1. BASIN OF THE FIRTH OF FORTH
Reference has already been made to the remarkable peculiarity in the development of the lower part of the Carboniferous system in this district.[476] Elsewhere throughout Scotland the Cement-stone group and the plateau lavas are immediately overlain by the Carboniferous Limestone series. But in the basin of the Firth of Forth a varied succession of strata, more than 3000 feet in thickness, intervenes between the Cement-stones and the Hurlet Limestone. The lower portion of this thick mass of sediment may represent a part of the Cement-stone group of other districts, but even if some deduction is made on this account there remain many hundred feet of stratified deposits, for which there does not appear to be any stratigraphical equivalent elsewhere in Scotland. The distinguishing features of this series of strata are the thick zones of white sandstone, with occasional bands of fine conglomerate, the abundant seams of dark shale, often highly carbonaceous (oil-shales), the cyprid limestones and the seams of coal. Such an association of deposits may indicate a more humid climate and more varied conditions of denudation and deposition than are presented by the typical Cement-stones. The muddy floor of the shallow water must, in many places, have supported a luxuriant growth of vegetation, which is preserved in occasional seams and streaks of coal. Numerous epiphytic ferns grew on the subærial stems and branches of the lycopodiaceous trees. Large coniferae clothed the higher grounds, from which the streams brought down copious supplies of sediment, and whence a flood now and then transported huge prostrate trunks of pine. In the lagoons animal life abounded. Cyprids swarmed to such a degree as to form by their accumulated remains bands of limestone, which in the well-known Burdiehouse seam sometimes attain a thickness of 70 feet. Fishes of many genera haunted the waters, for their scales, bones and coprolites are found in profusion among the shales and limestones.
[476] See Maclaren's "Geology of Fife and the Lothians," the Memoirs of the Geological Survey of Scotland, on Sheets 31 and 32, and my Memoir, already cited, Trans. Roy. Soc. Edin. vol. xxix. (1879) p. 437.
When the puys began their activity, this district was gradually dotted over with little volcanic cones. At the same time it was affected by the general movement of slow subsidence which embraced all Central Scotland. Cone after cone, more or less effaced by the waters which closed over it, was carried down and buried under the growing accumulation of sediment. New vents, however, continued to be opened elsewhere, throwing out for a time their showers of dust and stones, and then lapsing into quiescence as they sank into the lagoon. Two groups of volcanoes emitted streams of lava and built up two long volcanic ridges—those of Fife and West Lothian.
The occasional presence of the sea over the area is well shown by the occurrence of thin bands of limestone or shale, containing such fossils as species of Orthoceras, Bellerophon and Discina, which suffice to prove the strata to be stratigraphical equivalents of the Lower Limestone shale, and part of the Carboniferous Limestone of England ([Fig. 170]). Yet the general estuarine or freshwater character of the accumulations seems satisfactorily established, not only by the absence of undoubtedly marine forms from most of the strata, but by the abundance of cyprids and small ganoids, the profusion of vegetable remains, and the occasional seams of coal.
The portion of the Forth basin within which the puys are displayed extends from near Leven in Fife, on the north, to Crosswood Burn near the borders of Lanarkshire, on the south, a distance of about 36 miles, and from near Culross in Fife and the line of the Almond River between Stirlingshire and Linlithgowshire, on the west, to the island of Inchkeith on the east, a distance of about 16 miles ([Map IV.]). But these limits do not precisely mark the original boundaries of the eruptions. To the north and south, indeed, we can trace the gradual dying out of the volcanic intercalations, until we reach ground over which no trace of either lavas or tuffs can be detected. To the east, the waters of the Firth conceal the geology of a considerable area, the island of Inchkeith with its bedded lavas and tuffs showing that these rocks extend some way farther eastwards than the position of that island. But in Midlothian there is no evidence that any of the puy-eruptions took place to the east of the line of the Pentland Hills. On the west side, the volcanic rocks dip under the Millstone Grit and Coal-measures, so that we do not know how far they extend in that direction. But as the Carboniferous Limestone series, when it rises again to the surface on the west side of the Stirlingshire coal-field, is destitute of included lavas and tuffs, the westward limit of the eruptions cannot lie much beyond the line of the River Almond. We shall probably be within the mark if we set down the original area over which puys broke out and spread abroad their lavas and tuffs as covering about 300 square miles of the lagoons and jungles of Central Scotland.
