vol. iv. (1874) p. 123.]
[Illustration: Fig. 120.--Section across part of the Clyde Plateau to
the west of Bowling (reduced from Sheet 6 of the Horizontal Sections of
the Geological Survey of Scotland).
1. "Ballagan Beds"; 2. White sandstone; 3. Tuffs, 600 feet thick,
with a thin sheet of andesite; 4. Andesite sheets, 500 feet;
5. Stratified tuffs with thin coals, shales, fireclays and
plant-remains, 500 or 600 feet; 6 6. A series of andesite-lavas,
about 1500 feet thick, enclosing a thin coal-seam at *; 7.
Stratified tuffs, 200 feet; 8. Shales with plants and coaly
seams, 150 feet; 9. Base of another andesite series, which must
be some hundreds of feet thick; 10 and 11. Necks of agglomerate.
]
_Upper Limits and Original Areas and Slopes of the Plateaux._--Where
the highest members of the volcanic series can be seen passing
conformably under the overlying Carboniferous strata they are
frequently found to be mainly composed of fine tuffs, the last feeble
efforts of the plateau-volcanoes having consisted in the discharge
of showers of ashes. These materials were mingled with a gradually
increasing proportion of ordinary mechanical sediment, which finally
overspread and buried the volcanic tracts of ground, as these slowly
sank in the general subsidence of the region. The characteristic
corals, crinoids and shells of the Carboniferous Limestone begin to
appear in these ashy sediments. There is thus an insensible passage
from volcanic detritus into fossiliferous shales and limestones.
Examples of this gradation may be seen in many natural sections along
the flanks of the Ayrshire plateau from above Kilbirnie to Strathavon.
It is still possible to fix in some quarters the limits beyond which
neither the lavas nor the tuffs extended, and thus partially to map out
the original areas of the plateaux. For example, in certain directions
the Carboniferous formations can be followed continuously downward
below the Main Limestone, without the intervention of any volcanic
material, or with only a slight intermixture of fine volcanic lapilli,
such as might have been carried by a strong wind from some neighbouring
active vents. By this kind of evidence and by the proved thinning-out
of the materials of the plateau, we can demonstrate that in the north
of Ayrshire the southern limits of the great volcanic bank did not
pass beyond a line drawn from near Ardrossan to Galston. We can show,
too, that the lavas of the Campsie Fells ended off about a mile beyond
Stirling before they reached the line of the Ochil heights, and that
the _coulées_ which flowed from the Solway vents did not quite join
with those from the Berwickshire volcanoes.
[Illustration: Fig. 121.--Diagram illustrating the thinning away
southwards of the lavas of the Clyde Plateau between Largs and
Ardrossan. Length about 10 miles.
1. Upper Old Red Sandstone; 2. Sandstones, shales, etc., with "Ballagan
Beds"; 3. Tuffs; 4. Andesite lavas; 5. Carboniferous Limestone series.]
Moreover, evidence enough remains to enable us to form a tolerably
clear conception of the original average slopes of the surface of some
of the plateaux. Thus in the great escarpment above Largs and the high
ground eastward to Kilbirnie the volcanic series, as already stated,
must be at least 1500 feet thick. This thick mass of lavas and tuffs
thins away southwards and probably disappears a short distance south
from Ardrossan in a space of about ten miles (Fig. 121). The original
southward slope of the plateau would thus appear to have been about 1
in 35. Again, the northward slope of the same plateau may be estimated
from observations in the Campsie Fells. We have seen that above Kilsyth
the total depth of the volcanic sheets is about 1000 feet, while to the
westward it is much thicker. From the top of the Meikle Bin (1870 feet)
above Kilsyth north-eastwards to Causewayhead, where the whole volcanic
series has died out, is a distance of 12 miles, so that the slope of
the surface of erupted materials on this side was about 1 in 63 (Fig.
122).
Judging from the sections exposed along the faces of the escarpments,
we may infer that the volcanic sheets had a tolerably uniform surface
which sloped gently away from the chief vents, but with local
inequalities according to the irregularities of the lava-streams
that were heaped up round the vents and flowed outward in different
directions and to various distances from them. At the beginning, these
flat volcanic domes were certainly subaqueous. While they were being
formed, continuous subsidence appears to have been in progress. But the
great thickness of the volcanic accumulations, as in the Kilpatrick
and Renfrewshire areas, and the paucity of ordinary sedimentary strata
among them, make it not improbable that at least their higher parts
rose above the water. Where this was the case there may have been
considerable degradation of the lava-banks before these were reduced
or were by subsidence submerged beneath the water-level. Evidence of
this waste is probably to be recognized in the bands of conglomerate,
occasionally of considerable thickness, which, particularly in some
parts of Ayrshire, intervene between the top of the volcanic group and
the Hurlet Limestone. As I shall have occasion to point out further
on, there seems to be some amount of evidence in favour of the view
that a considerable interval of time elapsed between the close of the
plateau-eruptions and the date of that widespread depression which led
to the deposition of the Hurlet Limestone over the whole of Central
Scotland. If such an interval did occur it would include a prolonged
abrasion of any projecting parts of the plateaux, and the production
and deposition of volcanic conglomerate.
