CHAP. XX 
THE DUNEATON ERUPTIVE CENTRE 
329 
lie near the base of the Old Ked Sandstone, and if we could bore among the 
overlying andesites we should probably meet with the Upper Silurian shales 
among or conformably beneath the red passage-beds, as in the Lesmahagow 
district. 
The andesitic lavas creep over the upturned denuded edges of these 
strata and sweep round the flanks of Tinto. This conspicuous hill reaches 
a height of 2335 feet above the sea, and consists of the felsitic rocks already 
described (p. 278). Seen from many points of view it rises as a graceful cone, 
distinguislied from all the other eminences around it by the pinkish colour of 
its screes. Ui reality it forms a continuous ridge which runs in an east and 
west dii’ection for about five and a half miles, with a l)readth of about a 
mile. Some part at least, and possibly the whole of this oblong mass, is in 
the form of a sill or laccolite which dips towards the north. Conglomerates 
and sandstones plunge undei' it on the southern side, and similar sandstones 
overlie it on the north. If there be a neck in this mass, as one might infer 
from the shape of the hill, its precise limits are concealed. The rock does 
not break through the andesites, and may l)elong to an earlier period of 
eruptivity than the lavas immediately around it. There were othei’, though 
smaller, vents in the immediate neighbourhood. Besides the cone of 
Quothquan just referred to, another may be marked by the felsite boss 
which overlooks the village of Douglas, four miles to the south-west of the 
Tinto ridge, while a third rises into a low rounded hill close to the village 
of Symington. 
The lavas spread out again to the south-west of Tinto in a group of 
hills, until they are interrupted by a fault which brings in the Douglas 
coal-field.^ This dislocation abruptly terminates the Biggar volcanic band 
in a south-westerly direction, after extending for a length of 2 6 miles, with 
a breadth of sometimes as much as five miles. 
7. The Dimeaton Centre 
Among the high lileak muirlands on the confines of the three counties of 
Lanark, Ayr and Dumfries, traversed by the Duneaton Water, a distinct 
volcanic area may be traced.’^ Its boundaiies, however, cannot be satis- 
factorily fixed. It is overspread with Carboniferous rocks both to the 
north-east and south-west, so that its rocks are only visible along a strip 
about seven miles long and two miles broad. On the north-western side its 
low’er members are seen lying iuterstratified among the sandstones and con- 
glomerates which thence pass down conformably into the Upper Silurian 
series (Fig. 94). But although we thus get below the volcanic series we meet 
with no vents or sills among the lower rocks. On the south-east side the 
highest lavas and tuffs are overlain by some 5000 feet of red sandstones and 
conglomerates (2, 3), which completely bury all traces of the volcanic history. 
1 See Explanation to Sheet 23 of the Geological Siu-vey of Scotland (1873), p. 15. This 
gvonnd was mapped and described by Mr. B. N. Beach. 
^ This area was mapjjed by Mr. B. N.Peaoh in Sheet 15 of the Geological Surv'ey of Scotland, 
and is described by him in the accompanying Memoir. 
