CHAP. XII 
ARENIG ERUPriONS—SCOTLAND 
201 
During the progress of the Geological Survey along the southern borders 
of the Highlands, a remarkable group of rocks has been observed, intervening 
as a narrow interrupted strip between the schistose masses to the north and 
the great boundary-fault which brings the Old Bed Sandstone in vertical 
strata against them. Between Cortachy in Borfarshire and Stonehaven on 
the east coast, these rocks have been mapped by Mr. G. Barrow, who has 
carefully worked out their relations. They appear again between Callander 
and Loch Lomond, where their extent and structure have been mapped by 
Mr. C. T. Clough. For the purpose of our present inqixiry two chief 
features of interest are presented by these rocks. They include a group 
of sedimentary strata among which occur bands of jasper or chert contain- 
ing radiolaria, and one of their most conspicuous members is a series of 
volcanic rocks consisting chiefly of dolerites and basalts, some of which have 
been much crushed and cleaved, but in which vesicular structures can still 
occasionally be recognized. 
The striking resemblance of both the aqueous and igneous members of 
this marginal strip of rocks along the Highland border to the Arenig cherts 
and their accompanying lavas in the south of Scotland, the remarkable 
association of the same kinds of material in the same order of sequence, the 
occurrence of radiolaria in the siliceous bands in both regions, furnish 
strong presumptive evidence that a strip of Arenig rocks has been wedged 
in against the Highland schists. 
In many respects, these dull green diabasic lavas of the Highland 
border resemble those of the Ayrshire coast. In particular, the same 
peculiar sack-like or pillow-shaped masses are conspicuous in the Forfar- 
shire ravines. As in Ayrshire, igneous materials underlie the cherts 
which are doubled over and repeated by many successive folds. Un- 
fortunately, it is only a narrow strip of these probably Arenig lavas that has 
been preserved, and no trace has been detected of tuffs, agglomerates or 
necks. If, however, we may regard the rocks as truly of Arenig age, they 
furnish interesting additional proof of the wide extent of the earliest Silurian 
volcanoes. The distance between the last Arenig volcanic outcrop in the 
Southern uplands and the band of similar lavas along the margin of the 
Highlands is about 50 miles. If the volcanic ejections were continuous 
across the intervening tract, the total area over which the lavas and tuffs of 
the Arenig volcanoes were distributed must be increased by at least 6000 
square miles in Scotland. 
But it is in the north of Ireland that this northern extension of what 
may probably be regarded as an Arenig series of volcanic rocks attains its 
greatest development. Of this Irish prolongation a brief account is given 
in Chapter xiv., where the whole of the Silurian volcanic rocks of the 
island are discussed. 