[Illustration: Fig. 122.--Diagram illustrating the thinning away
eastwards of the lavas of the Clyde Plateau in the Fintry Hills. Length
about 12 miles.
1. Upper Old Red Sandstone; 2. White sandstone, blue shales and
cement-stones ("Ballagan Beds"); 3. Andesite sheet, about 100
feet thick; 4. Tuffs (250 feet), with an included band of
ashy sandstone containing plant-remains; 5. Andesite lavas;
6. Carboniferous Limestone series, which to the east lies
immediately on the Upper Old Red Sandstone.
]
2. VENTS
We have now to consider the external forms, internal contents and
distribution of the vents from which the material of the plateaux was
discharged. In the Carboniferous system these interesting relics of
former volcanoes are far more distinctly defined and better preserved
than in older geological formations. Moreover, in Scotland, they are
laid bare to greater advantage, both inland and along the sea-coast,
and may indeed be studied there as typical illustrations of this kind
of geological structure.
[Illustration: Fig. 123.--View of the two necks Dumgoyn and Dumfoyn,
Stirlingshire, taken from the south.
These two necks form a conspicuous feature in front of and below the
lava plateau, a portion of which is shown on the right hand. The
ground-plan of the same necks is shown in Fig. 124.]
[Illustration: Fig. 124.--Ground-plan of Plateau-vents near
Strathblane, Stirlingshire, on the scale of 6 inches to a mile.]
In external form the necks connected both with the plateaux and the
puys generally rise from the surrounding ground as isolated, rounded,
conical or dome-shaped prominences, their details of contour depending
mainly upon the materials of which they consist. When these materials
are of agglomerate, tuff or other readily disintegrated rock, the
surface of the domes is generally smooth and grass-covered. Where, on
the other hand, they consist wholly or in part of dolerite, basalt,
diabase, andesite, trachyte or other crystalline rock, they present
more irregular rocky outlines. Illustrations of some of those varying
forms are given in Figs. 23 and 123. In rare instances the vent is
marked at the surface not by a hill but by a hollow, as in the great
neck in the heart of the Campsie Fells (Fig. 128).
[Illustration: Fig. 125.--Ground-plans of double and triple necks in
the Plateau series, on the scale of 6 inches to a mile.
A. Barwood Hill and Ravenscraig, east of Dumbarton, double vent. B. The
Knock Hill, Largs, Ayrshire, double vent (see Fig. 23). C. Dumbowie and
Dumbuck, east of Dumbarton, triple vent.]
As regards their ground-plan, which affords a cross-section of the
original volcanic funnel, the plateau-vents present considerable
variety. The simplest cases are those in which the form is
approximately circular or somewhat elliptical. Here the outline
corresponds to the cross-section of a single and normal orifice. Some
examples of this simple type are given in Fig. 124, which represents
a group of vents on the edge of the Clyde plateau near Strathblane.
The two larger necks here shown are the same which appear in the
view in Fig. 123.[434] Where two vents have been successively opened
close to each other, or where the same vent has shifted its position,
the ground-plan may be greatly modified. In some instances the double
funnel can be distinctly traced. Thus in the conspicuous Knock Hill
above Largs in Ayrshire (Fig. 125, B) there are two conjoined necks,
and such appears to be also the structure shown by the ground-plan of
the neck of Barwood Hill and Raven's Craig, east of Dumbarton (Fig.
125, A).[435] But more complex forms occur which point to a still
larger number of coalescing necks. A group of hills to the east of
Dumbarton gives the ground-plan shown in C, Fig. 125, where traces may
be detected of three separate vents. Still more irregular are long
narrow dyke-like masses of tuff or agglomerate which have probably
risen along lines of fissure (Fig. 22, No. 1). The most striking
example of these, however, occur in association with the puys and will
be described in later pages.
[Footnote 434: The illustrations in Figs. 124 and 125 are taken from
the field-maps of the Geological Survey on the scale of 6 inches to a
mile. The ground represented in Fig. 124 was mapped by Mr. R. L. Jack.]
[Footnote 435: These ground-plans are likewise taken from the
field-maps of the Geological Survey. A and C were mapped by Mr. Jack, B
by myself. The shaded parts are intrusive andesites and dolerites; the
dark bars in A and C being dolerite dykes of much later date than the
necks. The dotted portions mark tuff and agglomerate.]
