3. Black shales.
2. Limestone (Burdiehouse).
1. Sandstones, shales and thin limestones forming the strata at Burntisland through which the sills of that district have been injected ([Fig. 159]).
The phenomena of sills are abundantly developed among the Carboniferous rocks of the basin of the Firth of Forth, and some of the more remarkable examples in this district have been already cited. Taking now a general survey of this part of the volcanic history, I may observe that if the sills are for a moment considered simply as they appear at the surface, and apart from the geological horizons on which they lie, they form a wide ring surrounding the Falkirk and Stirlingshire coal-field.
Beginning at the Abbey Craig, near Stirling, we may trace this ring as a continuous belt of high ground from Stirling to the River Carron. Thence it splits up into minor masses in different portions of the Carboniferous system, and doubtless belonging to different periods of volcanic disturbance, but yet sweeping as a whole across the north-eastern part of the Clyde coal-field, and then circling round into Stirlingshire and Linlithgowshire. There are no visible masses to fill up the portion of the ring back to Abbey Craig. But through Linlithgowshire and the west of Edinburghshire a number of intrusive sheets form an eastward prolongation of the ring. Large as some of these sheets are at the surface, for they sometimes exceed two or three square miles in area, a much larger portion of their mass is generally concealed below ground. Mining operations, for example, have proved that in the south-east of Linlithgowshire areas of intrusive rock which appear as detached bosses or bands at the surface are connected underneath as portions of one continuous sill, which must be several square miles in extent.
Fig. 172.—Section across the Fife band of Sills.
1. Upper Old Red Sandstone; 2. Calciferous Sandstones; 3. Carboniferous Limestone series; 4. Millstone Grit; 5. Coal-measures; 6. Dolerite Sills. f, Fault.
But it is in Fife that the sills reach their greatest development among the Carboniferous rocks of Scotland ([Fig. 172]). A nearly continuous belt of them runs from the Cult Hill near Saline on the west, to near St. Andrews on the east, a distance of about 35 miles. This remarkable band is connected with a less extensive one, which extends from Torryburn on the west, to near Kirkcaldy on the east. In two districts of the Fife region of sills, a connection seems to be traceable between the intrusive sheets and volcanic vents, at least groups of necks are found in the midst of the sills. One of these districts is that of the Saline Hills already described, the other is that of Burntisland. In the latter case the evidence is especially striking, for the vents are connected above with bedded lavas and tuffs, while below lie three well-marked sills ([Fig. 159]).
It is certainly worthy of remark that sills are generally absent from those areas where no traces of contemporaneous volcanic activity are to be found. No contrast in this respect can be stronger than that between the ground to the east and west of the old axis of the Pentland Hills. In the western district, where the puys are so well displayed, sills abound, but in the eastern tract both disappear.
Another question of importance in dealing with the history of these sills is their stratigraphical position. By far the larger proportion of them lies in the Carboniferous Limestone series. Thus the great sill between Stirling and Kilsyth keeps among the lower parts of that series. On the same general horizon are the vast sheets of dolerite which stretch through Fife in the chain of the Cult, Cleish, and Lomond Hills on the one side, and in the eminences from Torryburn to Kinghorn on the other, though the intrusive material sometimes descends almost to the Old Red Sandstone. In Linlithgowshire and Edinburghshire, as well as in the south of Fife, the sills traverse the Calciferous Sandstone groups.