368 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
Other can still be traced. They are entirely confined, as I have said, to the 
southern halt of Scotland (see Ma]) IV.). In noting their situations we are once 
more hi ought face to face with the reinarkahle tact, so stiikingly manifested 
in the geological history of Britain, tliat volcanic action has lieen apt to recur 
again and again in or near to the same areas. The Carboniferous volcanic 
lilateaux were poured out from vents, some ot which not impossibly rose 
among tlie extinct vents of the Old Bed Sandstone. Another fact, to which 
also I have already alluded as partially recognizable in the records of Old Bed 
Sandstone volcanism, now liecomes increasingly evident — the tendency of 
volcanic vents to be opened along lines of valley rather than over tracts of hill, 
dlie vents that supplied the materials of the largest of the Carboniferous 
volcanic plateaux liroke forth, like the Old Bed Sandstone volcanoes, along 
tlie broad Midland Valley of Scotland, between the ridge of the Highlands 
on the north and that of tlie Southern Uplands on the south. Others 
appeared in the long liollow between the southern side of these uplands, 
and the Cheviot Hills and hills ot the Lake District. It is not a question 
of the rise of volcanic vents merely along lines of fault, but over broad 
tracts ot low ground rather than on the surrounding or neighbouring 
heights. It can easily be shown that this distrilmtion 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 
Did Bed Sandstone, these higher grounds are singularly free from traces of 
necks which, had any vents ever existed there, would certainly have re- 
mained as memorials of them. The following summary of the position and 
exteirt ol the Plateaux will afford some idea of their general characters : 
1. The Clyde Plateau. — The chief plateau rises into one of the most 
conspicuous features in the scenery of Central Scotland. Beginning at Stir- 
ling, it forms the table-laud of the Fintry, Kilsyth, Campsie and Kilpatrick- 
Hills, stretching westwards to the Clyde near Dumbarton. It rises again on 
the soutli side of that river, .sweeping southwards into the hilly moorlands 
which range from Greenock to Ardrossan, and spreading eastwards along the 
high watershed between Benfrew'shire, Ayrshire, and Lanarkshire to Galston 
and Strathavon. But it is not confined to the mainland, for its prolongation 
can he traced down the broad expanse of the Firth of Clyde by the iLands 