Connected with their ground-plan is the relative size of the
plateau-vents. On the whole they are larger than those of the puy
series. The simple circular or elliptical type presents the smallest
necks, some of them not exceeding 100 feet in diameter. The more
complex forms are generally also of larger dimensions. By much the
largest vent or connected group of vents is that which lies among the
uplands of Misty Law in the heart of the Renfrewshire part of the Clyde
plateau, where a connected mass of tuff and agglomerate now occupies a
space of about 4 miles in length by 2½ miles in breadth (Fig. 129). It
has not been found possible, however, to trace the boundaries of the
separate vents of this tract, nor to distinguish the material of the
necks from that which surrounds them. Another large mass which from its
shape may be conjectured to represent more than one vent is the great
tract north of Melrose, which measures 8800 by 4200 feet.[436]
[Footnote 436: The following measurements are, like those in the text,
taken from the field-maps of the Geological Survey. Carewood Rig, on
the borders of Roxburghshire and Dumfriesshire, 7000 × 2400 feet; the
great vent in the middle of the Campsie Fells, 5200 × 2600; Black Law,
between Bedrule and Jedburgh, 3400 × 1600; Dumgoyn, Strathblane, 2300 ×
1300; Rubers Law, 1500 × 1000; Minto Hill (south), 2300 × 1650; Minto
Hill (north), 1500 × 1100; Doughnot Hill, Kilpatrick range, 1000 × 700;
four of the smallest agglomerate vents along the northern escarpment
of the Clyde plateau between Strathblane and Fintry, 500 × 450, 450 ×
400, 250 × 100, 200 × 200; Pike Law, Arkleton, Tarras Water, 500 × 500;
Harwood, Stonedge, 5 miles S.E. from Hawick, 500 × 300; Arkleton Burn,
Dumfriesshire, 400 × 100; Dalbate Burn, 250 × 120.]
The distribution of the necks can best be understood from the maps
of the Geological Survey, where they have been carefully indicated.
As might have been expected, they are not found outside the original
limits within which it may be reasonably inferred that the lavas
and tuffs were erupted. They occur most abundantly and attain their
largest size in and around the districts where the plateaux are most
extensively developed. No doubt a large number of them are concealed
under these plateaux. A few appear at the surface among the lavas and
tuffs, but by far the largest number now visible have been revealed by
denudation, the escarpments having been cut back so as to lay bare the
underlying rocks through which the necks rise. Thus, along the flanks
of the great escarpment that extends from near Stirling by Fintry and
Strathblane to Dumbarton, more than two dozen of agglomerate necks
may be counted in a distance of about sixteen miles, while if the
necks of lava-form material are included, the number of vents must be
about fifty. Nowhere in Scotland do such necks form a more conspicuous
feature in the scenery as well as the geology than they do between
Fintry and Strathblane, where, standing out as bold isolated hills in
front of the escarpments, their conical and rounded outlines present
a striking contrast to the terraced escarpments behind them. I would
especially refer again to the two remarkable cones of Dumfoyn and
Dumgoyn above Strathblane (Figs. 123, 124, 127). Along the west front
of the hills between Gourock and Ardrossan seventeen agglomerate-vents
occur in a distance of sixteen miles. In Roxburghshire a group of large
agglomerate-necks is dotted over the Silurian country around Melrose
and Selkirk[437] (see Fig. 130).
[Footnote 437: In this region and farther southward, besides the
plateau-eruptions, a later group of puys is to be seen, and it is
difficult to discriminate between the necks belonging to the two
groups. Those which lie to the east are probably connected with the
plateaux, those to the west with the puys. The latter are referred to
on p. 475.]
[Illustration: Fig. 126.--Ground-plan of tuff-neck, shore east of
Dunbar.
The surrounding rocks are sandstones, which are much hardened round
the vent in the zone marked by the short divergent lines. The arrows
mark the direction of dip. See "Geology of East Lothian," _Mem. Geol.
Survey_, p. 44.]
From the evidence of these necks it is plain that the volcanic
materials of the plateaux must in each case have been supplied not from
great central orifices, but from abundant vents standing sometimes
singly, with intervening spaces of several miles, often in groups of
four or five within a single square mile.
In the interior of the country, it is seldom possible to examine the
actual junction of necks with the rocks through which they rise, the
boundary-line being usually obscured by debris or herbage. On the
coast, the vents of the plateaux have not been bared by the sea so
fully as in the case of the much younger series of the east of Fife
to be described in later pages. But where the East Lothian plateau
touches the shore, the waves have laid bare a number of its minor
vents, which have thus been dissected in ground-plan on the beach. As
an illustration of these vents an example is given in Fig. 126, from
the shore east of Dunbar. Here the sandstones, which are inclined
in an easterly direction at 20° to 25°, are pierced by an irregular
mass of tuff. It is observable that in this instance long tongue-like
projections of the sandstones protrude into the neck; more frequently
the material of a neck sends veins or dykes into the surrounding walls.
A volcanic chimney would seem to have been often much shattered and
fissured in the course of the volcanic explosions, and the fragmentary
material has fallen or been injected into the rents thus caused. As a
rule, the rocks immediately around the Carboniferous necks are more or
less indurated, as in this instance from the Dunbar shore.
The materials which have filled up the vents connected with the
plateau-eruptions generally consist of (_a_) agglomerates or tuffs, but
occasionally of (_b_) some kind of lava, and frequently (_c_) of both
these kinds of rock combined.
(_a_) _Necks of Agglomerate or Tuff._--These materials vary greatly
in the nature and relative proportions of their constituents. Usually
the included blocks and lapilli are pieces of andesite, diabase,
basalt or other lava, like the rocks of the plateaux. But with these
occur also fragments probably detached from the sides of the funnels
through which the explosions took place, such as pieces of greywacke,
sandstone, limestone and shale. Considerable induration may be observed
among these non-volcanic ingredients. In some cases, as in that of the
occurrence of pieces of granite referred to on p. 382, the stones have
probably been brought up from some considerable depth. In others it is
easy to see that the blocks have slipped down from some higher group
of strata now removed from the surrounding surface by denudation. Some
striking illustrations of this feature will be cited from necks of the
puy-series in the south of Roxburghshire (p. 476).
