CHAPTER XX 
VOLCANOES OF THE LOWER OLD BED SANDSTONE OF “ LAKE 
CALEDONIA ” — Continued 
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 Eed 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 
0 . 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 Maelaren,^ and the earlier 
geognostical papers of Jameson.^ 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. 
When in these earlj^ 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. Lot until after more 
than twenty years, largely spent in the investigation of volcanic rocks else- 
where, had I an opportunity of resurveying the ground and bringing to its 
' J. Sketch of the Ucolocjy of Fife and the Lothians, 1839. The detailed descriptious hi tins 
work are aceonipained 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. 
•■i 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. 
