Now and then the eruptions of tuff have consisted of extremely fine

volcanic dust, which, mingling with water, took the form of a compact

mudstone, as in the case of the Houston Marls (p. 423), which remind

one of a volcanic mud. But in most localities the discharge of

tuff, though for a time it may have completely obscured the ordinary

contemporaneous sedimentation, was intermittent, so that in the

intervals between successive showers of detritus, the deposition of

non-volcanic sediment went on as usual. Hence it is that bands of tuff,

whether they lie among lavas or among sedimentary formations, are apt

to contain interstratifications of sandstone, shale, limestone or other

detrital deposit, and to pass insensibly into these. The extremely

gentle gradation from volcanic into non-volcanic sediment, and the

occasional reappearance of thin partings of tuff bring vividly before

the mind the slow dying out of volcanic energy among the Carboniferous

lagoons.

[Illustration:

Fig. 150.--Section in old quarry, west of Wester Ochiltree,

Linlithgowshire. Calciferous Sandstone series.

]

The comparatively quiet character of the volcanic explosions, and the

contemporaneous undisturbed deposition of sediment during the earlier

part of the puy period, are exemplified in many sections throughout the

areas above enumerated, as will be more fully illustrated in subsequent

pages. Two typical examples may suffice for this general statement of

the characters of the discharges of tuff in the puy-eruptions. In the

Linlithgowshire quarry represented in Fig. 150, where about ten feet of

strata have been exposed, a black shale (1) of the usual carbonaceous

character, so common in the Oil-shale series of this region, may be

seen at the bottom of the section. It is covered by a bed of nodular

bluish-grey tuff (2) containing black shale fragments. A second black

shale (3) is succeeded by a second thin band of fine pale yellowish

tuff (4). Black shale (5) again supervenes, containing rounded

fragments of tuff, perhaps ejected lapilli, and passing up into a layer

of tuff (6). It is evident that we have here a continuous deposit of

black shale which was three times interrupted by showers of volcanic

dust and stones. At the close of the third interruption, the deposition

of the shale was renewed and continued, with sufficient slowness to

permit of the segregation of thin seams and nodules of clay ironstone

round the decomposing organic remains of the muddy bottom (7). A fourth

volcanic interlude now took place, and the floor of the water was once

more covered with tuff (8). But the old conditions of deposit were

immediately afterwards resumed (9); the muddy bottom was abundantly

peopled with ostracod crustaceans, while many fishes, whose coprolites

have been left in the mud, haunted the locality. At last, however, a

much more serious volcanic explosion took place. A coarse agglomeratic

tuff (10), with blocks sometimes nearly three feet in diameter, was

then thrown out, and overspread the lagoon.[459]

[Footnote 459: See _Geol. Surv. Memoir of Edinburgh_, p. 45. These

tuffs are further described on pp. 465 _et seq._]

The second illustration may be taken from the admirable coast-section

between Burntisland and Kinghorn, where the number of intercalations

of tuff is very great. Besides thicker well-marked bands, successive

innumerable thin layers occur there among the associated zones of

sedimentary strata which separate the sheets of basalt. The character

of these tuff-seams may be inferred from the following details of less

than two feet of rock at Pettycur Point:--

Tuff 1·5 inch.

Limestone 0·2 "

Tuff 0·5 "

Shale 0·2 "

Tuff 0·1 "

Shale and tuff 1·0 "

Shale 0·2 "

Limestone 0·5 "

Shale full of volcanic dust 3·5 "

Shaly limestone 1·5 "

Laminated tufaceous limestone 2·0 "

Limestone in thin bands, with thin laminæ of tuff 0·8 "

Granular tuff 0·6 "

Argillaceous limestone, with diffused tuff 0·9 "

Fine granular tuff 0·7 "

Argillaceous limestone, with diffused tuff 1·5 "

Laminated limestone 0·1 "

Limestone, with parting of granular tuff in middle 0·9 "

Tufaceous shale 2·0 "

Limestone 0·4 "

Shaly tuff 1·25 "

Laminated limestone 0·1 "

Tuff 1·2 "

-----

21·65 inches.

[Illustration: Fig. 151.--Ejected volcanic block in Carboniferous

strata, Burntisland.

1. Brown shaly fire-clay with rootlets, about five inches; 2.

Impure coal, five or six inches, pressed down in its upper

layers by the impact and weight of the stone; 3. Green crumbling

ashy fire-clay, one foot, with its lower layers pressed down by

the stone while the upper layers rise over it, showing that the

stone fell at the time when half this seam was deposited. The

fire-clay passes up into dark greenish and black ashy shale (4)

about six inches thick and containing plant-remains. The stone

is a pale diabase weighing about six or eight pounds.

]

Such a section as this brings vividly before the mind a long-continued

intermittent feeble volcanic action during pauses between successive

outbursts of lava. In such intervals of quiescence, the ordinary

sediment of the lagoons accumulated, and was mixed up with the debris

supplied by occasional showers of volcanic dust. In this Fife volcanic

series, thin layers of sandstone, streaked with remains of the

Carboniferous vegetation; beds of shale full of cyprid-cases, ganoid

scales, and fragmentary ferns; thin beds of limestone, and bands of

fire-clay supporting seams of coal, are interleaved with strata of tuff

and sheets of basalt. Now and then a sharp discharge of larger stones

is seen to have taken place, as in the case of the block many years ago

described by me as having fallen and crushed down a still soft bed of

coal (Fig. 151).[460]

[Footnote 460: _Geol. Mag._ vol. i. p. 22. This Fife coast-section is

given in full at p. 470.]

[Illustration: Fig. 152.--View of volcanic agglomerate becoming finer

above. East end of Kingswood Craig, two miles east from Burntisland.]

The Fife coast-section from which these details are taken supplies

almost endless instances of the varying characters of the pyroclastic

materials of the puy-eruptions. The very same cliff, bank or reef will

show at one point an accumulation of excessively coarse volcanic debris

and at another thin laminæ of the finest dust and lapilli. These rapid

gradations are illustrated in Fig. 152, which is taken from the east

end of the Kingswood Craig. The lower part of the declivity is a coarse

agglomerate which passes upward into finer tuff.

Besides the thin partings and thicker layers of tuff which,

intercalated among the sedimentary strata of the Carboniferous system,

mark a comparatively feeble and intermittent volcanic activity, we meet

in some localities with examples where the puys have piled up much

thicker accumulations of fragmentary material without any intercalated

streams of lava, or interstratified sandstone, shale or limestone. Thus

the widespread Houston marls above described reach a thickness of some

200 feet. The vents of the Saline Hills in Fife covered the sea-floor

with volcanic ashes to a depth of several hundred feet. In the north of

Ayrshire the first eruptions of the puys have formed a continuous band

of fine tuff traceable for some 15 miles, and in places at least 200

feet thick.

Where volcanic energy reached its highest intensity during the time

of the puys, not only tuffs but sheets of lava were emitted, which,

gathering round the vents, formed cones or long, connected banks and

ridges. Of these there are four conspicuous examples in Scotland--the

hills of the Burntisland district, the Bathgate Hills, the ground

between Dalry and Galston in north Ayrshire, and a broken tract in

Liddesdale. Nowhere in the volcanic history of this country have even

the minutest details of that history been more admirably preserved than

among the materials erupted from puys in these respective districts.