The lava blocks in the tuffs and agglomerates are usually rounded or
subangular. Pear-shaped blocks, or flattened discs, or hollow spherical
balls are hardly ever to be observed, though I have noticed a few
examples in the tuffs of Dunbar. A frequent character of the blocks
is that of roughly rounded, highly amygdaloidal pieces of lava, the
cellular structure being specially developed in the interior, and the
cells on the outside being often much drawn out round the circumference
of the mass. Such blocks were probably torn from the cavernous,
partially consolidated, or at least rather viscous, top of a lava
column. Most of the stones, however, suggest that they were produced
by the explosion of already solidified lava, and were somewhat rounded
by attrition in their ascent and descent. The vents filled with such
materials must have been the scene of prolonged and intermittent
activity; successive paroxysms resulting in the clearing out of the
hardened lava column in the throat of the volcano, and in the rise of
fresh lava, with abundant ejection of dust and lapilli.
[Illustration: Fig. 127.--Section across the vents Dumgoyn and Dumfoyn,
and the edge of the Clyde plateau above Strathblane, Stirlingshire.
1. Upper Old Red Sandstone; 2. Shales, cement-stones and sandstones
("Ballagan beds"); 3. White sandstone; 4. Andesite lavas;
5. Agglomerate (shown by the dotted portions), traversed by
intrusive diabase. _f_, Fault. _D._ Late dolerite dyke.
]
Necks formed entirely of agglomerate are abundant among the vents
connected with the plateaux. As examples of them I may refer to the
series already mentioned as fronting the escarpment of the Clyde
plateau from Fintry to Largs. Another interesting group rises through
the Silurian and Old Red Sandstone rocks to the west of the escarpment
of the Berwickshire plateau, that near Melrose forming one of the
largest in Scotland.
[Illustration: Fig. 128.--Section through the large vent of the Campsie
Hills.
1. Andesite lavas; 2. Agglomerate and tuff; 3. Trachytic and andesitic
intrusive rocks.]
[Illustration: Fig. 129.--Diagrammatic section across the central vent
of the Clyde plateau in Renfrewshire.
1. Andesite lavas; 2. Agglomerates and fine tuffs often much altered;
3. Dykes of trachytic and andesitic rocks; 4. Later dykes of dolerite
and basalt.]
Illustrations of the varying structure of these vents are given in the
accompanying figures. In Fig. 127, a section is drawn through the two
necks Dumgoyn and Dumfoyn, which have already been shown in outline and
in ground-plan. The relation of these two vents to the neighbouring
plateau to the right can here be seen. Fig. 128 gives a section taken
through the great vent of the Campsie Hills, with the minor adjacent
necks of Dungoil, Bin Bairn, and the Meikle Bin.
The diagram in Fig. 129 is meant to convey in a general way what
appears to be the structure of the central vent of the Renfrewshire
plateaux, to be afterwards referred to. But, as already mentioned, the
limits of the various rocks are too much obscured to allow an accurate
delineation to be given of their areas and relations to each other. The
Berwickshire plateau supplies abundant interesting examples of tuff
necks which rise through the Old Red Sandstone many miles distant from
the edge of the lavas. This structure is shown in Fig. 130.
[Illustration: Fig. 130.--Section across Southern Berwickshire to show
the relation of the volcanic plateau to the vents lying south from it.
1. Upper Silurian strata; 2. Upper Old Red Sandstone; 3. The volcanic
plateau; 4. Agglomerate and tuff of the vents; 5. Basalt and dolerite;
6. Lower Carboniferous strata.]
Indications may occasionally be observed of an agglomerate vent having
been first occupied by one kind of material and then, after being in
great measure cleared out by explosions, having been subsequently
filled up with another. As an example of this structure I may cite
again the double neck of the Knock Hill a little to the north of Largs,
of which the outline is shown in Fig. 23, and the ground-plan in Fig.
125, B. This hill rises from the red sandstone slopes that front the
great Ayrshire plateau and forms a conspicuous cone the top of which
is rather more than 700 feet above the sea. Its summit commands a
remarkably extensive and interesting panorama of the scenery of the
Clyde, but to the geologist perhaps the most striking feature in the
landscape is the range of terraced hills behind, mounting up into the
great vents of the Renfrewshire uplands. On these declivities the
successive lava-streams that have built up the plateau can be seen
piled over each other for a thickness of more than 1000 feet, and
presenting their escarpments as parallel lines of brown crag with green
slopes between.
The Knock has had its upper part artificially dressed, for lines of
trench have been cut out of its rocks by some early race that converted
the summit of the hill into a strongly intrenched camp. From the apex
of the cone the ground falls rapidly westward into a hollow, beyond
which rises a lower rounded ridge of similar materials. It is possible
that this western ridge may really form part of the main hill, but
the grass-covered ground does not afford sufficient exposures of the
rocks to settle this point. From the contours of the surface, it may
be inferred that there are two closely adjacent vents, and that the
western and lower eminence is the older of the two. This hill or
ridge consists partly of a coarse agglomerate, and partly of veins and
irregular protrusions of a dark, compact, slightly cellular lava. The
stones in the fragmental rock are different olivine-basalts, or other
basic lavas, and sandstones. The paste is rough, loose and granular.
