The Southern Chain—The Pentland Volcano—The Biggar Centre—The Duneaton Centre—The Ayrshire Volcanoes.
We have now to note the leading features of the groups of volcanic rocks distributed along the southern line of vents already described. At least four different centres of eruption may be observed on that line. Their mutual limits are, on the whole, better seen than those of the northern line, for from the north-eastern to the south-western end of the volcanic belt the Old Red Sandstone and rocks of older date are almost continuously exposed at the surface. The encroaching areas of Carboniferous formations in Lanarkshire and Ayrshire interrupt but do not entirely conceal the volcanic tracts.
II. THE SOUTHERN CHAIN OF VOLCANOES IN "LAKE CALEDONIA"
5. The Pentland Volcano
Beginning at the north-east end of the line we first come upon the classic area of the Pentland Hills, for the study of which the geologist is prepared by the admirable description of Charles Maclaren,[363] and the earlier geognostical papers of Jameson.[364] The area mapped in detail is represented in Sheet 32 of the Geological Survey of Scotland, published in 1859, and described in the Memoir accompanying that sheet.
[363] A Sketch of the Geology of Fife and the Lothians, 1839. The detailed descriptions in this work are accompanied with a map and two plates of sections. In the map all the volcanic rocks are represented by one colour. In the sections the bedding of the rocks is shown, and an indication is given of the succession of their chief varieties.
[364] See specially Mem. Wernerian Soc. vol. ii.; also MacKnight in vol. i. The account of the Pentland Hills by Hay Cunningham in vol. vii. (1838) is clear but brief.
When in these early days I surveyed this ground I found it extremely difficult to understand. Being then myself but a beginner in geology, and the study of old volcanic rocks not having yet advanced much beyond its elementary stage, I failed to disentangle the puzzle. Not until after more than twenty years, largely spent in the investigation of volcanic rocks elsewhere, had I an opportunity of resurveying the ground and bringing to its renewed study a wider knowledge of the subject. A new edition of the map was issued in 1892, and I shall here embody in my summary the chief results obtained in the course of this revision.
The most obvious features in the Pentland area are the marked development of the volcanic rocks at the north end of the chain, their rapid diminution and disappearance towards the south-west, the abrupt truncation of the bedded masses by the line of craggy declivity which forms the northern termination of the hills, and lastly, the continuation of the volcanic series northward in a totally different form in the lower eminences of the Braid Hills.
The length of the whole volcanic tract is about eleven miles; its breadth at the widest northern part is four miles, but from that maximum it dwindles southwards and dies out in seven miles. Its western side is in large measure flanked by the unconformable overlap of the Upper Old Red Sandstone and Lower Carboniferous formations, though in some places the base of the volcanic series is seen. The eastern boundary is chiefly formed by a large fault which brings down the Carboniferous rocks against the volcanic ridge. At the northern end, this ridge plunges unconformably under the Upper Old Red Sandstone of the southern outskirts of Edinburgh.