CHAP. XXV 
CLOSE OF THE PLATEAU-ERUPTIONS 
411 
part by the continued subsidence of the region. The downward movement 
may possibly for a time have been accelerated, especially in some districts. 
Thus the Hnrlet Limestone, though usually not more than five or six feet 
thick, increases locally to a much greater thickness. At Petersfield, near 
Bathgate, for example, it is between 70 and 80 feet in depth, while at 
Beith, in North Ayrshire, it increases to 100 feet (Fig. 137), which is the 
thickest mass of Carboniferous Limestone known to exist in Scotland. At 
' 5 
ri(!. 137. — Section across tlie wlge of the Cly<le ])liitean, sontli-cast of Beitli. 
1 Plateau-lavas: Tuffs and volcanic cotiglunierates ; 3. Hnrlp.t Limestone ; 4. Cual-beaniig strata above the 
limestone ; 1). Dolerite dyke, 
both of these localities the limestone lies upon a series of volcanic rocks, 
and we may perhaps infer that the subsidence advanced there somewhat 
more rapidly or to a greater extent, so as to form hollows in which the 
limestone could gather to an abnormal depth. The water would appear to 
have become for a time tolerably free from mechanical sediment. The 
limestone is hence comparatively pure, and is extensively quarried all over 
the country for industrial purposes. Tt is a crinoidal rock, abounding in 
many species of corals, brachiopods, lamellibranchs, and gasteropods, with 
trilobites, cephalopods, and fishes. 
A variable thickness of strata intervenes between the top of the volcanic 
series and the Main Limestone. Sometimes these deposits consist in large 
measure of a mixture of ordinary sandy and muddy material with the 
washed-down tuff of the cones, and probably with volcanic dust and lapilli 
NW. 
Fig. 138.— Section across tlie upper part of the Clyde plateau at Kilbiruie, Ayrshire. 
1 1 Plateau-lavas ; 2 2. Tuffs ; 3 3. Ilurlet Lime.stone ; 4 lilack-banil Ironstone. //. Fanils. 
thrown out by the latest eruptions. Thus along the ilaidv of the hills from 
Barrhead to Strathavon, yellow and green ashy sandstones, giits and con- 
glomerates are succeeded by oi'chnary sandstones, black shales and ironstones, 
while here and there true volcanic tuff and conglomerate make their appear- 
ance.^ Further w^est, in the Kilbirnie district, the limestone lies directly on 
the tuffs that rest upon the andesites (Figs. 137, 138). 
“ Explanation of Sheet 22, Gml. Sun-. Scotland, i'. 12. 