Lava-cones, answering to solitary tuff-cones among the fragmental

eruptions, do not appear to have existed, or, like some of those in

the great lava-fields of Northern Iceland and Western America, must

have been mere small heaps of slag and cinders at the top of the

lava-column, which were washed down and effaced during the subsidence

and entombment of the volcanic materials. The lavas never occur without

traces of fragmentary discharges. Two successive streams of basalt may

indeed be found at a given locality without any visible intercalation

of tuff, but proofs of the eruption of fragmental material will

generally be observed to occur somewhere in the neighbourhood,

associated with one or both of them, or with other lavas above or below

them.

Where the phenomena of the puys have been most typically developed,

lavas and tuffs succeed each other in rapid succession, with numerous

or occasional interstratifications of ordinary sediment. Perhaps the

most complete and interesting example of this association is to be

found on the coast between Burntisland and Kirkcaldy, where, out of a

total thickness of rock which may be computed to be between 1500 and

2000 feet, it will probably be a fair estimate to say that the igneous

materials constitute four-fifths, or from

[Illustration: Fig. 153.--Alternations of basalt and tuff with shale,

etc., Kingswood Craig, Burntisland.] 1200 to 1600 feet. The lavas

are varieties of basalt ranging in character from a black compact

columnar to a dirty green earthy cellular or slaggy rock. Each separate

flow may be on the average about 20 or 30 feet in thickness. Columnar

and amorphous sheets succeed each other without any interposition

of fragmentary material (Fig. 171). But along the junctions of the

separate flows layers of red clay, like the bole between the basalts

of the Giant's Causeway, may frequently be noticed. The characteristic

slaggy aspect of the upper parts of these ancient _coulées_ is

sometimes remarkably striking. The full details of this most

interesting section will be given in later pages (p. 470). But some of

its more characteristic external features may be understood from the

views which are presented in Figs. 152, 153, 170, 171.

The general bedded character of the volcanic series is well shown in

Fig. 153, which represents the alternations of lavas and tuffs in

the Kingswood Craig two miles to the east of Burntisland. The harder

basalts will be seen to project as bold crags while the tuffs and

other stratified deposits between them give rise to grassy slopes and

hollows. A nearer view of the alternation of lavas and tuffs with

non-volcanic sedimentary deposits is supplied in Fig. 170, which is

taken from a part of the Fife coast a little further to the east than

the last illustration. Here one of the limestones of the Carboniferous

Limestone series is overlain with shale and tuff, which, being easily

disintegrated, have been cut away by the waves, leaving the lava above

to overhang and fall off in blocks. The columnar structure of some of

the basalts of this coast is well brought out in Fig. 171, which shows

further how the columns sometimes merge into an amorphous part of the

same sheet.

These Fife basalts illustrate admirably the peculiarities of the sheets

of lava which are intercalated among the Carboniferous strata. They

show how easy it generally is to discriminate between such sheets and

intrusive sills. The true lavas are never so largely crystalline, nor

spread out in such thick sheets as the sills; they are frequently

slaggy and amygdaloidal, especially towards the top and bottom, the

central portion being generally more fine-grained and sometimes

porphyritic. Where most highly cellular they often decompose into a

dull, earthy, dirty-green rock. Where they form a thick mass they are

usually composed of different beds of varying texture. Except the

differences between the more compact centre and the slaggy layer above

and below, the bedded lavas do not present any marked variation in

composition or structure within the same sheet. A striking exception to

this rule, however, is furnished by the Bathgate "leckstone" already

described.[461] This mass forms a continuation of the great basaltic

ridge of the Bathgate Hills, and though its exact relations to the

surrounding strata are concealed, it appears to be an interbedded and

not an intrusive sheet. The remarkable separation of its constituent

minerals into an upper, lighter felspathic layer, and a lower, heavier

layer, rich in olivine, augite and iron-ores, is a structure which

might be more naturally expected to occur in a sill. An instance

of its development in an undoubted sill will be described further

on. Nevertheless, if we follow the trend of the volcanic band of the

Bathgate Hills southward for only two miles beyond the picrite quarry,

we find in the Skolie Burn a rock in many respects similar, and

quarried for the same purpose of building oven-soles. This "leckstone"

is there seen to be surmounted by a group of calcareous shales and thin

limestones. The section laid bare in the stream is represented in Fig.

154. Immediately above the diabase, which is highly cellular, lies a

green felspathic sandstone or shale containing detached fragments of

the amygdaloid together with _Lingulæ_ and other shells. There seems no

reason to doubt that this is a true interstratified lava.[462]

[Footnote 461: _Trans. Roy. Soc. Edin._ xxix. (1879) p. 504.]

[Footnote 462: _Trans. Roy. Soc. Edin._ xxix. (1879), pp. 505-507.]

[Illustration: Fig. 154.--Section of the upper surface of a diabase

("leckstone") sheet, Skolie Burn, south-east of Bathgate.

1. Slaggy diabase; 2. Green sandy shale and shaly sandstone

containing _Lingulæ_, also pieces of slag from the underlying

lava, which are completely wrapped round in the sediment;

3. Yellow calcareous shelly sandstone; 4. Dark shale with

_Spiriferæ_, etc.; 5. Bed of blue crinoidal limestone; 6. Clays

and thin coal; 7. Black and blue calcareous shales and thin

limestones.

]

Where the puys attained their greatest development in Scotland, they

rose in the shallow lagoons, and here and there from deeper parts of

the sea-bottom, until by their successive discharges of lavas and tuffs

they gradually built up piles of material, which, in the Linlithgow and

Bathgate district, may have been nearly 2000 feet in thickness. It must

be remembered, however, that the eruptions took place in a subsiding

area, and that even the thickest volcanic ejections, if the downward

movement kept pace with the volcanic activity, need not have grown into

a lofty volcanic hill. Indeed, largely as the lavas and tuffs bulk in

the geology of some parts of Central Scotland, their eruption does not

seem to have seriously interfered with the broader physical changes

that were in progress over the whole region. Thus the subsidence which

led to the spread of a marine and limestone-making fauna over much of

Central Scotland included also the volcanic districts. The limestones,

formed of crinoids, corals and other marine organisms, extended over

the submerged lavas and tuffs, and were even interstratified with them.

While the volcanic materials are found to replace locally the ordinary

Carboniferous sedimentary strata, it is interesting in this regard to

note that, during pauses in the volcanic activity, while the subsidence

doubtless was still going on, some groups of sandstones, shales or

limestones extended themselves across the volcanic ridges so as to

interpose, on more than one platform, a mass of ordinary sediment

between the lavas or tuffs already erupted and those of succeeding

discharges, and thus to furnish valuable geological chronometers by

which to define the stratigraphical horizons of the successive phases

of volcanic energy.

[Illustration: Fig. 155.--Section across the volcanic ridge of the

Linlithgow and Bathgate Hills, showing the intercalation of limestones

that mark important stratigraphical horizons.

