Among the sandstones and shales that cover the limestone, bands of tuff make their appearance, indicating the renewal of volcanic activity. These are immediately surmounted by another thick pile of basalt-sheets. Subsequently, during pauses in the eruptions, while the general subsidence continued, renewed deposits of sediment spread over the submerged volcanic bank. One of the longest periods of quiescence was that during which the coals and even the Index Limestone of Bathgate crept northwards over the sunken lavas and tuffs. But the whole of the ridge does not seem to have disappeared at that time under water, at least these intercalated strata have not been traced across the thick pile of volcanic material near Linlithgow. During the final period of eruption, the outpouring of lava and discharge of ashes, neither in united thickness nor in horizontal extent, equalled those which had preceded them. When the volcanoes ceased their activity, the area continued to sink, and over the submerged lavas marine organisms crowded the sea-floor, so as to build up the Calmy Limestone. After that time volcanic action seems to have become extinct in this region, for no trace of any intercalated lava or tuff has been detected either in the overlying Millstone Grit or in the Coal-measures. The total thickness of rock in the Linlithgowshire volcanic ridge is about 2200 feet. It will probably not be an exaggeration to place the proportion of lava and tuff in that depth of material at nearly 2000 feet.
The northern or Fifeshire district over which lavas were abundantly erupted stretches along the coast from Aberdour to Kirkcaldy and inland to near Lochgelly, as well as seawards to Inchkeith. It may comprise an area of about 30 square miles. In many respects this is the most important locality in Britain for the study of Carboniferous volcanic history. The sea has cut an admirable coast-section in which the structures of the rocks are laid bare. The bottom and top of the whole volcanic series can be seen. The vents and their relation to the lavas and tuffs that were emitted from them may easily be made out; while the interstratification of well-known seams of rock in the Scottish Carboniferous system permits the sequence and chronology of the whole volcanic series to be traced with great clearness.
Most of these features have already been described in foregoing pages, for the district is a typical one for the study of Carboniferous volcanic phenomena. Thus the group of vents about Burntisland has been illustrated by the Binn of Burntisland rising among the bedded lavas and tuffs. The characters of the Carboniferous basalt-sheets have been enumerated, together with their intercalated layers of tuff and bole, and their fine partings of ashy material that was thrown out over the lagoons during the intervals between two outbursts of lava. But it may be of service if I insert here a detailed section of the whole volcanic series as it is displayed along the coast-section between Burntisland and Kinghorn. The lowest intercalated lavas of that section lie a little above the horizon of the Burdiehouse Limestone, and are thus probably rather earlier than those of Linlithgowshire. The highest reach up to the base of the Hurlet Limestone. The volcanic energy manifestly died out here long before it ceased on the south side of the Firth. Yet so vigorous was its activity while it continued, that it piled up one of the thickest masses of volcanic material anywhere to be seen among the puy-eruptions of the British Isles. The following tabular statement of the alternations of material in this great mass in descending order, was drawn up by me on the ground many years ago, before the construction of fortifications and other changes partly concealed the rocks.
Fig. 171.—Columnar basalt, Pettycur, Kinghorn, Fife. (From a photograph taken for the Geological Survey by Mr. R. Lunn.)
Section of the Volcanic Series below the Hurlet or Main Limestone on the Coast of Fife, west of Kinghorn, in descending order[477]
[477] The succession of rocks in this interesting coast-section was briefly given by Dr. P. Neill in his translation of Daubuisson's Basalts of Saxony, Edinburgh, 1814, note f, p. 215. He was secretary of the Wernerian Society, and in his enumeration the Wernerian terminology is used without a hint that any single band in the whole series is of volcanic origin.
75. Reddish and white sandstones.
74. Shale with hard ribs of limestone and ironstone nodules. Fossils abundant.
73. Limestone, crinoidal, 8 or 9 feet.