298 
VOLCANOES OF THE LOWER OLD RED SANDSTONE 
BOOK V 
now and then to overlap those from another, but the two series remain 
distinct. 
Reasoning from these data and studying the distribution of the various 
volcanic areas, we are led to recognize the former existence of two parallel 
chains of vents, running along the length of the lake at a distance from 
each other of somewhere about twenty miles. They' may' be conveniently' 
distinguished as the northern and the southern chain. 
Ihe northern band runs from the coast-line near Stonehaven south-west- 
ward through the Sidlaw and Ochil Hills. It is then abruptly truncated by a 
large fault and by the unconformable superposition of the Carboniferous forma- 
tions. But 60 miles further to the south-west, where the Old Bed Sandstone 
comes out on the west side of the Firth of Clyde, a continuation of the volcanic 
band has recently been detected by Mr. W. Gunn of the Geological Survey in 
the Island of Arran. Twenty-five miles still further in the same direction a 
much ampler development of the volcanic rocks occurs to the south of 
Campbeltown in Cantyre. If we cross the 22 miles of sea that separate 
the Argyllshire coast-sections from those of Bed Bay' in Ireland, we find 
near Cushendall a repetition of the Scottish volcanic conglomerates, 
while still further along the same persistent line, some 50 miles into the 
interior, the hills of Tyrone include sheets of lava precisely like those 
of Central Scotland. The total length of this northern chain of volcanoes 
is thus not much less than 250 miles, and as its north-eastern end 
is now cut oft' by the ISTorth Sea it must have been still longer. It ran 
parallel to the north-western coast-line of the lake, at a distance which, 
over the site of the Midland Valley' of Scotland, seems to have varied from 
10 to 20 miles, but which greatly' lessened further to the south-west. 
At a distance of some twenty' miles to the south of the northern belt, the 
second parallel chain of volcanoes ran in a nearly' straight line, which is now 
traceable from the southern suburbs of Edinburgh to the coast of Ayrshire, 
a distance of about 7 5 miles, but as its north-eastern end is concealed 
by Carboniferous formations, and its south-western passes under the sea, its 
true length is probably considerably more. 
If the areas which present evidence of distinct and independent vents 
are grouped according to their positions on these two lines, they naturally 
arrange themselves as in the following list : — 
I. Northern Chain or Volcanoes 
1. The Montrose Centre. 
2. The Sidlaw and Ochil Group. 
3. The Arran and Cantyre Centre. 
4. The Ulster Centres. 
II. Southern Chain of Volcanoes 
5. The Pentland Volcano. 
6. The Biggar Centre. 
7. The Duneaton Centre. 
8. The Ayrshire Group. 