1. Houston Coal; 2. Houston Marls and tuffs; 3. Interstratified

sheets of basic lavas with occasional tuffs and intercalations

of shale, sandstone, etc.; 4. Tartraven Limestone; 5. Hurlet

Limestone with tuffs, shales and sandstones above and below;

6. Wardlaw Limestone; 7. Index Limestone; 8. Highest band of

tuff--upward limit of the volcanic series; 9 9. Volcanic necks;

10. Sill of basalt; 11. Levenseat or Castlecary Limestone; 12.

Millstone Grit; 13. Base of Coal-measures; 14. Thick doleritic

sill; 15. Dolerite dyke (? Tertiary).

]

The volcanic banks or ridges not improbably emerged as islets out

of the water, and were sometimes ten miles or more in length. Their

materials were supplied from many separate vents along their surface,

but probably never attained to anything approaching the elevation

which they would have reached had they been poured out upon a stable

platform. This feature in the history of the volcanic ridges is

admirably shown by the fact just referred to, that recognizable

stratigraphical horizons can sometimes be traced right through the

heart of the thickest volcanic accumulations. One of the largest areas

of basalts and tuffs connected with the puys is that of the Linlithgow

and Bathgate Hills, where, as already remarked, a depth of some 2000

feet of igneous rocks has been piled up. Yet several well-known seams

of stone can be traced through it, such as the Hurlet Limestone and the

Index Limestone (Fig. 155). Only at the north end, where the volcanic

mass is thickest and the surface-exposures of rock are not continuous,

has it been impossible to subdivide the mass by mapping intercalations

of sedimentary strata across it. It would thus seem that, even where

the amplest accumulations gathered round the puys, they formed low flat

domes, rather than prominent hills, which, as subsidence went on and

the tuff-cones were washed down, gradually sank under water, and were

buried under the accumulating silt of the sea-floor.

As a detailed illustration of the manner in which the growth of

organically-formed limestones and the deposit of ordinary sediment took

place concurrently with the occasional outflow of lava-streams over the

sea-bottom, I may cite the section presented in another Linlithgowshire

quarry (Fig. 156). At the bottom of the group of strata there exposed,

a pale amygdaloidal, somewhat altered basalt (A) marks the upper

surface of one of the submarine lavas of the period. Directly over it

comes a bed of limestone (B) 15 feet thick, the lower layers of which

are made up of a dense growth of the thin-stemmed coral _Lithostrotion

irregulare_. The next stratum is a band of dark shale (C) about two

feet thick, followed by about the same thickness of an impure limestone

with shale seams (D). The conditions for coral and crinoid growth

were evidently not favourable, for this argillaceous limestone was

eventually arrested first by the deposit of a dark mud, now to be seen

in the form of three or four inches of a black pyritous shale (E), and

next by the inroad of a large quantity of dark sandy mud and drift

vegetation, which has been preserved as a sandy shale (F), containing

_Calamites_, _Producti_, ganoid scales and other traces of the life of

the time. Finally, a great sheet of lava, represented by the uppermost

amygdaloid (G), overspread the area, and sealed up these records of

Palæozoic history.[463]

[Footnote 463: _Geol. Surv. Mem._ "Geology of Edinburgh," p. 58.]

[Illustration: Fig. 156.--Section in Wardlaw Quarry, Linlithgowshire.]

Among the phenomena associated with the Carboniferous volcanoes mention

may, in conclusion, be made of the evidence for the former existence

of thermal springs and saline sublimations or incrustations. Among the

plateau-tuffs of North Berwick, as has been already pointed out (p.

390), a fœtid limestone has been quarried, which bears indications

of having been deposited by springs, probably in connection with the

volcanic action of the district. The lower limestones of Bathgate

furnish abundant laminæ of silica interleaved with calcareous matter,

the whole probably due to the action of siliceous and calcareous

springs connected with the active puys of that district. Some

portions of the limestone are full of cellular spaces, lined with

chalcedony.[464] A saline water has been met with among the volcanic

rocks to the west of Linlithgow, in a bore which was sunk to a depth of

348 feet in these rocks without reaching their bottom. The water that

rose from the bore-hole was found to contain as much as 135 grains of

chloride of sodium in the gallon. It is not improbable that this salt

was originally produced by incrustations on the Carboniferous lavas

immediately after their eruption, as has happened so often in recent

times at Vesuvius, and that it was then buried under succeeding showers

of tuff and streams of lava.[465]

[Footnote 464: _Ibid._ p. 49, _et seq._]

[Footnote 465: _Proc. Roy. Soc. Edin._ vol. ix. p. 367. Besides

chloride of sodium the water contained also chlorides of calcium,

magnesium and potassium, carbonates of lime and magnesia, sulphate of

lime, and other ingredients in minute proportions.]

_Subsequent Dislocation of Bedded Lavas and Tuffs._--As the

interstratified volcanic materials were laid down in sheets at the

surface, they necessarily behave like the ordinary sedimentary strata,

and have undergone with them the various curvatures and fractures

which have occurred since Carboniferous times. Notwithstanding their

volcanic nature, they can be traced and mapped precisely as if they

had been limestones or sandstones. This perfect conformability with

the associated stratified rocks is strikingly seen in the case of the

sheets of lava which lie imbedded in the heart of the great volcanic

ridge of Linlithgowshire. The overlying strata having been removed from

their surface for some distance, and the ground having been broken by

faults, these volcanic rocks might at first be taken for irregular

intrusive bosses, but their true character is that shown in Fig. 157,

where by a succession of faults, with a throw in the same direction,

the upper basalts of Bonnytoun Hill are gradually brought down to the

level of the Firth of Forth.

[Illustration: Fig. 157.--Section from Linlithgow Loch to the Firth of

Forth.]

iii. SILLS, BOSSES AND DYKES

One of the characteristic features of Central Scotland is the great

number, and often the large size and extraordinary persistence, of

the masses of eruptive, more or less basic material, which have been

injected among the Carboniferous strata. The precise geological age

of these intrusions cannot, of course, be more exactly defined than

by stating that they are younger than the rocks which they traverse,

though in many cases their association with the necks, lavas and

tuffs is such as to show that they must be regarded as part of the

Carboniferous volcanic phenomena.

Sills.--With regard to the sills I have been led, for the following

reasons, to connect the great majority of them with the puys, though

some are certainly of far later date, while others should possibly be

assigned to the plateaux.

In the first place, the sills obviously connected with the plateaux

are in great measure intermediate, or even somewhat acid rocks, while

those of the puy series are much more basic. It is hardly possible,

however, in all cases to decide to which series a particular sill

should be assigned. This difficulty is particularly manifest in the

western part of Midlothian, where the plateau of that district exhibits

such frequent interruption, and where it often consists only of a

single basaltic sheet. To the west of it lie the abundant puys with

their lavas and tuffs, and between the two volcanic areas numerous

sills of dolerite and diabase make their appearance. In the difficulty

of deciding to which series these sills should be referred, it will be

convenient to consider them with those of the puys.

[Illustration: Fig. 158.--Section across the Campsie Fells illustrating

the contrast between the sills below and above the plateau-lavas.

1. Upper Old Red Sandstone; 2. "Ballagan Beds"; 3. Tuffs; 4. Lavas of

the Campsie district of the Clyde plateau; 5 5. Necks belonging to the

plateau volcanic series; 6. Trachytic sills belonging to the plateau;

7. Carboniferous Limestone series; 8. Dolerite sills cutting the

Carboniferous Limestone series. _f_, Fault.]

