396 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
instances the vent is marked at the surface not by a hill but by a hollow, 
as in the great neck in the heart of the Campsie Fells (Fig. 128). 
As regards their ground-plan, which affords a cross-section of the original 
Fig. 125. — (Iroiuid-plans of double .aiid triple necks iii the Plateau .series, on the .scale of 
6 incliey to a mile. 
A. Banvood Hill and Kavenseraif;, east of Pumbartoii, double vent. B. The Knock Hill, Largs, AvTsliire double 
vent (see Fig. 23). C. Dumbowio and Dumbuck, east of Dumbarton, triple vent. ’ 
volcanic tunnel, the plateau-veiits present considerable variety. The simplest 
cases are those in which the form is approximately circular or somewhat 
elliptical. Here the outline corresponds to the cross-section of a single and 
normal orifice. Some examples of this simple type are given in Fig. 124, 
which represents a group of vents on the edge of the Clyde plateau near Strath- 
blane. The two larger neck.s here .shown are the same which appear in the 
