CHAP. XXV 
PLATEAU-VENTS IN SCOTLAND 
401 
-4 
great vent of the Campsie Hills, with the minor adjacent necks of Dungoil, 
Bin Bairn, and the Meikle Bin. 
The diagram in Fig 129 is meant to convey in a 
general way what appears to be the structure of the 
central vent of the Eenfrewshire plateaux, to be after- 
wards referred to. But, as already mentioned, the limits 
of the various rocks are too much obscured to allow an 
accurate delineation to be given of their areas and rela- 
tions to each other. The Berwickshire plateau supplies 
abundant intei’esting examples of tuff necks which rise 
through the Old Bed Sandstone many miles distant from 
the edge of the lavas. This structure is shown in 
Fig. IdO. 
Indications may occasionally be observed of an 
agglomerate vent having been first occupied by one kind 
of material and then, after being in great measure cleared 
out by explosions, having been subsequently filled up 
with another. As an example of this structure I may 
cite again the double neck of the Knock Hill a little 
to the north of Largs, of which the outline is shown 
in Fig. 23, and the ground-plan in Fig. 125, B. This 
hill rises from the red sandstone slopes that front the 
great Ayrshire plateau and forms a conspicuous cone 
the top of which is rather more than 700 feet above the 
sea. Its summit commands a remarkably extensive and 
interesting panorama of the scenery of the Clyde, but 
to the geologist pei'haps the most striking feature in the 
landscape is the range of terraced hills behind, mounting 
up into the great vents of the Eenfrewshire uplands. 
On these declivities the successive lava-streams that have 
built up the plateau can be seen piled over each other 
for a tliickness of more than 1000 feet, and presenting 
their escarpments as parallel lines of brown crag with 
green slopes between. 
The Knock has had its upper part artificially dressed, 
for lines of trench have been cut out of its rocks by some 
early race that converted the summit of the hill into a 
strongly intrenched camp. From the apex of the cone 
the ground falls rapidly westward into a hollow, beyond 
which rises a lower rounded ridge of similar materials. 
It is possible that this western ridge may really form 
part of the main hill, but the grass-covered ground does 
not afford sufficient exposures of the rocks to settle this 
point. From the contours of the surface, it may be 
inferred that there are two closely adjacent vents, and 
that the western and lower eminence is the older of the two. This 
VOL. I 
Fig. 130. — Section across Southern Berwickshire to show the relation of the volcanic plateau to the vents lying south from it. 
Upper Silurian strata ; 2. Upper Old Red yandstone ; 3. The volcanic plateau ; 4. Agglomerate and tuff of the vents ; 5. Basalt and dolerite ; 0. Lower Carboniferous strata. 