A remarkable illustration of the contrast in petrographical character

between the typical sills of the plateaux and those of the puys is

furnished by the chain of the Campsie Fells, where, on the north

side, among the Calciferous Sandstones which emerge from under the

andesitic lavas of the Clyde plateau, many intrusive sheets and bosses

of trachytic material may be seen, while on the southern side come the

great basic sills which, from Milngavie by Kilsyth to Stirling, run

in the Carboniferous Limestone series (Fig. 158). A similar contrast

may be observed in Renfrewshire between the trachytic sills below the

plateau-lavas south of Greenock and the basic sills above these lavas

in the Carboniferous Limestone series around Johnstone and Paisley.

In the second place, the more basic sills, as a rule, appear on

platforms higher in stratigraphical position than the plateaux, and

wherever this is their position there cannot be any hesitation in

deciding against their association with the older phase of volcanic

activity.

In the third place, the basic sills often occur in obvious connection

with the vents or bedded lavas and tuffs of the puy series. A

conspicuous example of this dependence is supplied by the intrusive

sheets of Burntisland, underlying the basalts and tuffs of that

district in the immediate neighbourhood of some of the vents from which

these bedded rocks were erupted (Fig. 159).

In the fourth place, even where no visible vents appear now at the

surface near the sills, the latter generally occupy horizons within

the stratigraphical range indicated by the interbedded volcanic rocks.

It must be remembered that all the Carboniferous vents were deeply

buried under sedimentary deposits, and that large as is the number of

them which has been exposed by denudation, it is probably much smaller

than the number still concealed from our view. The sills are to be

regarded as deep-seated parts of the volcanic protrusions, and they

more especially appear at the surface where the strata between which

they were injected crop out from under some of the higher members of

the Carboniferous system. Thus the remarkable group of sills between

Kilsyth and Stirling (Fig. 158) may quite possibly be connected with

a group of vents lying not far to the eastward, but now buried under

the higher parts of the Carboniferous Limestone, Millstone Grit and

Coal-measures. Again, the great series of sills that gives rise to

such a conspicuous range of hills in the north and middle of Fife

may have depended for its origin upon the efforts of a line of vents

running east and west through the centre of the county, but now buried

under the Coal-measures. Some vents, indeed, have been laid bare in

that district, such as the conspicuous groups of the Saline Hills

and the Hill of Beath, but many more may be concealed under higher

Carboniferous strata further east.

[Illustration: Fig. 159.--Section showing the position of the basic

sills in relation to the volcanic series at Burntisland, Fife.

1. Calciferous Sandstone series; 2. Burdiehouse Limestone; 3.

Sandstones, shales and tuffs; 4. Basalts and tuffs, with

intercalations of sandstone, shale and limestone; 5. Agglomerate

of the Binn of Burntisland neck; 6. Basalt dyke; 7. Dyke and

sill; 8 8 8. Three sills.

]

In the fifth place, the materials of which the sills consist link them

in petrographical character with those that proceeded from the puys.

The rocks of the intrusive sheets in West Lothian, Midlothian and Fife

are very much what an examination of the bedded lavas of the puys in

the same region would lead us to expect. There is, of course, the

marked textural difference between masses of molten rock which have

cooled very slowly within the crust of the earth and those which have

solidified with rapidity at the surface, the sills being for the most

part much more coarsely crystalline than the lavas, and more uniform in

texture throughout, though generally finer at the margins than at the

centre. There is likewise the further contrast arising from differences

in the composition of the volcanic magma at widely-separated periods

of its extravasation. At the time when the streams of basalt flowed

out from the puys its constitution was comparatively basic, in some

localities even extremely basic. Any sills dating from that time may

be expected to show an equal proportion of bases. But those which were

injected at a long subsequent stage in the volcanic period may well

have been considerably more acid.

In actual fact the petrographical range of the sills reasonably

referable to the puy-eruptions varies from picrite or limburgite to

dolerite without olivine. The great majority of these sheets in the

basin of the Firth of Forth, where they are chiefly displayed, are

dolerites (diabases), sometimes with, but more frequently without,

olivine. They include all the more coarsely crystalline rocks of the

region, though occasionally they are ordinary close-grained basalts.

Their texture may be observed to bear some relation to their mass,

so far at least as that, where they occur in beds only two or three

feet or yards in thickness, they are almost invariably closer-grained.

A cellular or amygdaloidal texture is seldom to be observed among

them, and never where they are largely crystalline. This texture is

most often to be found in thin sills which have been injected among

carbonaceous shales or coals. These intrusive sheets are generally

finely cellular, and more or less decayed ("white trap").

[Illustration: Fig. 160.--Sills between shales and sandstones, Hound

Point, Linlithgowshire.]

Differences of texture may often be observed within short distances

in the same sill, and likewise considerable varieties in colour and

composition. The most finely crystalline portions are, as usual, those

along the junction with the stratified rocks, the most crystalline

occurring in the central parts of the mass. A diminution in the size

of the crystalline constituents may be traced not only at the base,

but also at the top of a sheet, or at any intermediate portion which

has come in contact with a large mass of the surrounding rock. A good

illustration is supplied by the intrusive sheet at Hound Point (Fig.

160), to the east of South Queensferry, where some layers of shale

have been involved in the igneous rock, which becomes remarkably

close-grained along the junction.[466] This change in texture and

absence of cellular structure form a well-marked distinction between

these sheets and those which have flowed out at the surface as true

lava-streams.

[Footnote 466: See Hay Cunningham's "Essay," p. 66, and plate ix.; and

_Geol. Survey Memoir_ on "Geology of Edinburgh," p. 114.]

Some of the larger doleritic sills display a somewhat coarsely

crystalline texture in their central portions, and occasionally

present a notable micropegmatitic aggregate, which plays the part of

interstitial substance enclosing the other minerals. Mr. Teall has

referred to the frequent occurrence of this structure in the coarser

parts of the Whin Sill of the north of England.[467] It occurs also in

a marked degree in the Ratho sill and in some portions of the great

doleritic sill of which the crags of Stirling form a part.[468]

[Footnote 467: _British Petrography_, p. 208.]

[Footnote 468: Mr. H. W. Monckton. _Quart. Journal Geol. Soc._ vol. li.

(1895), p. 482.]

But beside the differences in texture, mainly due to varying rates of

cooling, the sills sometimes exhibit striking varieties of composition

in the same mass of rock. These variations are more especially

noticeable among the larger sills, and particularly where the material

is most markedly basic. The special type of differentiation, so

noticeable in the Bathgate diabase and picrite mass already alluded

to, is likewise well exhibited in an intrusive sheet or group of

sheets, recently exposed at Barnton, in the cutting of a railway from

Edinburgh to Cramond[469] (Fig. 161). The intrusive nature of the

several bands of igneous rock which occur here is made quite evident

by the alteration they have produced upon the shales with which they

have come in contact. It is the uppermost and most extensive of these

sills which specially deserves notice, for the differentiation of its

constituents. It stretches along the cutting for several hundred yards

at an angle of dip of about 15°. At the western or upper part of the

mass its actual contact with the superincumbent sedimentary strata is

not visible, but as the igneous rock is there a good deal finer in

grain than elsewhere, its upper surface cannot be many feet distant.