The sandstone fragments are much indurated and sometimes bleached.
The Knock itself is formed mainly of a remarkably coarse and strikingly
volcanic agglomerate. Round the outside, and particularly on the
south-east, the rock is finer in texture, compact, and gravelly, or
like a mudstone, with few or no imbedded blocks, dull-green to red in
colour, and breaking with a clean fracture which shows angular lapilli
of various basalts or diabases. At the southern end of the neck, where
the surrounding red sandstone can be seen within a few feet of the
tuff, the latter is bright red in colour, and contains much debris of
red sandstone and marl. Possibly this finer tuff, which is traceable
as an irregular band round the outside of the neck, may mark an older
infilling of the vent than the agglomerate of the centre; but there
is no sharp line to be drawn between the two, though a hollow can
sometimes be traced on the surface where they join.
The agglomerate of this locality is one of the most characteristic
among the plateau-necks of the Clyde region. Its blocks sometimes
measure from two to three feet in diameter. They consist almost
wholly of a dark crystalline porphyritic olivine-basalt. These blocks
are subangular in form, often with clean-fractured surfaces. Though
occasionally slightly cellular, they are never slaggy so far as I could
see, nor are any true scoriæ to be noticed among them. The blocks
suggest that they were derived from the disruption of an already
solidified mass of lava. The agglomerate is entirely without any trace
of stratification.
Through this tumultuous accumulation of volcanic debris some irregular
veins of olivine-basalt, sometimes glassy in structure, have been
injected, and reach nearly to the summit of the hill. This intrusive
material resembles generally some of the dark intrusive masses in the
Dumbartonshire necks. Like these, it exhibits a tendency to assume a
more or less distinctly columnar structure, its columns having the same
characteristic wavy sides and irregular curvature. The intrusive rocks
in the two eminences of the Knock may be paralleled among the stones
in the agglomerate. The neck on its north-eastern side rises steeply
from the red sandstones which it pierces, but which, although they are
much jointed and broken, are not sensibly indurated. Unfortunately the
actual junction of the igneous and sedimentary rocks is concealed under
herbage.
As a rule, the fragmental materials of the plateau-necks are quite
unstratified. Their included blocks, distributed irregularly through
the mass, have evidently undergone little or no assortment after
they fell back into the vents. Occasionally, however, a more or less
distinct bedding of the agglomerate or tuff may be observed, the
layers having a tendency to dip inward into the centre. One of the
most conspicuous examples of this structure is to be found in the
hill of Dumbuck, to the east of Dumbarton. This neck, which forms
so prominent a feature in the landscape, presents a precipitous
face towards the south, and allows the disposition of its component
materials to be there seen. The agglomerate consists of a succession
of rudely stratified beds of coarser and finer detritus, which on both
sides are inclined towards the centre, where a plug of fine-grained
olivine-basalt has risen and spread out into a columnar sheet above
(Fig. 131). In general form this basalt resembles such intrusions as
that of Largo Law, to be afterwards described (Fig. 226), where what
may have been the hollow or bottom of the crater is filled with basalt.
[Illustration: Fig. 131.--Section of south end of Dumbuck Hill. East of
Dumbarton.]
[Illustration: Fig. 132.--Section across the East Lothian plateau to
show the relative position of one of the necks.
1. Lower Carboniferous sandstones and shales; 2. Red and green tuffs
with a seam of limestone (_l_); 3. Band of basic sheets at the base of
the lavas; 4. Trachytes; 5. Phonolite neck.]
(_b_) _Necks of Andesite, Trachyte, Dolerite, Diabase, or other massive
Rock._--When the vents have been filled by the uprise of some molten
rock, it is generally, as we have seen, of a more acid character than
the ordinary lavas of the plateaux. Frequently it consists of some
variety of trachyte or andesite, commonly of a dull yellow or grey
tint and waxy lustre. Good examples may be seen among the remarkable
group of necks on either side of the valley north of the village of
Strathblane and in those above Bowling. The three great necks in East
Lothian, already alluded to,--Traprain Law (Figs. 132, 133), North
Berwick Law (Fig. 109), and the Bass Rock (Fig. 110)--are masses
of phonolite and trachyte, obviously related to the trachytes of
the adjacent plateau. A smaller but very perfect instance of a vent
similarly filled is to be seen in the same neighbourhood on the shore
to the east of North Berwick Law.[438]
[Footnote 438: See "Geology of East Lothian," _Geological Survey
Memoir_, p. 40.]
Examples occur where the funnels of eruption have been finally sealed
up by the rise of more basic material, and this has happened even in
a district where most of the lava-form necks consist of trachyte or
some other intermediate lava. Thus, in the Campsie Fells, several such
bosses appear, of which the most conspicuous forms the hill of Dungoil
(1396 feet, Fig. 128). Further west, among the Kilpatrick Hills, bosses
of this kind are still more numerous. The group of bosses near Ancrum
and Jedburgh is mainly made up of olivine-dolerites and olivine-basalts
(Fig. 130). This more basic composition of itself suggests that
these bosses may be connected rather with the puy- than with the
plateau-eruptions.
