404 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
(fi) Necks of Composite Character . — lu not a few examples, the vents 
have been filled with agglomerate which has been pierced by a ping or veins 
of lava-form mateiial. Many illustrations of this composite structure may 
be ol)served along the west front of the great escarpments from Fintry to 
Ardrossau (see Figs. 124, 125, 127 and 128). In that region the intruded 
rock is often a dull yellowish or grey trachytic or andesitic material. 
Olivine-basalt is the chief rock intruded in the vents in the Dumbarton 
district. Among the Eoxburghshire vents, where the injected material is 
commonly olivine-basalt or dolerite, it occasionally happens, as in Hubers Law, 
that the uprise of the lava has almost entirely cleared out or concealed the 
agglomerate, and in some of the bosses, where no agglomerate is now to be 
seen, the basalt may have taken its place (Fig. 130). 
The largest and most interesting vents connected with this type of 
Carboniferous volcano, are those which occur within the limits of the 
plateaux, where they are still surrounded with lavas and tuffs that prob- 
ably came out of them. Of these by far the most extensive and remark- 
able lies among the high moorlands of Kenfrewshire between Largs and 
Lochwiunoch, where the ground rises to more than 1700 feet above the 
sea (see Fig. 129). This area, as already remarked, is unfortunately much 
obscured with drift and peat, so that the limits of its rocks cannot be so 
satisfactorily traced as might be desired. I think it probable that several 
successive vents have here been opened close to each other, but their erupted 
ashes probably cannot be distinguished. Over a space measuring about four 
miles in length by two and a half in breadth, the rocks exposed at the 
surface are fine tuffs, breccias and coarse agglomerates, largely made up of 
trachytic, andesitic or felsitic material, and pierced by innumerable protrusions 
of various andesitic, trachytic or felsitic rocks in bosses and veins, as well 
as also by dykes of a more basic kind, such as dolerites and basalts. Some 
of the tuffs present a curiously indurated condition ; and they are frequently 
much decayed at the surface.^ Another large mass of tuff and agglomerate 
lies a little to the south-west of the main area. 
iVfter the explosions ceased, by which the vents were opened and the 
cones of debris were heaped up, heated vapours would in many cases, as in 
modern volcanoes, continue for a long while to ascend in the funnels. The 
experiments of Daubree on the effects of water and vapour upon silicates 
under great pressure and at a low red heat, have shown how gi-eat may 
be the lithological changes thereby superinduced. It is improbable that 
where a mass of tuff and lava, lying deep within a volcanic vent, was 
thoroughly permeated with constantly ascending heated vapours, it should 
escape some kind of change. I am inclined to attribute to this cause the 
frequent conversion of the sandstones round the walls of the vents into 
quartzite. The most remarkable example of metamorpliism within a vent 
which I have observed among the plateaux, occurs in the heart of the 
Campsie Fells, where, instead of forming a prominence, the neck is marked 
^ This tract of ground was mapped for the Geological Survey by Mr. R. L. Jack, now in charge 
of the Geological Survey of Queensland. See Sheet 31, Geological Survey of Scotland. 