The upper visible portion is a light well-crystallized dolerite with

a rudely bedded structure, the planes dipping westwards at 15°. About

20 or 30 feet below the upper visible termination of the mass, the

dark ferro-magnesian minerals begin rapidly to increase in relative

proportion to the pale felspar, and the rock consequently becomes

dark-greenish brown. The change is particularly noticeable in certain

bands which run parallel with the general dip. There is no definite

line between the pale and dark body of the rock, the two graduating

into each other and the darker part becoming deeper in colour, heavier

and more decomposing, until it becomes a true typical picrite. Even

in this ultra-basic portion the same rude bedding or banding may be

observed.

[Footnote 469: This rock has been described by Mr. J. Henderson and

Mr. Goodchild, _Trans. Geol. Soc. Edin._ vi. (1893) pp. 297, 301, and

by Mr. H. W. Monckton, _Quart. Journ. Geol. Soc._ l. (1894) p. 39. Mr.

Goodchild recognized the occurrence of picrite, and Mr. Monckton has

described the succession of rocks, and given a diagram of them.]

[Illustration: Fig. 161.--Section of Sill, Cramomd Railway, Barnton,

near Edinburgh.

1. Baked shale; 2. Sill of very felspathic dolerite about, nine

feet thick; 3. Baked shale, eight inches; 4. Dolerite showing

chilled fine-grained edge and adhering firmly to the shale

below; it rapidly passes up into (5) Picrite with white

felspathic veins (6); 7. Junction of picrite and dolerite with a

similar vein along the line of contact; 8. Large globular body

of dolerite enclosing a mass of picrite.

]

Veins in which felspar predominates over the darker minerals traverse

the rock, sometimes parallel with the bedding, sometimes across it.

They vary from less than an inch to a foot in width, sometimes dividing

and enclosing parts of the surrounding mass. But that they are on the

whole contemporaneous with the sill itself, and not long subsequent

injections, is shown by the way in which the dark ferro-magnesian

minerals project from the picrite into the veins and lock the two

together.

But besides these injections, which doubtless represent the last

and more acid portions of the magma injected into the basic parts

before the final consolidation of the whole, there are to be observed

irregular concretionary patches, of similar character to the veins,

distributed through the picrite. On the other hand, towards its base

the sill becomes a coarse dolerite round which the picrite is wrapped,

and which encloses a detached portion of that rock.

It is deserving of note that while the ultra-basic portion descends

almost to the very bottom of the sill, the lowest five feet show the

same change as occurs at the top of the mass. There the felspar

rapidly begins to predominate over the darker minerals, and the

dolerite into which the rock passes shows a fine-grained margin

adhering firmly to the shales on which it rests. This lower doleritic

band, showing as it does the effect of chilling upon its under surface,

may be due to more rapid cooling and crystallization, while in the

overlying parts the mass remained sufficiently mobile to allow of a

separation of the heavier minerals from the felspars, which appear in

predominant quantity towards the top. It must be frankly admitted,

however, that we are still very ignorant of the causes which led to

this separation of ingredients in a few sills, while they were entirely

absent or non-efficient in most of them.

The intrusive character of the Carboniferous sills of Central Scotland

and their contact-metamorphism have been fully described, and some

of them have become, as it were, "household words" in geology.[470]

Exposed in so many fine natural sections in the vicinity of Edinburgh,

they early attracted the notice of geologists, and furnished a

battle-ground on which many a conflict took place between the Plutonist

and Neptunist champions at the beginning of the present century.

[Footnote 470: See, for instance, Maclaren's _Geology of Fife and

the Lothians_, 1839; Hay Cunningham's _Essay_, previously cited;

_Geological Survey Memoir on the Geology of Edinburgh_ (Sheet 32),

1861; Mr. Allport, _Quart. Journ. Geol. Soc._ vol. xxx. (1874) p. 553;

Teall, _British Petrography_, p. 187; E. Stecher, _Contacterscheinungen

an schottischen Olivindiabasen_, Tschermak's _Mineralog. Mittheil._

vol. ix. (1887) p. 145; _Proc. Roy. Soc. Edin._ vol. xv. (1888) p. 160.]

As the sills frequently lie in even sheets perfectly parallel with

the bedding of the strata between which they have been injected, care

is required in some cases to establish that they are of intrusive

origin. One of the most obvious tests for this purpose is furnished by

the alteration they produce among the strata through which they have

made their way, whether these lie above or below them. The strata are

sometimes crumpled up in such a manner as to indicate considerable

pressure. They are occasionally broken into fragments, though this

may have been due rather to the effects of gaseous explosions than to

the actual protrusion of melted rock. But the most frequent change

superinduced upon them is an induration which varies greatly in

amount even along the edge of the same intrusive sheet. Sandstones

are hardened into quartzite, breaking with a smooth clear glistening

fracture. Coals are converted into a soft sooty substance, sometimes

into anthracite. Limestones acquire a crystalline saccharoid structure.

Shales pass generally into a kind of porcellanite, but occasionally

exhibit other types of contact-metamorphism. Thus below the thick

picrite sill at Barnton, near Edinburgh, the shales have assumed a

finely concretionary structure by the appearance in them of spherical

pea-like aggregates.

Another proof of intrusion is to be found in the manner in which sills

catch up and completely enclose portions of the overlying strata. The

well-known examples on Salisbury Crags (Fig. 162) are paralleled by

scores of other instances in different parts of the same region.

Moreover, sills do not always remain on the same horizon; that is,

between the same strata. They may be observed to steal across or break

through the beds, so as to lie successively between different layers.

No more instructive example of this relation on a small scale could

be cited than that of the intrusive sheet which has been laid open

in the Dodhead Limestone Quarry, near Burntisland. As shown in the

accompanying figure (Fig. 163), this rock breaks through the limestone

and then spreads out among the overlying shales, across which it passes

obliquely.

[Illustration: Fig. 162.--Intrusive dolerite sheet enclosing and sending

threads into portions of shale, Salisbury Crags, Edinburgh.]

Among the larger sills this transgressive character is seen to be

sometimes manifested on a great scale. Thus, along the important belt

of intrusive rocks that runs from Kilsyth to Stirling, the Hurlet

Limestone lies in one place below, in another above, the invading

mass, but in the intervening ground has been engulphed in it. Similar

evidence of the widely separate horizons occupied by different parts of

the same sill is supplied at Kilsyth, where the intrusive sheet lies

about 70 or 80 fathoms below the Index Limestone, while at Croy, in the

same neighbourhood, it actually passes above that seam.[471]

[Footnote 471: Explanation of Sheet 31, _Geological Survey of

Scotland_, §§ 43 and 83.]

[Illustration: Fig. 163.--Intrusive sheet invading limestone and shale,

Dodhead Quarry, near Burntisland.]

Other interesting evidence of the intrusive nature of the Carboniferous

dolerite sills of Central Scotland is supplied by the internal

modifications which the eruptive rock has undergone by contact with

the strata between which it has been thrust. These alterations, though

partly visible to the naked eye, are best studied in thin slices with

the aid of the microscope. Tracing the variations of an intrusive

dolerite outwards in the direction of the rocks which it has invaded,

we perceive change first in the augite. The large crystals and kernels

of that mineral grow smaller until they pass into a granulated form

like that characteristic of basalts. The large plates and amorphous

patches of titaniferous iron or magnetite give place to minute

particles, which tend to group themselves into long club-shaped bodies.

