172 PEOCEEDINGS Or THE GEOLOGICAL SOt^IEir. 



of Ireland, volcanic rocks are absent except in the Limerick area, 

 and over England in the same formations they are quite as rare. 



4. The sites of volcanic vents in all the geological systems 

 "wherein they occur have not usually been determined by any obvious 

 structure in the rocks now visible. They comparatively seldom 

 depend on ascertainable lines of fault, even when faults, probably 

 already existent, occur in their near neighbourhood. This inde- 

 pendence, to which, however, there are occasional marked exceptions, 

 comes out more particularly in the coal-fields pierced by vents, for 

 there mining operations have revealed the positions of many more 

 faults than can be traced at the surface. It may, of course, be asserted 

 that the sites of the vents have been fixed by dislocations or lines 

 of weakness in the terrestrial crust below the formations now visible 

 at the surface. Such an assertion, however, though possibly well 

 founded, does not seem capable of proof. 



But there is one striking connexion between the sites of these 

 vents and ancient topographical features to which I have several 

 times adverted. All through the long volcanic histor}^, as far back 

 as such features can be traced, we see that orifices of discharge 

 for the erupted materials have been opened along low grounds and 

 valleys. The great central hollow of the Scottish midlands was 

 a depression even as long ago as the time of the Lower Old Red 

 Sandstone, and though it has probably been several times since then 

 filled up, and more or less completely effaced, its ancient features 

 have been partially revealed by extensive denudation. This vast 

 depression, forty miles broad, between the Highland mountains on 

 the one side and the Southern Uplands on the other, was the chief 

 centre of volcanic activity in our region during the latter half of 

 Palaeozoic time. As I have shown, the vents of the Old Red Sand- 

 stone, Carboniferous, and Permian series are scattered all over it, 

 but few or none of them are to be found on the high grounds that 

 bound it. Again, in Tertiary time, the great outpouring of lava 

 took place in the long hollow that lay between the ridge of the 

 Outer Hebrides and the mainland of Scotland. But the most 

 striking example of the way in which the vents keep to the valleys 

 is that supplied by the Permian necks of Nithsdale and the neigh- 

 bouring valleys. These depressions are as old as Permian, and even 

 as Carboniferous time, but they appear to be entirely hollows of 

 erosion, for they have yielded no evidence that their direction has 

 been determined by lines of fault. 



5. Looking at the broader features of volcanic action during the 



