410 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
Palteozoic formations or in the puy - type which succeeded them. They 
consist in general of short lenticular sheets of andesite or trachyte, like the 
necks and dykes in proximity to which they commonly appear. The best 
area for the study of them is the ground which stretches out from tlie 
base of the great escarpments of the Campsie, Kilpatrick and Ayrshire 
Hills (big. 136), where, among the agglomerate-vents and abundant dykes, 
intrusive sheets have likewise been injected between the bedding-planes of 
the red sandstones. But these sheets are of comparatively trifling dimensions. 
\eiy few of them reach a mile in length, the great majority falling far 
short of that size. In the Cumbraes and in Bute, Mr. Gunn has observed 
Fig. 136.— Trachytic sills, Knockvadie, Kilpatrick Hills. 
a. Upper Old Red Sandstone ; 2. “ Rallagaii Beds ” ^ 3 Tuffs ; 4. L.was of the Plateau ; 5. Agglomerate of necks ; 
6- Irachyte sills ; i. Dolerite ilyko (? Tertiary). 
that the trachytic, olivine-basalt and dolerite dykes are apt to pass into 
intrusive sheets. That the sills, as well as the dykes and bosses of the 
same material, are not of older date than the lavas of the plateaux is proved 
by the manner in which they pierce these lavas, especially towards the 
bottom of the series. The general absence of basic sills, when we consider 
how thick a mass of tliese rocks has sometimes been poured out in the 
plateaux, is not a little remarkable. Only in the basin of the Birth of 
Joith do we encounter thick basic sills near the plateaux, such, for instance, 
as Salisbury Crags at Edinburgh. But it is doubtful wliether they ought 
not rather to be classed with the sills of the puys, to be afterwards 
described. 
Close of the Plateau-ekuptions 
The relative geological date wlien the eruptions of each plateau ceased 
can fortunately be determined with much more precision than the time of 
their beginning. The Hurlet Limestone, so well known as the lowest thick 
calcareous seam in the Carboniferous Limestone series, of which it is 
generally taken as the base, can be identified over the whole of Central 
Scotland, and thus forms an excellent stratigraphical horizon, from which the 
upward termination of the volcanic sheets underneath it can be measured. 
When the volcanic episode of the plateau-eruptions came to an end, 
such banks or cones as rose above the level of the shallow sea which then 
overspread Central Scotland were brought beneath the water, as I have 
already remarked, either by prolonged denudation or more probably in large 
