468 
SUMMARY A AW GENERAL DEDUCTIONS 
BOOK VIII 
again and again as lines of escape for volcanic energy, while close to them 
other portions of greater solidity have been persistently left intact. 
4. The sites of volcanic vents in all the geological systems wherein they 
occur in Britain have not usually been determined by any obvious structure 
in the rocks now visible. They comparatively seldom depend on ascertain- 
able lines of fault, even when faults, probably already existent, occur in their 
near neighbourhood. This independence, to which, however, there are 
occasional marked exceptions, comes out more particularly in the coal-fields 
pierced by vents, for mining operations have there revealed the positions of 
many more faults than can be traced at the surface. If the sites of the 
vents have been fixed by dislocations or lines of weakness in the terrestrial 
crust, these must generally lie below the formations now visible at the surface. 
There is one striking connection between the sites of the vents and 
ancient topographical features to which frequent reference has been made in 
the foregoing chapters. All through the long volcanic history, 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 rather 
than on ridges and hills. The great central hollow of the Scottish midlands 
was a depression even as long ago as the time of the Lower Old Bed Sand- 
stone, 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, 40 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 western Europe 
during the latter half of Palaeozoic time. The vents of the Old Eed 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 hollow that 
lay between the ridge of the Outer Hebrides and the mainland of Scotland. 
This wide and long tract of low ground was buried under upwards of 3000 
feet ol lava and tuff, but these materials were erupted from fissures and vents 
within its own border and not from the mountains on either side. 
But perhaps the most conspicuous example of any in which the vents keep 
to the valleys is that supplied by the Permian necks of Uithsdale and the 
neighbouring glens. These depressions are as old as Permian, and even as 
Carboniferous time, but they appear to he entirely hollows of erosion ; at 
least they have yielded no evidence that their direction has been determined 
by lines of fault. The chain of vents can be followed from the lowlands of 
Ayrshire up to the base of the Southern Uplands, down the wide valley cut 
by the Nith in these hills and up some of the tributary valleys, and though 
the volcanoes continued for some time in vigorous eruption, not a trace of 
any contemporary vent has yet been met with on the surrounding hills. 
While the position of volcanic vents in lines of valley may be generally 
due to guiding lines of fissure in the crust underneath, either within or below 
the rocks visible at the surface, there may sometimes be conditions in which 
other dominant causes come into play. The curious coincidence between 