Fig. 170.—Junction of amygdaloidal basalt with shales and limestone, Shore, half a mile east from Kinghorn, Fife. (From a photograph by Mr. R. Lunn.)
I have already shown that the range in geological time of the puy-eruptions in this district extends from a low horizon among the Calciferous Sandstones through the Carboniferous Limestone series, up to nearly the level of the Calmy Limestone, which lies not far from the top of that series. The vertical thickness of strata between these two stratigraphical limits, when there are no intercalated volcanic rocks, is probably more than 4000 feet.
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.
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.
72. Blue shale, becoming calcareous towards the top, where shells are plentiful.
71. Reddish false-bedded sandstones, with bands of reddish and blue shale.
70. Basalt in two sills separated by 2 or 3 feet of sandstone and shale.
69. Dark fissile sandy shale, passing up into white shaly sandstone, and including a thin parting of impure coal.
68. Limestone (Hurlet or Main Seam) in a number of bands having a united thickness of 25 feet. Abundant fossils.
67. Black shale becoming calcareous at top, and then enclosing abundant Productus, etc., 8 or 10 feet.
66. Red and green tufaceous marl and tuff. About 30 feet.
65. Basalt, the lower part strongly amygdaloidal.
64. Tufaceous red marl and tuff; comparatively coarse below, becoming finer above, 3 or 4 feet.
63. Basalt, earthy and amygdaloidal, with an irregular bottom involving masses of the shales below.
62. Dark calcareous shale and dull green tufaceous marly shale, 2 or 3 feet.
61. Crinoidal limestone in several bands with a united thickness of 10 feet.
60. Shale, 1 foot.
59. Fine green sandy tuffs in a number of bands of varying coarseness, about 6 feet.
58. Dark shale with abundance of Aviculopecten immediately under the tuffs above, 1½ feet.
57. Soft, light, marly shale with fragmentary plants, 1½ feet.
56. Dark fissile shale, full of fish-scales, plants, etc., 3 feet.
55. Basalt, rudely columnar, dark fine-grained in centre, becoming highly amygdaloidal and scoriaceous at bottom and top.
54. Basalt, like the sheet above, vesicular at top and bottom, with a parting of red clay on top.
53. Fissile rippled sandy shale, with plants, having a red and green marly parting at the top, 12 or 14 feet.
52. Basalts; a group of beds, probably in part sills, involving three bands of sandstone or quartzite.
51. Quartzite—a hard white altered sandstone, 2 to 3 feet.
50. Basalt, light green, earthy, amygdaloidal.
49. Sandstones and shales with plants, 25 feet.
48. Basalt, with a highly amygdaloidal central band. There may be several sheets here.
47. Green tufaceous shale and marl, 1 foot.
46. Basalt, dark, firm and amygdaloidal.
45. Sandstones and shales with plants.
44. Basalt forming west side of Kinghorn Bay, and including more than one sheet. The rock is very black, compact, irregularly columnar, with the usual amygdaloidal earthy band at the base, and forms the crag called the Carlinehead Rocks. An irregular and inconstant band of dull green tufaceous shale, sometimes 2 feet thick, serves to separate two of the basalt-sheets. Below it lies a remarkable scoriaceous almost brecciated basalt, which has been broken up on cooling in such a manner that at first it might be mistaken for a volcanic conglomerate.
43. Basalt, a compact black solid rock, with a vesicular and amygdaloidal bottom, about 40 feet. This sheet runs out into the promontory of Kinghorn Ness.
42. Basalt, firm, compact and highly amygdaloidal throughout, 15 feet.
41. Basalt, earthy, amygdaloidal and scoriaceous in the upper part, compact below.
40. Red tufaceous marl, clay or bole, a few inches thick.
39. Basalt: one of the most compact sheets of the whole series, about 40 feet. The top is formed of a thick zone of scoriaceous and brecciated material, the bottom is singularly uneven owing to the very irregular surface of the underlying bed.