The labradorite continues but little affected, except that its prisms,

though as defined, may not be quite so large. The interstitial glassy

groundmass remains in much the same condition and relative amount as in

the centre of the rock.

Along the line of contact, while the dolerite becomes exceedingly

close-grained, its felspar crystals are still quite distinct even up

to the very edge. But they become fewer in relative number, and still

smaller in size, though an occasional prism two or three millimetres

in length may occur. They retain also their sharpness of outline, and

their comparative freedom from enclosures of any kind. They tend to

range themselves parallel with the surface of the contact-rock. The

augite exists as a finely granular pale green substance, which might

at first be taken for a glass, but it gives the characteristic action

of augite with polarized light. It is intimately mixed through the

clear glass of the groundmass, which it far exceeds in quantity. The

iron oxides now appear as a fine granular dust, which is frequently

aggregated into elongated club-shaped objects, as if round some inner

pellucid or translucent microlite. In patches throughout the field,

however, the oxides take the form of a geometrically perfect network of

interlacing rods. This beautiful structure, described and figured by

Zirkel and others,[472] is never to be seen in any of the dolerites,

except close to the line of contact with the surrounding rocks. It

occurs also in some of the dykes. I have not succeeded in detecting any

microlites in the sandstones at the edge of a dolerite sheet, though I

have had many slices prepared for the purpose.

[Footnote 472: _Mikroskopische Beschaffenheit der Mineralien und

Gesteine_, p. 273; Vogelsang's _Krystalliten_.]

Where one of the dolerite sills has invaded sandstone, there is usually

a tolerably sharp line of demarcation between the two rocks, though it

is seldom easy to procure a hand-specimen showing the actual contact,

for the stone is apt to break along the junction-line. Where, however,

the rock traversed by the igneous mass is argillaceous shale, we may

find a thorough welding of the two substances into each other. In such

cases the dolerite at the actual contact becomes a dark opaque rock,

which in thin slices under the microscope is found to be formed of a

mottled or curdled segregation of exceedingly minute black grains and

hairs in a clear glassy matrix, in which the augite and felspar are not

individualized. But even in this tachylyte-like rock perfectly formed

and very sharply defined crystals of triclinic felspar may be observed

ranging themselves as usual parallel to the bounding surfaces of the

rock. These characters are well seen in the contact of the intrusive

sheet of dolerite with shale and sandstone at Hound Point (Fig. 160).

Another instructive example is furnished by the small threads which

proceed from the dolerite of Salisbury Crags, and traverse enclosed

fragments of shale (Fig. 162). Some of these miniature dykes are

not more than one-eighth of an inch in diameter, and may therefore

easily be included, together with part of the surrounding rock, in

the field of the microscope. The dolerite in these ramifications

assumes an exceedingly fine texture. The felspar is the only mineral

distinctly formed into definite crystals. It occurs in prisms of an

early consolidation, sometimes one-fifth of an inch long, and therefore

readily recognizable by the naked eye. These prisms are perfectly

shaped, contain abundant twin lamellæ, and show enclosures of the iron

of the base. They had been already completely formed at the time of

injection; for occasionally they may be observed projecting beyond

the wall of the vein into the adjacent shale or sandstone, and they

have ranged themselves parallel to the sides of the vein.[473] The

black ground, from which these large well-defined crystals stand out

prominently, consists of a devitrified glass, rendered dark by the

multitude of its enclosed black opaque microlites. These are very

minute grains and rudely feathered rods, with a tendency to group

themselves here and there into forms like portions of the rhombohedral

skeletons of titaniferous iron. So thoroughly fused and liquid has

the dolerite been at the time of its injection, that little threads

of it, less than 1/100 of an inch in diameter, consisting of the same

dark base, with well-defined felspars, may be seen isolated within the

surrounding sedimentary rock. Minute grains and rounded portions of the

latter may also be noticed in the marginal parts of the dolerite.

[Footnote 473: The infusibility of the felspar was well shown in some

experiments on the rocks of the neighbourhood of Edinburgh, made at

my request by Dr. R. S. Marsden, who subjected some of these rocks to

fusion at the laboratory of the University of Edinburgh. Microscopic

sections were prepared of the products obtained. The basalt of Lion's

Haunch is peculiarly instructive. Its large labradorite crystals have

resisted the intense white heat which, continued for four hours, has

reduced the rest of the minerals to a perfect glass. We can thus

well understand how large definite crystals of felspar should have

survived or appeared in dykes and veins while the rock was still

thoroughly liquid. The glass obtained from the Lion's Haunch rock is of

a honey-yellow, and contains translucent tufted microlites. The iron

forms beautiful dendritic films in the cracks. Altogether, the glass

presents a strong resemblance to the palagonitic substance so abundant

among the lapilli in the tuffs of the vents.]

It is thus evident that specimens taken from the edge of an intrusive

sheet, where the rock has rapidly chilled and solidified, represent to

us an earlier stage in the history of the whole mass than specimens

taken from its central portions. In fact, a series of samples collected

at short intervals from the outer contact to the inner mass shows, as

it were, the successive stages in the consolidation of the molten rock.

From the observations just described, it appears that the triclinic

felspars began to assume the shape of large definite crystals before

any of the other minerals. These felspars already existed when the

molten mass forced its way among the shales, for they can be seen

lying with their long axes parallel to the surface of shale, precisely

as, in the well-known flow-structures, they behave round a large

crystal embedded in the heart of a rock. A few feet from where the

consolidation was not so rapid, the iron oxides have grouped themselves

into incipient crystalline forms and skeleton crystals; the felspar

crystals abundantly occur, and the augite has been left in the finely

granular condition. Still further towards the interior of the mass, the

normal character of the dolerite is gradually assumed.[474]

[Footnote 474: For a further and more detailed investigation of the

contact phenomena of the Carboniferous doleritic sills of the basin of

the Firth of Forth, see the papers by Dr. Stecher, quoted on p. 451.]

[Illustration: Fig. 164.--Spheroidal weathering of dolerite sill,

quarry east of North Queensferry, Fife.]

Where a sill has been injected among carbonaceous shales and coals it

has undergone great alteration along the contact, and if the sheet is

only a few inches or feet thick, the change extends throughout its

whole mass. Black basalts and dolerites, in such circumstances, pass

into a substance like a white or pale yellow clay, which at first might

be mistaken for some band of fire-clay intercalated among the other

sediments. But evidence of actual intrusion may usually be found, as

where the igneous rock has caught up or broken through the adjacent

strata, besides altering them. Such "white traps," as they have been

called, have long been familiar in the coal-fields of Scotland and

Central England.

[Illustration: Fig. 165.--Two thin sills of "White Trap" injected into

black carbonaceous shale overlying the Hurlet Limestone, Hillhouse

Quarry, Linlithgow.

1. Hurlet Limestone; 2. Black shales; 3 3. Two sills of "White

Trap"; 4. Columnar Basalts.