(_c_) _Necks of Composite Character._--In not a few examples, the vents
have been filled with agglomerate which has been pierced by a plug
or veins of lava-form material. Many illustrations of this composite
structure may be observed along the west front of the great escarpments
from Fintry to Ardrossan (see Figs. 124, 125, 127 and 128). In that
region the intruded rock is often a dull yellowish or grey trachytic
or andesitic material. Olivine-basalt is the chief rock intruded in
the vents in the Dumbarton district. Among the Roxburghshire vents,
where the injected material is commonly olivine-basalt or dolerite, it
occasionally happens, as in Rubers Law, that the uprise of the lava has
almost entirely cleared out or concealed the agglomerate, and in some
of the bosses, where no agglomerate is now to be seen, the basalt may
have taken its place (Fig. 130).
The largest and most interesting vents connected with this type of
Carboniferous volcano, are those which occur within the limits of
the plateaux, where they are still surrounded with lavas and tuffs
that probably came out of them. Of these by far the most extensive
and remarkable lies among the high moorlands of Renfrewshire between
Largs and Lochwinnoch, where the ground rises to more than 1700 feet
above the sea (see Fig. 129). This area, as already remarked, is
unfortunately much obscured with drift and peat, so that the limits
of its rocks cannot be so satisfactorily traced as might be desired.
I think it probable that several successive vents have here been
opened close to each other, but their erupted ashes probably cannot be
distinguished. Over a space measuring about four miles in length by
two and a half in breadth, the rocks exposed at the surface are fine
tuffs, breccias and coarse agglomerates, largely made up of trachytic,
andesitic or felsitic material, and pierced by innumerable protrusions
of various andesitic, trachytic or felsitic rocks in bosses and veins,
as well as also by dykes of a more basic kind, such as dolerites and
basalts. Some of the tuffs present a curiously indurated condition; and
they are frequently much decayed at the surface.[439] Another large
mass of tuff and agglomerate lies a little to the south-west of the
main area.
[Footnote 439: This tract of ground was mapped for the Geological
Survey by Mr. R. L. Jack, now in charge of the Geological Survey of
Queensland. See Sheet 31, _Geological Survey of Scotland_.]
After the explosions ceased, by which the vents were opened and the
cones of debris were heaped up, heated vapours would in many cases,
as in modern volcanoes, continue for a long while to ascend in the
funnels. The experiments of Daubrée on the effects of water and vapour
upon silicates under great pressure and at a low red heat, have shown
how great may be the lithological changes thereby superinduced. It
is improbable that where a mass of tuff and lava, lying deep within
a volcanic vent, was thoroughly permeated with constantly ascending
heated vapours, it should escape some kind of change. I am inclined to
attribute to this cause the frequent conversion of the sandstones round
the walls of the vents into quartzite. The most remarkable example of
metamorphism within a vent which I have observed among the plateaux,
occurs in the heart of the Campsie Fells, where, instead of forming
a prominence, the neck is marked by a great hollow, measuring about
a mile in length and half a mile in breadth (Fig. 128).[440] It is
occupied mainly by a coarse tumultuous agglomerate, like that of other
necks in the same district, but with a matrix rather more indurated,
and assuming in certain parts a crystalline texture, so as to be
at first sight hardly distinguishable from some of the surrounding
andesites. Even in this altered condition, however, its included
fragments may be recognized, particularly blocks of sandstone which
have been hardened into quartzite. Numerous small veins of pink and
yellow trachyte traverse the agglomerate, and are found also cutting
the bedded andesites that encircle it.
[Footnote 440: See Explanation to Sheet 31, _Geological Survey of
Scotland_, par. 21 (1878).]
[Illustration: Fig. 133.--View of Traprain Law from the south, a
phonolite neck of the Garleton Plateau.]
3. DYKES AND SILLS
Intrusive masses both in the form of dykes and of sills are of frequent
occurrence in connection with the Carboniferous volcanic plateaux. From
the variety of their component materials it may be inferred that these
rocks belong to different ages of intrusion.
Dykes.--The great majority of the Dykes consist of trachyte or of
andesite, resembling in lithological characters the material of the
necks and doubtless connected with its uprise. There occur also dykes
of diabase, basalt or dolerite. Some of the latter, especially those
which run for many miles, cutting every rock in the districts in
which they occur, and crossing large faults without deviation, are
certainly long posterior to the plateau volcanic period. Whether the
small inconstant dykes of more basic composition, found in the same
districts with the trachytes, are to be looked upon as part of the
volcanic phenomena of the plateaux, is a question to which at present
no definite answer can be given. I shall have occasion to show that in
the next volcanic period the lavas that flowed from the puys are more
basic than most of those of the plateaux, and that they are associated
with more basic dykes and sills. In Roxburghshire, where it is so
difficult to distinguish between the denuded vents of the two periods,
the dark heavy olivine-basalts and dolerites of the bosses may possibly
belong rather to the later than to the earlier volcanic episode. And if
that be their true age, the dykes of similar material may be connected
with them. At the same time it must be remembered that the earliest
eruptions of the plateaux were markedly basic, that many vents in the
plateaux are pierced by basic intrusions, and that basic dykes may have
been associated with the uprise of the same magma.
