394 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
from them. At the beginning, these fiat volcanic domes were certainly 
subaqueous. While tliey were being formed, continuous .subsidence appears 
to have been in progress. Tint the great thickness of the volcunic accumu- 
lations, as in the Kilpatrick and lienfrewshire areas, and the paucity of 
ordinary sedimentary strata among them, make it not improbable that at least 
their higher piirts rose above the water. Where this was tlie case there 
may have been considerable degradation of the lava-banks before these were 
reduced or were by subsidence submerged beneath the water-level. Evidence 
w StTomndHill {W76Ff) 
Fia. VI2. — Diagraiii illustratiiis; tlie tlumiing away eastw.anls of the lavas of the Clyde 
Plateau iii the Fiiitry Hill.s. Length about ]-2 mile.s. 
1. Upper Old Red Saiiilstoiie ; 2. White .sandstone, blue shales anil cement-stones (“ Ilallogan Beds ”) • 3. Andesite 
sheet, about lOO feet Oiick ; 4. ruffs (2.W feet), with an Included hand of asliy sandstone containing plant- 
remain^ o. Andesite lava.s ; D. Carboniferous Limestone aeries, wliicli to Hie east lies iinniediately on the 
of this -waste is probably to be recognized in the bands of conglomerate, 
occasionally of considerable thickness, which, particularly in some parts of 
Ayrshire, intervene between the top of the volcanic group and the Ilurlet 
Limestone. As I shall have occasion to point out further on, there seems 
to be some amount of evidence in favour of the view tluit a considerable 
interval of time elapsed between the close of the plateau-eruptions and the 
date of that widespread depression which led to the deposition of the Hurlet 
Limestone over the whole of Central Scotland. If such an interval did 
occur it would include a prolonged abrasion of any projecting parts of the 
plateaux, and the prodnetiou and deposition of volcanic conglomerate. 
2. VENTS 
We have now to consider the external forms, internal contents and 
distribution of the vents from which the material of the plateaux was dis- 
charged. In tlie Carboniferous system these interesting relics of former 
volcanoes are far more distinctly defined and better preserved than in older 
geological formations. Moreover, in Scotland, they are laid bare to greater 
advantage, both inland and along the sea-coast, and may indeed be studied 
there as typical illustrations of this kind of geological structure. 
In external form the necks connected both with the plateaux and the 
puys generally rise from the surrounding ground as isolated, rounded, conical 
or dome-shaped prominences, their details of contour depending mainly upon 
the materials of wdiich they consist. When these materials are of agglomer- 
ate, tuff or other readily disintegrated rock, the surface of the domes is 
generally smooth and grass-covered. Where, on the other hand, they consist 