]

As a good illustration of the behaviour of such thin sills among

carbonaceous shales I give here a section (Fig. 165) exposed in the

old limestone quarry of Hillhouse, south of Linlithgow. At the bottom

lies the Hurlet Limestone which has once been extensively mined at this

locality. Above it comes a group of black shales which in turn are

surmounted by a sheet of beautifully columnar basalt. The shales seem

at first sight to include two layers of pale fire-clay, each only a few

inches in thickness. Closer inspection, however, will show that these

two thin intercalations are really sills which, though on the whole

parallel with the bedding of the shale, may be seen to cut across it,

and even at one place to send a finger into it. The upper example may

also be observed to diminish rapidly in thickness in one direction.

The dimensions of the sills vary within tolerably wide limits. Although

here and there the injected material dwindles down to an inch or less

in thickness, running away even into threads, it more usually forms

sheets of considerable depth. The rock of Salisbury Crags, for example,

is fully 150 feet thick at its maximum. That of Corstorphine Hill is

probably about 350 feet. The great sheet which runs among the lower

limestones from Kilsyth by Denny to Stirling has been bored through to

a depth of 276 feet, but as the bore started on the rock, and not in

overlying strata, some addition may need to be made to that thickness.

The spheroidal weathering so characteristic of basic eruptive rocks

is nowhere more characteristically displayed than among the great

doleritic sills of the basin of the Firth of Forth. As an illustration

of this structure an example is taken here from the large sheet at

North Queensferry (Fig. 164).

While one is struck with the great size and extent of some of the sills

connected with the puys, as compared with the small and local sheets

underneath the plateaux, there is a further fact regarding them that

[Illustration: Fig. 166.--Dyke cutting the agglomerate of a neck. Binn

of Burntisland.] deserves remark--their capricious distribution.

Their occurrence seems to have little or no relation to the measure

of volcanic energy as manifested in superficial eruptions. Thus in

the north of Ayrshire, where a long band of lavas and tuffs, pointing

to vigorous activity, lies at the top of the Carboniferous Limestone

series, and where the strata underneath it are abundantly exposed

at the surface, the sills occur as thin and inconstant bands in the

central and eastern parts of the district only. The bedded lavas and

tuffs at the head of the Slitrig Water have no visible accompaniment of

sills. On the other hand, in the Edinburgh and Burntisland districts,

the sills bear a large proportion to the amount of bedded lavas and

tuffs, while in the Bathgate and Linlithgow district, where the

superficial eruptions were especially vigorous and prolonged, the sills

are of trifling extent.

It would seem from these facts that the extent to which the crust of

the earth round a volcanic orifice is injected with molten rock, in the

form of intrusive sheets between the strata, does not depend upon the

energy of the volcano as gauged by its superficial outpourings, but on

other considerations not quite apparent. Possibly, the more effectively

volcanic energy succeeded in expelling materials from the vent, the

less opportunity was afforded for subterranean injections. And if the

protrusion of the sills took place after the vents were solidly sealed

up with agglomerate or lava, it would doubtless often be easier for

the impelled magma to open a way for itself laterally between the

bedding-planes of the strata than vertically through the thick solid

crust. The size and extent of the sills may thus be a record of the

intensity of this latest phase of the volcanic eruptions.

[Illustration: Fig. 167.--Boss of diabase cutting the Burdiehouse

Limestone and sending sills and veins into the overlying shales.

Railway cutting, West Quarry, East Calder, Midlothian.

1. Burdiehouse Limestone; 2. Shales; 3. Diabase.]

* * * * *

Bosses.--The rounded, oval or irregularly shaped masses of igneous rock

included under this head are found in some cases to be only denuded

domes of sills, as, for example, in the apparently isolated patches

in the oil-shale district of Linlithgowshire, which have been found

to unite under ground. (Compare Fig. 157). In other instances, bosses

possibly, or almost certainly, mark the position of volcanic funnels,

as at the Castle Rock of Edinburgh, Dunearn Hill, Burntisland, and

Galabraes, near Bathgate. But many examples occur which can only be

regarded as the exposed ends of irregular bodies of molten material

which has been protruded upwards into the

[Illustration: Fig. 168.--Side of columnar basalt-dyke in the same

agglomerate as in Fig. 166.] Carboniferous formations. The area

between Edinburgh and Linlithgow and the hills of the north of Fife

furnish many examples.

The connection between bosses and intrusive sheets is instructively

exhibited in a railway cutting to the west of Edinburgh, where the

section shown in Fig. 167 may be seen. In the space of a few yards

no fewer than four distinct bands of diabase traverse the shale,

thickening rapidly in one direction and uniting with a large boss of

more coarsely crystalline material. Such connections must exist in

all sills, for the material injected as a sheet between stratified

formations cannot but be united to some column, dyke or irregular

protrusion which descends to the parent magma in the interior. But it

is very rarely that the geologist is permitted to see them.

* * * * *

[Illustration: Fig. 169.--Dyke rising through the Hurlet Limestone and

its overlying shales. Silvermine Quarry, Linlithgowshire.]

Dykes take a comparatively unimportant place in the eruptive phenomena

of the puys. They occur in some numbers, but on a small scale, among

the tuff vents, and there they can without much hesitation be set down

as part of the phenomena of eruption through these pipes. The Binn

of Burntisland, which has been so often referred to in this Chapter,

may again be cited as a typical vent for the display of this series

of dykes (Figs. 149 and 159). Two additional illustrations from this

locality are here given. In Fig. 166 a dyke of compact black basalt is

seen on the right hand running up the steep slopes of the agglomerate.

Some of these dykes are distinctly columnar, the columns diverging from

the walls on each side. Where the encasing agglomerate has been removed

by the weather, the side of the dyke presents a reticulated network of

prism-ends. A narrow basalt-dyke of this character near the top of the

Binn vent is represented in Fig. 168.

But dykes also occur apart from vents and without any apparent relation

to these. They are sometimes associated with sills and bosses in such a

manner as to suggest that the whole of these forms of injected material

belong to one connected series of intrusions. Among the Bathgate Hills,

for example, from which I have already cited instances of sills and

a boss, the section represented in Fig. 169 occurs. Yet in this same

district there is a group of large east and west dykes which cut all

the other rocks including the bedded lavas and tuffs, and must be of

later date than the highest part of the Coal-measures (Fig. 155).

It is difficult to ascertain the age of the dykes which rise through

the Carboniferous formations at a distance from any interbedded sheets

of lava and tuff, or from any recognizable vent. The south-east and

north-west dykes, increasing in number as they go westward, and

attaining a prodigious development in the great volcanic area of Antrim

and the Inner Hebrides, are probably of Tertiary date.[475] Others may

possibly be Permian, while a certain number may reasonably be looked

upon as Carboniferous. In petrographical characters the latter resemble

the dolerites and basalts (diabases) of the finer-grained sills.

[Footnote 475: These are fully described in Chapters xxxiv. and xxxv.]

CHAPTER XXVIII

ILLUSTRATIVE EXAMPLES OF THE CARBONIFEROUS PUYS OF SCOTLAND

The Basin of the Firth of Forth--North Ayrshire--Liddesdale.

Though many of the geological details of each of the Scottish districts

of Puys have been given in the foregoing pages, it will be of advantage

to describe in connected sequence the structure and geological history

of a few typical areas. By far the fullest and most varied record of

this phase of volcanic activity has been preserved in the basin of

the Firth of Forth; but the north of Ayrshire and the district of

Liddesdale furnish also many interesting characteristics.

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.

[Footnote 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

[Illustration: 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.)] 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.

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.