38. Basalt more or less scoriaceous throughout, especially at the bottom, the vesicles being drawn out round the slag-like blocks.
37. Green tufaceous shales with bands of fine green tuff, 7 to 8 feet. The lower bands consist of a gravelly tuff passing up into a fine volcanic mudstone, with scattered lapilli of basalt an inch or more in diameter.
36. Basalt, with an upper, earthy and highly amygdaloidal portion, 30 feet.
35. Tufaceous sandstone and shale, 6 to 8 feet.
34. Basalt, in a thick bed, having an earthy, slaggy top and a scoriaceous bottom.
33. Basalt, very slaggy below with a compact centre.
32. Basalt, like that below it.
31. Basalt, firm, compact, black rock, with a rough, green earthy band, from 6 inches to a foot, at the bottom, and becoming again very slaggy at the top.
30. Green shale like that below the underlying limestone, a few inches in thickness.
29. Coarse, green, sandy tufaceous limestone, averaging 1 foot in thickness.
28. Black shale with plants, 12 or 14 feet, becoming green and tufaceous at the top.
27. Basalt—the most striking of the whole section—a fine compact black olivine-bearing rock, beautifully columnar, 30 feet. The columns reach to within a foot of the bottom of the bed and cease about 10 feet from the top, the upper portion of the bed being massive, with vesicles which are drawn out parallel to the bedding of the series. The lowest part of the bed is a broken brecciated band, 3 or 4 inches thick. (See [Fig. 171].)
26. Black shale with fragmentary plants, 3 feet.
25. Basalt, with plentiful olivine, 12 to 16 feet. The base is not highly scoriaceous, but finely vesicular. Towards the top it becomes green, earthy and roughly brecciated. It partly cuts out the tuff underneath.
24. Tuff, green, fine-grained and well-stratified, consisting chiefly of fine volcanic dust, but becoming coarser towards the top, where it contains lapilli and occasional bombs of highly vesicular lavas.
23. Black carbonaceous shale, 3 feet; approaching to the character of an impure coal in the lower part, and becoming more argillaceous above with some thin nodular calcareous bands.
22. Green tuff, 12 feet, well stratified and fine-grained, with minute lapilli of highly vesicular basic lavas; becomes shaly at the bottom.
21. Basalt, compact, amygdaloidal, with highly vesicular upper surface, 20 feet.
20. Basalt, hard, black and full of olivine; an irregular bed 3 to 6 feet thick.
19. Basalt, dull brownish-green to black, full of kernels and strings of calcite, and showing harder and softer bands parallel with upper and under surfaces, which give it a stratified appearance.
18. Basalt, some parts irregularly compact, others earthy and scoriaceous. The distinguishing feature of this bed is the abundance of its enclosed fragments of shale, ironstone and limestone, which here and there form half of its bulk. The roughly scoriaceous upper portion is especially full of these fragments. In the ironstone balls coprolites may be detected, and occasional pieces of plant-stems are embedded in the basalt. This lava has evidently broken up and involved some of the underlying strata over which it flowed. This rock overhangs Pettycur Harbour.
17. Shales and limestone bands more or less tufaceous, 8 to 10 feet, with plants, cyprids, etc. The intercalation of fine partings of tuff in this band has been already cited on [p. 438], as an illustration of the feeble intermittent character of many of the volcanic explosions between successive outflowings of lava.
Owing to a change in the direction of strike the rocks now wheel round and for a time run nearly parallel with the coast-line, while they are partly concealed by blown sand and herbage. The shales and limestones just mentioned are not constant, and are soon lost, but about a quarter of a mile westward a band of tuff begins on the same horizon or near it, and increases in thickness towards the west, where it is associated with other sediments. The shore ceases to furnish a continuous section, so that recourse must be had to the craggy slopes immediately to the north, where the rocks can be examined on a cliff face ([Fig. 153]). There the tuff just referred to, together with some overlying bands of sandstone, is seen to pass under the group of basalts last enumerated. It is a green, stratified rock, perhaps 60 feet thick at its maximum, but dying out rapidly to north-west and south-east. It encloses balls of basalt and subangular and rounded fragments of sandstone, limestone and shale. A mass of coarse volcanic agglomerate which is connected with it and cuts across the ends of some of the basalts below, probably marks the position of the vent from which the tuff was ejected ([Fig. 152]).