The dykes occur in considerable numbers and in two distinct positions,
though these may be closely related to each other: 1st, among the rocks
outside and beneath the plateau-lavas, or cutting these lavas; and 2nd,
in and around the vents.
1. Among the rocks which emerge from under the Carboniferous volcanic
plateaux, dykes are sometimes to be observed in considerable numbers.
They may be compared to the far more extensive series connected
with the Tertiary basalt-plateaux, like which they may have had a
close relation to the actual building up of the successive sheets
of andesite, trachyte and basalt that were erupted at the surface.
They are particularly well developed in the Clyde plateau, where
by extensive denudation they have been admirably exposed. I would
especially refer to those that traverse the tract of red sandstones
which underlie the volcanic series along the flanks of the great
escarpments from Fintry to Strathblane and Dumbarton, and between
Gourock and Ardrossan. These dykes have been dissected by the sea along
both sides of the estuary of the Clyde and in the islands of Cumbrae.
In these islands and in Bute they have recently been mapped in great
detail for the Geological Survey by my colleague, Mr. W. Gunn, who has
supplied me with notes of his observations on the subject, from which
the following summary is compiled.
"There are at least four distinct groups of intrusive rocks in the
Greater Cumbrae. The oldest of these is trachytic in character, and
occurs both as dykes and sheets, which run generally in the same E.N.E.
direction. The rock is usually pinkish in colour, sometimes grey or
purplish. A specimen from the dyke of the Hawk's Nest, north of Farland
Point, analyzed by Mr. Teall, was found to contain 11 per cent of
alkalies, principally potash, while the percentages of lime and iron
were very low. Sometimes these rocks are fine in grain with only a few
porphyritic orthoclase crystals, though numerous small crystals of this
mineral are found with the aid of the microscope. These red trachyte
dykes are almost confined to the Upper Old Red Sandstone, rarely
entering the overlying white Calciferous Sandstones, and never invading
the plateau-lavas. They are therefore probably of early Carboniferous
age.
"The next group follows the same general direction, but clearly
traverses the trachytes, and must therefore be of later date. The dykes
of this group are the most numerous of the whole, the greater part of
the island being intersected by them. In the north-east corner about
40 of them may be counted in half a mile of coast-line, some being of
large size. All of them which can be clearly made out are porphyritic
olivine-basalts of the type of the Lion's Haunch at Arthur's Seat.
They are generally grey in colour and finer at the edges than in the
centre, which is often coarsely porphyritic and amygdaloidal. Olivine
seems always characteristic, but has often been replaced by hæmatite
or calcite. In Bute a good many dykes have been mapped to the north of
Kilchattan Bay resembling this basalt series of Cumbrae, and running in
the same direction. But they appear to be all porphyritic andesites.
The second group of dykes, though it cuts the first and is thus
proved to be later in date, is nevertheless confined within the same
stratigraphical limits. It may thus belong nearly to the same period of
intrusion.
"The dykes of the third group are dolerites without olivine, and
follow on the whole an east and west direction. They cut both of the
two foregoing sets of dykes, and likewise the lavas of the plateau.
They must thus belong to a far later period of intrusion. They may be
connected with other dykes and sills on the mainland, which traverse
the Coal-measures, and would thus be not older than late Carboniferous
or Permian time.
"The fourth group of dykes intersects all the others, and is probably
of Tertiary age. The prevalent direction of these dykes in the Cumbraes
is N.N.W." The Tertiary dykes are more fully described in Chapters
xxxiv. and xxxv.
The great group of tuffs which underlies the lavas of the East Lothian
plateau is traversed by numerous dykes and sills, of which many good
examples may be seen in the coast-cliffs of North Berwick. Among these
rocks are beautiful olivine-basalts with singularly fresh olivine, as
on the shore at North Berwick. Some of them are still more basic, as in
the case of a limburgite intrusion at the Gin Head, Tantallon Castle.
[Illustration: Fig. 134.--Veins and dykes traversing the agglomerate
and tuff of the great Renfrewshire vent.]
2. In the necks, dykes are sometimes abundant, and they may be observed
occasionally to traverse the surrounding lavas. They consist of similar
materials to those found outside the plateaux. Some of the larger necks
are intersected by a network of dykes and veins. The great vent or
group of vents among the uplands of Renfrewshire, already described
(Fig. 129), furnishes some admirable examples of this characteristic
volcanic feature. An illustration from that locality forms the subject
of Fig. 134. The agglomerate which fills the large hollow among the
Campsie Hills may be quoted as another illustration (Fig. 128). Further
instances will be found in some of the sections given in preceding
pages (see Figs. 124, 125, 127). The general aspect of a dyke in the
volcanic series is shown in Fig. 135.
[Illustration: Fig. 135.--"The Yellow Man," a dyke in volcanic tuff
and conglomerate on the shore a little east of North Berwick.]
