CHAPTER XXV 
GEOLOGICAL STEUCTUKE OF THE CAE150NIFEE0US A'OLCANIC 
PLATEAUX OF SCOTLAXH 
1. Bedded Lavas and Tuffs; Upper Limits and Original Areas and Slopes of tlie 
Plateaux- 2. Vents; Necks of Agglomerate and Tnff ; Necks of Massive Rock, 
Composite Necks ; 3. Dykes and Sills; 4. Close of tl.e Plateau-eruptions. 
The structure of the various plateaux presents a general smiilarity. with many 
local variations. Each plateau is built up entirely, or almost entirely, of sheets 
of volcanic material, the intercalations of ordinary sedimentary layers being, 
for the most part, few and uniniportant, and usually occurring either towards 
the base or the top of the volcanic series, though at a few localities inter- 
stratifications of shale and sandstone, marking pauses in the eruptions, 
occur throughout that series. The vents of eruption are in some instances 
still to be recognized on the plateaux themselves. More usually they occur 
on the lower ground flanking the volcanic escarpments, where they have 
been laid bare by denudation. Dykes, tliough seldom abundant, are asso- 
ciated with the plateaux, while the sills which may mark the latest mani- 
festations of volcanic energy, though not developed on so large a scale as 
among the Cambrian and Silurian volcanoes, can nevertheless be distinctly 
rBCo^^nizcd. 
It is a question of some interest to determine the geological date of the 
commencement of the plateau-eruptions by fixing the precise stratigraphical 
horizon on which the base of the volcanic series rests. I have already 
referred to the fact that this base does not always he on the same platlorm 
among the Lower Carboniferous formations. In Eerwickshire, as above 
mentioned, the earliest eruptions appear to have taken place before the 
close of the Upper Old Bed Sandstone period, ihese are the earliest of 
the whole series. In Cantyre, the lowest lavas and tuffs come directly upon 
the sandstones, marls and cornstones of the Upper Old Red Sanclstoim. In 
Stirliimshire, llenfrewshire and Ayrshire several hundred leet of the Cenieiit- 
stone o-roup are sometimes interposed between the bottom of the volcanic 
rocks mid the top of the Old Bed Sandstone. Tliis divergence doubtless 
indicates that the eruptions began earlier in some districts than in others 
But there were also probably unequal terrestrial movements preceding, and 
