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.[367] This dislocation abruptly terminates the Biggar volcanic band in a south-westerly direction, after extending for a length of 26 miles, with a breadth of sometimes as much as five miles.
[367] See Explanation to Sheet 23 of the Geological Survey of Scotland (1873), p. 15. This ground was mapped and described by Mr. B. N. Peach.
7. The Duneaton Centre
Among the high bleak 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.[368] Its boundaries, however, cannot be satisfactorily 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 lower members are seen lying interstratified among the sandstones and conglomerates 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.
[368] This area was mapped by Mr. B. N. Peach in Sheet 15 of the Geological Survey of Scotland, and is described by him in the accompanying Memoir.
The volcanic series in this limited district reaches an estimated thickness of 4000 feet, built up of purple and green slaggy andesites, dark heavy diabases (melaphyres) and tuffs, with abundant interstratification of sandstone, especially towards the base. One of its chief features of interest is the manner in which it exhibits, better, perhaps, than can be found in any of the other volcanic areas, the frequent and rapid alternations of lavas and tuffs with sandstone and conglomerate. In this part of the region the volcanic discharges were obviously frequent and intermittent, while at the same time the transport and deposition of sediment were continuous. This sediment consisted largely, indeed, of volcanic detritus mixed with ordinary sand and silt. That these conditions of sedimentation were not wholly inimical to animal life is shown by the occasional occurrence of worm-burrows in the ashy sandstones.[369]
[369] Memoir on Sheet 15 Geol. Surv. Scotland (1871), p. 22.
Fig. 94.—Section across the Duneaton volcanic district from the head of the Duneaton Water to Kirklea Hill.
1. Silurian strata; 2. Lower Old Red Sandstone and conglomerate; 3. Coarse conglomerate; 4. Andesite-lavas; 5. Stratified tuffs; 6. Spango granite; 7. Upper Old Red Sandstone.
The thick accumulation of sandstones and conglomerates above the main mass of lavas has been derived almost wholly from the waste of the volcanic rocks (3). Blocks of andesite, well rounded and often from six to twelve inches in diameter, may be seen in the remarkable band of coarse conglomerate which runs as a nearly continuous ridge from the Nith to the Clyde—a distance of more than twenty miles. Nothing impresses the geologist more, as he wanders over this district, than the evidence of the prodigious waste which the volcanic series underwent before it was finally buried. Some part of the detritus may have been supplied, indeed, by occasional discharges of fragmental matter, as has already been suggested in the case of the Ochil and Montrose conglomerates. But the nature of the pebbles in these masses of ancient shingle shows them to be not bombs, but pieces worn away from sheets of lava.