[Illustration: 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]

[Footnote 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.

[Illustration: 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.

[Illustration: 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.

]

2. NORTH OF AYRSHIRE

In this part of the country another group of puys and their associated

tuffs and lavas may be traced from near Dairy on the west, to near

Galston on the east (Map IV.). The length of the tract is about sixteen

miles, while its breadth varies from about a furlong to nearly a mile

and a half. I have had occasion to allude to this marked band of

volcanic materials which here intervenes between the Carboniferous

Limestone and the Coal-measures, and from its position appears to

mark the latest Carboniferous volcanoes. Its component rocks reach a

thickness of sometimes 600 feet, and as they dip southwards under the

Coal-measures, they may extend for some distance in that direction.

They have been met with in borings sunk through the northern part of

the Irvine coal-field. Even what of them can be seen at the surface,

in spite of the effects of faults and denudation, shows that they

form one of the most persistent platforms of volcanic rock among the

puy-eruptions of Scotland.

Where best developed this volcanic band has a zone of tuff at the

bottom, a central and much thicker zone of bedded basalts, and an

upper group of tuffs, on which the Coal-measures rest conformably. A

few vents, probably connected with it, are to be seen at the surface

between Fenwick and Ardrossan. But others have been buried under the

Carboniferous sedimentary rocks, and, as already described, have been

discovered in the underground workings for coal and ironstone (p.

434). These mining operations have, indeed, revealed the presence of

far more volcanic material below ground than would be surmized from

what can be seen at the surface. Here and there, thin layers of tuff

appear in brook-sections, indicating what might be conjectured to have

been trifling discharges of volcanic material. But the prosecution of

the ironstone-mining has proved that, at the time when the seam of

Black-band Ironstone of that district was accumulated, the floor of

the shallow sea or lagoon where this deposition took place was dotted

over with cones of tuff, in the hollows between which the ferruginous

and other sediments gathered into layers. That seam is in one place

thick and of good quality; yet only a short distance off it is found

to be so mixed with fine tuff as to be worthless, and even to die out

altogether.[478]

[Footnote 478: See Explanation of Sheet 22, _Geol. Surv. of Scotland_,

pars. 29, 33, 45.]

3. LIDDESDALE

A remarkable development of puys lies in that little-visited tract

of country which stretches from the valleys of the Teviot and Rule

Water south-westwards across the high moorland watershed, and down

Liddesdale. Through this district a zone of bedded olivine-basalts and

associated tuffs runs in a broken band which, owing to numerous faults

and extensive denudation, covers now only a few scattered patches of

the site over which it once spread. The geological horizon of this

zone lies in the Calciferous Sandstones, many hundred feet above the

position of the top of the plateau-lavas (Map IV.).

So great an amount of material has been here removed by denudation that

not only has the volcanic zone been bared away, but the vents which

supplied its materials have been revealed in the most remarkable manner

over an area some twenty miles long and eight miles broad. Upwards of

forty necks of agglomerate may be seen in this district, rising through

the Silurian, Old Red Sandstone, and lowest Carboniferous rocks. It

fills the geologist with wonder to meet with those stumps of old

volcanoes far to the west among the Silurian lowlands, sometimes fully

ten miles away from the nearest relic of the bedded lavas connected

with them.[479] That these vents, though they rose through ground which

at the time of their activity was covered with the volcanic series

of the plateaux, do not belong to that series, but are of younger

date, has been proved in several cases by Mr. Peach. He has found

that among the blocks composing their agglomerates, pieces of the

sandstones, fossiliferous limestones and shales of the Cement-stone

group, overlying the plateau-lavas, are to be recognized. These vents

were therefore drilled through some part at least of the Calciferous

Sandstones, which are thus shown to have extended across the tract

dotted with vents. After the volcanic activity ceased, fragments of

these strata tumbled down from the sides into the funnels. Denudation

has since stripped off the Calciferous Sandstones, but the pieces

detached from them, and sealed up at a lower level in the agglomerates,

still remain. Mr. Peach's observations indicate to how considerable

an extent sagging of the walls of these orifices took place, with the

precipitation not merely of blocks, but of enormous masses of rock,

into the volcanic chimneys. In one instance, between Tudhope Hill and

Anton Heights, a long neck, or perhaps group of necks, which crosses

the watershed, shows a mass of the red sandstone many acres in extent,

and large enough to be inserted on the one-inch map, which has fallen

into the vent (Fig. 175).

[Footnote 479: They have been recognized and mapped by Mr. B. N.

Peach for the Geological Survey. See Sheets 11 and 17, _Geol. Surv.

Scotland_.]

[Illustration: Fig. 174.--Section showing the connection of the two

volcanic bands in Liddesdale.

1. Upper Silurian strata; 2. Upper Old Red Sandstone; 3. The lavas

of the Solway plateau; 4. Agglomerate neck with lava plug,

belonging to the plateau system; 5. Calciferous Sandstone

series; 6. Thick Carboniferous Limestones; 7. Tuff, and 8.

Lavas, of the upper volcanic band, connected with the puys; 9.

Agglomerate neck with lava plug belonging to the puy-system; 10.

Basic sill.

]

[Illustration: Fig. 175.--Diagram to show the position of a mass of

Upper Old Red Sandstone which has fallen into the great vent near

Tudhope Hill, east of Mosspaul.

1. Upper Silurian strata; 2. Outlier of Upper Old Red Sandstone; 2´.

Large mass of this formation in the vent; 3. Agglomerate of the neck

with andesite intrusion (4).]

The materials ejected from the Liddesdale vents include both basaltic

lavas and tuffs. The former are sometimes highly vesicular, especially

along the upper parts of the flows. They are thickest towards the

north, and in Windburgh Hill attain a depth of at least 300 or 400

feet. In that part of the district they form the lower and main part of

the volcanic series, being there covered by a group of tuffs. But a few

miles southwards, not far to the west of Kershopefoot, they die out.

The tuffs then form the whole of the volcanic band which, intercalated

in a well-marked group of limestones, can be followed across the moors

for some six miles into the valley of the Esk, where an interesting

section of them and of the associated limestone and shales is exposed

(Fig. 174). At Kershopefoot, a higher band of basic lava overlies the

Kershopefoot limestone, and can be traced in scattered patches both on

the Scottish and English side of the Border.

END OF VOL. I.

_Printed by_ R. & R. Clark, Limited, _Edinburgh_.

[Illustration:

TO ACCOMPANY SIR ARCHIBALD GEIKIE'S "ANCIENT VOLCANOES OF BRITAIN"

Map IV.

MAP OF THE CARBONIFEROUS VOLCANOES OF SCOTLAND

English Miles

EXPLANATION OF COLOURING

_Basalts and thin Tuffs_ } _Puy Series_

_Thicker Sheets of Tuff_ }

_Lavas and thin Tuffs_ } _Plateau Series_

_Thicker Sheets of Tuff_ }

_Vents filled with Agglomerate

or Tuff_

_Basic Sills and Bosses_

_Intermediate and acid Sills

and Bosses_

The Edinburgh Geographical Institute

Copyright

J. G. Bartholomew.

]

[Illustration: MAP 1

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Transcriber Note

Minor typos corrected. Several tables were rotated so they will fit

in the limits of the text format. Text rearranged to avoid split

paragraphs at full page images.