CHAPTER XVII 
DISTRIBUTION OF THE VOLCANIC CENTRES IN THE LOWER. OLD RED SAND- 
STONE CHARACTERS OF THE MATERIALS ERUPTED BY THE VOLCANOES 
i. DISTRIBUTION OF VOLCANIC CENTRES 
The ami within whicli volcanic rocks belonging to the Lower Old Red Sand- 
stone appear is one of the most extensive regions over which the volcanic 
eruptions of any geological period can be traced in the British Isles (Map I.). 
Its northern limit reaches as far as the islet of Uya in Shetland, and its 
southern appears in England in the Cheviot Hills — a distance of about 
250 miles. But volcanic rocks of probably corresponding age occur even 
as i'ar to the south as the hills near Killarney. The most easterly 
margin of this area is defined by the Xorth Sea on the coast of Berwick- 
shire, and its extreme western boundary extends to near Lough Erne in 
the north of Ireland — a distance of some 230 miles. If w'e include the post- 
Silurian bosses and dykes, like those of Shap, and likewise the Devonian 
volcanic rocks of Devon and Cornwall, as contemporaneous with those of 
the Old Bed Sandstone, the area of eruption will be greatly enlarged. 
But leaving these out of account for the present, and confining our atten- 
tion to the Lower Old Bed Sandstone series, we find that, within the wide 
limits over which the volcanic rocks are distributed, a number of distinct 
and often widely separated centres of eruption may be traced. Taking 
these as they lie from north to south, we may specially enumerate the 
following : — 
1. Tlie Shetland and Orkney Islands, together with the basin of the 
^loray Firth. This region includes several distinct volcanic groups, of 
which the most northerly extends through the centre to the north-w'estern 
headlands of the mainland of Shetland, another lies in the island of Shapin- 
shay, one of the Orkneys, while at least two can be recognized on the south 
side of the Moray Firth. To this wide region of Old Bed Sandstone I 
have given the general designation ol “ Lake Orcadie. 
2. The basin of Lome, on the west of the mainland of Argyllshire, 
extendino- from Loch Oreran to Loch Melfort and the hills on the west side 
of Loch Awe. 
1 2'mns. Iluy. !^oc. Edin. vol. xxviii. (1878), p. 354. 
