The vents from which the volcanic materials were ejected, so far as they are now to be observed at the surface, may be divided into two groups, one lying to the north, the other to the south of the Firth of Forth. The northern or Fife group may be followed over an area 15 miles long, and about three miles broad. Some fifteen separate vents may be recognized in it, distributed chiefly at the two ends of the belt, a cluster of about six rising around Burntisland, while another of nearly as many appears at Saline. The characters of some of these necks have been already given in the foregoing pages.
The southern or West Lothian group includes about a dozen vents which are scattered over an area of some 60 square miles, extending from the coast-line between Borrowstounness and Queensferry southwards to Bathgate and Uphall. In this group Binns Hill, a mile long by almost half a mile broad, and rising to a height of nearly 300 feet above the sea, forms the most prominent individual. But the vents are generally smaller in the southern than in the northern group.
The manner in which the vents have been left filled with volcanic material has been described in previous pages. Most of them are occupied by tuff or agglomerate. In many cases the neck consists entirely of pyroclastic detritus, as in most of the vents of eastern Linlithgowshire and many of those in Fife. Not infrequently a column of basalt has risen in the funnel and solidified there, as exemplified by Binns Hill and Saline Hill, or the basalt has filled rents in the tuff and now appears in dykes like those on the Binn of Burntisland (Figs. [148], [149], [159], [166], [168]).
But it is possible that in some cases vents may be represented by bosses of basalt or dolerite, unaccompanied by any agglomerate or tuff. Perhaps the best example of this suggested origin is supplied by Galabraes Hill, which rises through the Hurlet limestone and the volcanic series of the Bathgate Hills, about a mile north-east from the town of Bathgate. This eminence rises to a height of 940 feet above the sea, and consists of a rudely elliptical boss of basalt, measuring 3500 feet in its greater and 3000 feet in its minor axis. It disrupts the sedimentary and volcanic series, which can be traced up to it on all sides. Some of the smaller circular or elliptical bosses in eastern Linlithgowshire and western Fife may perhaps belong to the same category. But undoubtedly most of the intrusive basalts and dolerites of this volcanic region are sills.
Over the greater part of the district, only fine tuffs were ejected. These occur as interstratifications among the ordinary sediments, and vary from mere thin partings, marking the feeblest and briefest explosions, up to continuous accumulations several hundred feet thick. As an example of the least vigorous emission of tuff I may refer to the sections already given on pp. [437], [438]. The thicker bands are well illustrated by that which lies some way above the Houston Coal, between Drumcross and West Broadlaw in Linlithgowshire, and by the great mass of tuff which occurs immediately below the Calmy Limestone on the River Avon near Linlithgow Bridge, and which may be 300 feet thick.
It is a striking characteristic of the tuffs that they may be met with in their solitary beds intercalated in the midst of ordinary sediments at a distance from any other trace of volcanic activity, their parent vents not being visible. I may cite in illustration an interesting case in the Swear Burn, near the southern end of the volcanic district. A band of tuff about ten feet thick lies there intercalated in a group of dark shales and thin coals. Below it there is a seam of coal four inches thick, and among the blue shales overlying it is another coal ten inches thick. The tuff is pale green, almost white in colour, fine in texture, like a volcanic mud, while some of its component beds, one foot in thickness, are made up of fine laminæ and are even false-bedded. We might infer that the volcanic vent lay at some distance, so that only the finest dust fell over the swamps in which the coal-vegetation was accumulating, but for the presence of occasional blocks of basalt one foot in diameter scattered through the tuff. When the eruptions ceased, the deposition of ordinary mud and the accumulation of plant-remains went on as before, and animal life crowded in again over the spot, for between the partings of the coal above the tuff, abundant fragments of eurypterids and scorpions may be found.
One of the most extensive volcanic discharges of fragmentary material was that which produced the "Houston marls" already referred to. These strata appear to mark a peculiar phase in the volcanic history of the Lower Carboniferous rocks of the Firth of Forth, when exceedingly fine ash, or perhaps even volcanic mud, was erupted in considerable quantity. The "marls" attain in some places a thickness of nearly 200 feet, and can be traced through most of the eastern part of Linlithgowshire, over an area of perhaps more than 50 square miles. This volcanic platform, which has been followed in mining for oil-shale, is one of the most extensive among the puy-eruptions. The material, which probably came from one or more vents among the Bathgate Hills, is not always of equal fineness, but passes into and even alternates with ordinary granular tuff. Thus in the Niddry Burn, above Ecclesmachan, the dull sage-green and brownish red Houston marls contain a few inconstant layers of green tuff, in which may be noticed pieces of black shale and lapilli of the usual basic pumice. Not far to the west, beyond Wester Ochiltree, and thus probably nearer to the active vents that ejected the dust and ashes, the Houston marls are replaced by or include a bedded granular tuff or basalt-agglomerate, which lies above the 2-feet coal of the district. The matrix of this rock is in part a dull green granular mudstone, wrapping round the lapilli and ejected stones, which, when they fall out under the action of the weather, leave casts of their forms behind them. The enclosed fragments vary in size up to blocks three feet in diameter, and consist in great measure of a compact volcanic grit, composed of a fine mud mixed with minute fragments of black shale, grains of sand and flakes of mica. There are likewise blocks of cement-stone and shale. Thin courses of black shale interlaminated with the tuff show its bedding.
The thickest and most continuous accumulations of tuff occur round some of the larger tuff cones, particularly round the Saline Hills, and in the Burntisland district. In the first-named area the copious eruptions of fragmentary material brought the volcanic history there to an end; but around Burntisland they were only the prelude to a prolonged and varied series of discharges.
I have already remarked that in the area of the puys of the Forth-basin, while the majority of the vents were tuff-cones, and emitted only fragmentary discharges, there were two well-marked tracts where lavas were poured out extensively and during a long geological interval. One of those lies in the southern, the other in the northern area.
The southern or Linlithgowshire lava-ridge forms now what are known as the Bathgate and Linlithgow Hills. The lavas extend for about 14 miles from north to south, dying out in both directions, while their present visible breadth is about three miles at its widest part. The highest summit reaches a height of about 1000 feet above the sea. The structure of this long ridge reveals an interesting record of volcanic eruptions. It consists mainly of sheets of basalt, sometimes separated by layers of tuff ([Fig. 155]). But on one or two horizons the volcanic rocks cease, and ordinary sedimentary deposits take their place. As has been already stated, the Main or Hurlet Limestone can be traced through the heart of the volcanic masses. This seam attains there an exceptional thickness of as much as 70 to 80 feet, and is nowhere more abundantly fossiliferous. During its deposition there seems to have been a subsidence of the area, together with a cessation of volcanic activity for a time. The crinoids, corals, brachiopods, bryozoa, lamellibranchs, gasteropods, cephalopods and fishes, which swarmed in the clear water, built up a thick calcareous layer above the lavas and tuffs of the sea-bottom.