Fig. 166.—Dyke cutting the agglomerate of a neck. Binn of Burntisland.
While one is struck with the great size and extent of some of the sills connected with the puys, as compared with the small and local sheets underneath the plateaux, there is a further fact regarding them that deserves remark—their capricious distribution. Their occurrence seems to have little or no relation to the measure of volcanic energy as manifested in superficial eruptions. Thus in the north of Ayrshire, where a long band of lavas and tuffs, pointing to vigorous activity, lies at the top of the Carboniferous Limestone series, and where the strata underneath it are abundantly exposed at the surface, the sills occur as thin and inconstant bands in the central and eastern parts of the district only. The bedded lavas and tuffs at the head of the Slitrig Water have no visible accompaniment of sills. On the other hand, in the Edinburgh and Burntisland districts, the sills bear a large proportion to the amount of bedded lavas and tuffs, while in the Bathgate and Linlithgow district, where the superficial eruptions were especially vigorous and prolonged, the sills are of trifling extent.
It would seem from these facts that the extent to which the crust of the earth round a volcanic orifice is injected with molten rock, in the form of intrusive sheets between the strata, does not depend upon the energy of the volcano as gauged by its superficial outpourings, but on other considerations not quite apparent. Possibly, the more effectively volcanic energy succeeded in expelling materials from the vent, the less opportunity was afforded for subterranean injections. And if the protrusion of the sills took place after the vents were solidly sealed up with agglomerate or lava, it would doubtless often be easier for the impelled magma to open a way for itself laterally between the bedding-planes of the strata than vertically through the thick solid crust. The size and extent of the sills may thus be a record of the intensity of this latest phase of the volcanic eruptions.
Fig. 167.—Boss of diabase cutting the Burdiehouse Limestone and sending sills and veins into the overlying shales. Railway cutting, West Quarry, East Calder, Midlothian.
1. Burdiehouse Limestone; 2. Shales; 3. Diabase.
Bosses.—The rounded, oval or irregularly shaped masses of igneous rock included under this head are found in some cases to be only denuded domes of sills, as, for example, in the apparently isolated patches in the oil-shale district of Linlithgowshire, which have been found to unite under ground. (Compare [Fig. 157]). In other instances, bosses possibly, or almost certainly, mark the position of volcanic funnels, as at the Castle Rock of Edinburgh, Dunearn Hill, Burntisland, and Galabraes, near Bathgate. But many examples occur which can only be regarded as the exposed ends of irregular bodies of molten material which has been protruded upwards into the Carboniferous formations. The area between Edinburgh and Linlithgow and the hills of the north of Fife furnish many examples.
Fig. 168.—Side of columnar basalt-dyke in the same agglomerate as in [Fig. 166].