Fig. 212.—Small neck in Calciferous Sandstones a little east from the "Rock and Spindle," two and a half miles east from St. Andrews.
The shape of the Fife vents is, as usual, generally circular or oval; but is subject to considerable irregularity. The coast-section between Largo and St. Monans exposes many ground-plans of them, and permits their irregularities to be closely examined. The accompanying figure ([Fig. 211]) exhibits some characteristic forms. Eccentricities of outline no doubt arose from the irregular way in which the rocks yielded to the forces of explosion during the piercing of a volcanic orifice. This is often well shown by the veins and nests of tuff or agglomerate which have been forced into the rents or sinuosities of the orifices. In other cases, however, it is probable that, as among the Ayrshire necks, and those of Carboniferous age already cited, what appears now as one volcanic neck was the result of a shifting of the actual funnel of discharge, so that the neck really represents several closely adjacent vents. The case of Largo Law has been already noticed. The necks at Kellie Law ([Fig. 213]) show clearly the same structure, the Law itself (1) probably consisting of two contiguous vents, while a third (2) forms a smaller cone immediately to the east. Such a slight lateral displacement of the vent has been noticed at many Tertiary and recent volcanic orifices. In the island or peninsula of Volcanello, for example, three craters indicate successive shiftings of the vent, the most perfect of them marking the latest and diminishing phase of volcanic activity ([Fig. 214], compare [Fig. 29, vol. i., p. 70]).
Fig. 213.—Plan of volcanic necks at Kellie Law, east of Fife, on the scale of three inches to one mile.
1, Kellie Law (tuff); 2, Carnbee Law (tuff); 3, 4, 5, small tuff necks; B B, basalt dykes and bosses; c c, coal-seams; l, limestone; f, fault. The arrows mark the dip of the strata through which the necks have been drilled.
Fig. 214.—Plan of the craters in Volcanello, Lipari Islands.
The Fife necks vary from only a few yards up to perhaps 4000 feet in diameter. One of the smallest and most completely exposed occurs on the shore at Newark Castle, near St. Monans. It measures only 60 yards in length by about 37 yards in breadth. A ground-plan of it is given in Fig 224. Still smaller is the neck at Buddo Ness, on the coast east of St. Andrews, which measures only 20 yards across.
From the way in which the vents have been dissected by the sea along the Fife coast, the geologist is enabled to study in minute detail the effects of the volcanic operations upon the strata through which the funnels have been drilled. Considerable variation may be observed in the nature and amount of change. Sometimes the orifice has been made without any noticeable alteration of the sandstones, shales and limestones, which retain their dip and strike up to the very wall of the chimney. Usually there is more or less jumbling and crushing of the stratification, and often a considerable amount of induration. As a typical example of these effects I give a section from the margin of the neck of tuff on the east side of Elie Harbour ([Fig. 215]). Here the sandstones and shales (a) have been doubled over and dragged down against the tuff (b). They have likewise been hardened into a kind of quartzite, and this alteration extends for about 20 to 30 feet from the edge of the neck.
Fig. 215.—Section of the strata at the edge of the volcanic vent on the east side of Elie Harbour.