16. Black and grey shales forming a thin band at the summit of King Alexander's Crag.
15. Basalt, dark compact rock, with an upper and lower highly scoriaceous and amygdaloidal band, 15 feet.
14. Black shales, tufaceous green shales, sandstone, and 6 inches of coal, forming a group of strata about 12 feet thick between two basalts; plants and cyprids abundant. (The coal seam is shown in [Fig. 151].)
13. Basalt, dull, earthy and highly amygdaloidal, with abundant calcite in kernels and veins; about 15 feet, but varying in thickness.
12. Basalt, forming a well-marked bed from 12 to 25 feet thick. It is a compact black olivine-bearing rock, sparingly amygdaloidal, but showing the usual dull green, earthy scoriform base. The upper surface is singularly irregular, having, in flowing, broken up into large clinker-like blocks, which are involved in the immediately overlying basalt. The bottom also is very uneven, for the basalt has in some places cut out the underlying shales, so as to rest directly upon the basalt below.
11. Black shale, varying up to 6 inches, but sometimes entirely removed by the overlying lava-stream.
10. Basalt, containing large irregularly spheroidal masses of hard black finely vesicular material enclosed in more earthy and coarsely vesicular rock. The vesicles are sometimes elongated parallel to the bedding, but have often been drawn out round a spheroid; some of them measure nearly a foot in length by 2 or 3 inches in breadth. The upper surface is uneven and coarsely amygdaloidal.
9. Basalt, hard black, with abundant olivine, and a columnar structure.
8. Green shale, 6 inches to 1 foot, much baked and involved in the overlying basalt.
7. Basalt, dull-green, earthy, amygdaloidal, varying from 10 to 40 feet in thickness.
6. Blue shale, disappearing where the basalt above it unites with that below.
5. Basalt with olivine, forming a thick irregular bed, which in some places is black and compact, in others green, earthy and amygdaloidal. The upper part is particularly cellular.
4. Sandstones forming a thick group of beds which have long been quarried for building-stone at the Grange and elsewhere.
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.
If the horizons of the sills furnished any reliable clue to their age, it might be inferred that the rocks were all intruded during the Carboniferous period, and as most of them lie beneath the upper stratigraphical limit of the puy-eruptions, the further deduction might be drawn that they are connected with these eruptions. I have little doubt that in a general sense both conclusions are well-founded. But that there are exceptions to the generalization must be frankly conceded. On close examination it will be observed that the same intrusive mass sometimes extends from the lower into the upper parts of the Carboniferous groups. Thus, in the west of Linlithgowshire, a large protrusion which lies upon the Upper Limestones, crosses most of the Millstone Grit, and reaches up almost as high as the Coal-measures. Again, in Fife, to the east of Loch Leven, a spur of the great Lomond sill, crossing the Carboniferous limestone, advances southward into the coal-field of Kinglassie, In Stirlingshire and Lanarkshire numerous large dolerite sheets have invaded the Millstone Grit and Coal-measures, including even the upper red sandstones, which form the top of the Carboniferous system in this region. It is thus obvious that if the puy-eruptions in the basin of the Forth ceased towards the close of the deposition of the Carboniferous Limestone series, there must have been a subsequent injection of basic lava on a gigantic scale in central Scotland. I shall recur to this subject in [Chapter xxxi.]
Fig. 173.—Section across the Upper Volcanic Band of north Ayrshire. Length about four miles.
1. Andesite lavas of the Clyde Plateau; 2. Tuffs closing the Plateau volcanic series; 3. Hurlet Limestone; 4. Carboniferous Limestone series with coal-seams; 5. Lower tuff zone of the Upper volcanic band; 6. Basic lavas; 7. Upper tuff zone; 8. Basic sill; 9. Coal-measures.