The Ancient Volcanoes of Great Britain, Volume 1 (of 2) - Archibald Geikie

Arranged in tabular form the stratigraphical and geographical distribution of the two great volcanic types of the Carboniferous system in Scotland will be more easily followed. I have therefore drawn up the accompanying scheme:—

Plateau-type.

Puy-type.

Coal Measures

Millstone Grit

Carboniferous Limestone Series.

Castlecary Limestone

Calmy "

Index "

Hurlet "

Calciferous Sandstone Series.

Burdiehouse Limestone
and Oil-shale Group

Cement-stone Group

Red Sandstones passing down
into Upper Old Red Sandstone

CHAPTER XXIV
CARBONIFEROUS VOLCANIC PLATEAUX OF SCOTLAND

I. The Plateau-type restricted to Scotland—i. Distribution in the Different Areas of Eruption—ii. Nature of the Materials erupted.

In the division of the Plateaux I group all the more copious eruptions during the Carboniferous period, when the fragmentary materials generally formed but a small part of the discharges, but when the lavas were poured out so abundantly and frequently as to form lava-fields sometimes more than 2000 square miles in area, and to build up piles of volcanic material sometimes upwards of 3000 feet in thickness. As already remarked, this phase of volcanic action, especially characteristic of the earlier part of the Carboniferous period across the south of Scotland, but not found elsewhere in the same system in Britain, preceded the type of the Puys. Its eruptions extended from about the close of the Old Red Sandstone period through that section of Carboniferous time which was marked by the deposition of the Calciferous Sandstones, but they entirely ceased before the accumulation of the Main or Hurlet Limestone, at the base of the Carboniferous Limestone Series of Scotland. Its stratigraphical limits, however, are not everywhere the same. In the eastern part of the region, the lavas appear to be intercalated with, and certainly lie directly upon, the Upper Old Red Sandstone containing scales of Bothriolepis and other characteristic fishes, and they are covered by the Cement-stone group of the Calciferous Sandstones. In the western district a considerable thickness of Carboniferous strata sometimes underlies the volcanic sheets. On the other hand, the type of the Puys, although it appeared in Fife, Linlithgowshire and Midlothian during the time of the Calciferous Sandstones, attained its chief development during that of the Carboniferous Limestone, and did not finally die out in Ayrshire until the beginning of the deposition of the Coal-measures.

i. DISTRIBUTION OF THE PLATEAUX

Notwithstanding the effects of many powerful faults and extensive denudation, the general position of the Plateaux and their independence of each other can still be traced. They are entirely confined, as I have said, to the southern half of Scotland (see [Map IV.]). In noting their situations we are once more brought face to face with the remarkable fact, so strikingly manifested in the geological history of Britain, that volcanic action has been apt to recur again and again in or near to the same areas. The Carboniferous volcanic plateaux were poured out from vents, some of which not impossibly rose among the extinct vents of the Old Red Sandstone. Another fact, to which also I have already alluded as partially recognizable in the records of Old Red Sandstone volcanism, now becomes increasingly evident—the tendency of volcanic vents to be opened along lines of valley rather than over tracts of hill. The vents that supplied the materials of the largest of the Carboniferous volcanic plateaux broke forth, like the Old Red Sandstone volcanoes, along the broad Midland Valley of Scotland, between the ridge of the Highlands on the north and that of the Southern Uplands on the south. Others appeared in the long hollow between the southern side of these uplands, and the Cheviot Hills and hills of the Lake District. It is not a question of the rise of volcanic vents merely along lines of fault, but over broad tracts of low ground rather than on the surrounding or neighbouring heights. It can easily be shown that this distribution is not the result of better preservation in the valleys and greater denudation from the higher grounds, for, as has been already remarked in regard to the volcanoes of the Old Red Sandstone, these higher grounds are singularly free from traces of necks which, had any vents ever existed there, would certainly have remained as memorials of them. The following summary of the position and extent of the Plateaux will afford some idea of their general characters:—

Fig. 107.—View of the escarpment of the Clyde Plateau in the Little Cumbrae, from the south-west.

1. The Clyde Plateau.—The chief plateau rises into one of the most conspicuous features in the scenery of Central Scotland. Beginning at Stirling, it forms the tableland of the Fintry, Kilsyth, Campsie and Kilpatrick Hills, stretching westwards to the Clyde near Dumbarton. It rises again on the south side of that river, sweeping southwards into the hilly moorlands which range from Greenock to Ardrossan, and spreading eastwards along the high watershed between Renfrewshire, Ayrshire, and Lanarkshire to Galston and Strathavon. But it is not confined to the mainland, for its prolongation can be traced down the broad expanse of the Firth of Clyde by the islands of Cumbrae to the southern end of Bute, and thence by the east of Arran to Campbeltown in Cantyre. Its visible remnants thus extend for more than 100 miles from north-east to south-west, with a width of some thirty-five miles in the broadest part. We shall probably not exaggerate if we estimate the original extent of this great volcanic area as not less than between 2000 and 3000 square miles.

CHAPTER XXIVCARBONIFEROUS VOLCANIC PLATEAUX OF SCOTLAND

CHAPTER XXIV
CARBONIFEROUS VOLCANIC PLATEAUX OF SCOTLAND