CHAP. .XXVII 
SILLS, BOSSES AND DYKES OF THE BUYS 
447 
A remarkable illustration of the contrast in petrographical 
between the typical sills of the platean.x and those 
of tlie pnys is furnished by the chain of the Campsie 
Fells, where, on the north side, among the Calciferons 
Sandstones which emerge from under the andesitic 
lavas of the Clyde plateau, many intrusive sheets and 
bosses of trachytic material may be seen, while on 
the soirthern side come the great basic sills wdiich, 
from Milngavie by Kilsyth to Stirling, run in the 
Carboniferous Tdmestone aeries (Fig. 1 58). A similar 
contrast may be observed in Kenfrewshire between 
the trachytic sills below the plateau-lavas south of 
Greenock and the basic sills above these lavas in the 
Carboniferous Limestone series around Johnstone and 
Paisley. 
In the second place, the more basic sills, as a 
rule, appear on platforms higher in stratigraphical 
position than the plateaux, and wherever this is 
their position there cannot be any hesitation in de- 
ciding against their association with the older phase 
of volcanic activity. 
In the third place, the basic sills often occur in 
obvious connection with the vents or bedded lavas 
and tuffs of the puy series. A conspicuous example 
of this dependence is supplied by the intrusive sheets 
of Burntisland, underlying the basalts and tuffs of 
that district in tlie immediate neighbourhood of some 
of the vents from which these bedded rocks were 
erupted (Fig. 159). 
In the fourth place, even where no visible vents 
appear now at the surface near the sills, the latter 
generally occupy horizons within the stratigraphical 
range indicated by the interbedded volcanic rocks. 
It must be remembered that all the Carboniferous 
vents were deeply buried under sedimentary deposits, 
and that large as is the number of them which has 
been exposed by denudation, it is probably much 
smaller than the number still concealed from our 
view’. The sills are to be regarded as deep-seated 
parts of the v’olcanic protrusions, and they more 
especially appear at the surface where the strata 
between which they w’ere injected crop out from 
under some of the higher members of the Carbon- 
iferous system. Thus the remark-able group of sills 
lietw’een Kilsyth and Stirling (Fig. 158) may cpiite 
possibly be connected with a group of vents lying 
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