feet, but is 

 generally much less ; while in the Mthsdale.and Annandale ground 

 the detached and much denuded areas show a still thinner develop- 

 ment. 



2. Vents. — We have now to consider the necks connected with the 

 lavas and tuffs here described, and extending far beyond these into 

 distant districts of Central Scotland. 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 erup- 

 tions took place. Immediately beyond them necks of precisely 

 similar character rise through the upper division of the Coal 

 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 

 this 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 all the connected groups that 

 retain the same general characteristics as parts of one geologically 

 synchronous series. 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 

 Nithsdale for a distance of some eighteen miles. If we traced 

 them down that valley to Sanquhar, without meeting with any ves- 

 tige of superficial outflows to mark their stratigraphical position, we 

 might possibly hesitate whether the ago 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 



