ANNIVERSAKT ADDRESS OF THE PRESIDENT. 1 23 



commonly appear. The best area for the study of them is the ground 

 which stretches out from the base of the great escarpments of the 

 Campsie, Kilpatrick, and Ayrshire Hills. Among the agglomerate- 

 vents and abundant dykes, intrusive sheets may likewise be observed, 

 where the igneous material has been injected between the bedding- 

 planes of the red sandstones. But these sheets are of comparatively 

 trifling dimensions. Very few of them reach a mile in length, the 

 great majority falling far short of that size. The general absence of 

 sills of porphyiite and olivine-basalt, when we consider how thick a 

 mass of these rocks has been poured out in the plateaux, is not a 

 little remarkable. That the felsitic 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. 



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 carried beneath the 

 water by the continued subsidence of the region. The downward 

 movement may possibly for a time have been accelerated, especially 

 in some districts. Thus we find that from the Campsie Pells into 

 Lanarkshire and Ayrshire the first traceable deposit which overlies 

 the volcanic series is the Hurlet Limestone with its associated strata. 

 This 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 I^orth Ayrshire, it increases to 100 feet, which is the 

 thickest mass of Carboniferous Limestone known to exist in Scot- 

 land. At both of these localities the limestone lies upon a series of 

 volcanic rocks, and we may infer that perhaps 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, for the limestone is extensively 

 quarried all over the country for industrial purposes. It is a 

 crinoidal rock, abounding in many species of corals, brachiopods, 

 lameUibranchs, and gasteropods, with trUobites, 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 



