CHAP. XXV 
LAVAS AND TUFFS OF THE PI^ATEAUX 
393 
Examples of tliis gradation may be seen in many natural sections along 
the flanks of the Ayrshire plateau from above Kilbirnie to Strathavon. 
It is still possible to fix in some quarters the limits beyond wine i 
neitlier the lavas nor the tiifls extended, and thus partially to map out 
the original areas of the plateaux. For example, in certain directions the 
Carboniferous formations can be followed continuously downward below the 
Main Limestone, without the intervention of any volcanic material, or wit i 
only a slight intermixture of fine volcanic lapilli, such as might have been 
■ carried by a strong wind from some neighbouring active vents. Ly this 
kind of evidence and by the proved thinning-oiit of the materials of the 
plateau, we can demonstrate that in the north of Ayrshire the southern 
limits of the great volcanic bank did not pass beyond a line drawn from near 
Ardrossan to Galston. LVe can show, too, that the lavas of the Canipsie 
Fells ended off about a mile beyond Stirling before they reached the line o 
the Ochil heights, and that the couUes which flowed from the Solway vents 
did not quite join with those from the Berwickshire volcanoes. 
Moreover, evidence enough remains to enable us to form a tolerab y 
clear conception of the original average slopes of the surface of some of 
RowantneMU (F04- ff) 
rog ijw.w 
Frdrossaru 
Fki 121,-niagi'aiu illnstrating the Ihiimiiig away southwavas of the lavas of the Clyde 
Plateau hetweeu Largs anil Ardrossan. Length about 10 imles. 
n upper 01,1 Kei,S.rastoae; .. " Andesite lavas ; 
the plateaux. Thus in the great escarpment above Largs and the high 
crrouiid eastward tp Kilbirnie the volcanic series, as already stated, must 
be at least 1500 feet thick. This thick mass of lavas and tuffs thins away 
southwards and probably disappears a short distance south from Ardrossan 
in a space of about ten miles (Fig. 121). The original southward slope of 
the plateau would thus appear to have been about 1 in o5 Again, the 
northward slope of the same plateau may be estimated fropi observations in 
the Campsie Fells. We have seen that above Kilsyth the total depth of 
the volcanic sheets is about 1000 feet, while to the westward it is m 
thicker. From the top of the Meikle Bin (1870 feet) above Kilsyth north- 
eastwards to Caiisewayhead, where the whole volcanic .series has died out, 
is a distance of 12 miles, so that the slope of the surface of erupted materials 
on this side was about 1 in 03 (lig. 122). 
Judmng from the sections exposed along the faces of the escarpments, 
we ma/ infer that the volcanic sheets had a tolerably uniform surface 
wliicli sloped gently away from the chief vents, but with local inequalities 
according to the iregularities of the lava-streams that were heaped up round 
the vents and flowed outward in different directions and to various distances 