The Sills associated with the plateau-type of Carboniferous volcanic
action form a less prominent feature than they do among the earlier
Palæozoic formations or in the puy-type which succeeded them. They
consist in general of short lenticular sheets of andesite or trachyte,
like the necks and dykes in proximity to which they commonly appear.
The best area for the study of them is the ground which stretches out
from the base of the great escarpments of the Campsie, Kilpatrick
and Ayrshire Hills (Fig. 136), where, among the agglomerate-vents
and abundant dykes, intrusive sheets have likewise been injected
between the bedding-planes of the red sandstones. But these sheets
are of comparatively trifling dimensions. Very few of them reach a
mile in length, the great majority falling far short of that size. In
the Cumbraes and in Bute, Mr. Gunn has observed that the trachytic,
olivine-basalt and dolerite dykes are apt to pass into intrusive
sheets. That the sills, as well as the dykes and bosses of the same
material, are not of older date than the lavas of the plateaux is
proved by the manner in which they pierce these lavas, especially
towards the bottom of the series. The general absence of basic sills,
when we consider how thick a mass of these rocks has sometimes been
poured out in the plateaux, is not a little remarkable. Only in the
basin of the Firth of Forth do we encounter thick basic sills near the
plateaux, such, for instance, as Salisbury Crags at Edinburgh. But it
is doubtful whether they ought not rather to be classed with the sills
of the puys, to be afterwards described.
[Illustration: Fig. 136.--Trachytic sills, Knockvadie, Kilpatrick
Hills.
1. Upper Old Red Sandstone; 2. "Ballagan Beds"; 3. Tuffs; 4. Lavas of
the Plateau; 5. Agglomerate of necks; 6. Trachyte sills; 7. Dolerite
dyke (? Tertiary).]
4. Close of the Plateau-eruptions
The relative geological date when the eruptions of each plateau ceased
can fortunately be determined with much more precision than the time
of their beginning. The Hurlet Limestone, so well known as the lowest
thick calcareous seam in the Carboniferous Limestone series, of which
it is generally taken as the base, can be identified over the whole of
Central Scotland, and thus forms an excellent stratigraphical horizon,
from which the upward termination of the volcanic sheets underneath it
can be measured.
When the volcanic episode of the plateau-eruptions came to an end, such
banks or cones as rose above the level of the shallow sea which then
overspread Central Scotland were brought beneath the water, as I have
already remarked, either by prolonged denudation or more probably in
large part by the continued subsidence of the region. The downward
movement may possibly for a time have been accelerated, especially in
some districts. Thus the Hurlet Limestone, though usually not more than
five or six feet thick, increases locally to a much greater thickness.
At Petersfield, near Bathgate, for example, it is between 70 and 80
feet in depth, while at Beith, in North Ayrshire, it increases to 100
feet (Fig. 137), which is the thickest mass of Carboniferous Limestone
known to exist in Scotland. At both of these localities the limestone
lies upon a series of volcanic rocks, and we may perhaps infer that
the subsidence advanced there somewhat more rapidly or to a greater
extent, so as to form hollows in which the limestone could gather
to an abnormal depth. The water would appear to have become for a
time tolerably free from mechanical sediment. The limestone is hence
comparatively pure, and is extensively quarried all over the country
for industrial purposes. It is a crinoidal rock, abounding in many
species of corals, brachiopods, lamellibranchs, and gasteropods, with
trilobites, cephalopods, and fishes.
[Illustration: Fig. 137.--Section across the edge of the Clyde plateau,
south-east of Beith.
1. Plateau-lavas; 2. Tuffs and volcanic conglomerates; 3. Hurlet
Limestone; 4. Coal-bearing strata above the limestone; 5. Dolerite
dyke.]
A variable thickness of strata intervenes between the top of the
volcanic series and the Main Limestone. Sometimes these deposits
consist in large measure of a mixture of ordinary sandy and muddy
material with the washed-down tuff of the cones, and probably with
volcanic dust and lapilli thrown out by the latest eruptions. Thus
along the flank of the hills from Barrhead to Strathavon, yellow
and green ashy sandstones, grits and conglomerates are succeeded by
ordinary sandstones, black shales and ironstones, while here and there
true volcanic tuff and conglomerate make their appearance.[441] Further
west, in the Kilbirnie district, the limestone lies directly on the
tuffs that rest upon the andesites (Figs. 137, 138).
[Footnote 441: Explanation of Sheet 22, _Geol. Surv. Scotland_, p. 12.]
[Illustration: Fig. 138.--Section across the upper part of the Clyde
plateau at Kilbirnie, Ayrshire.
1 1. Plateau-lavas; 2 2. Tuffs; 3 3. Hurlet Limestone; 4. Black-band
Ironstone. _f_ _f_. Faults.]
But perhaps the most striking contrast between adjacent localities
in regard to the distance between the limestone and the top of the
volcanic series is to be observed along the southern front of the
Campsie Fells. In spite of the abundant faults which have there so
broken up the regular sequence of the rocks, we can see that at
Banton and Burnhead the limestone lies almost immediately on the
volcanic series (Fig. 139). But a little to the westward, sandstones,
conglomerates, shales and thin limestones begin to intervene between
the volcanic series and the Hurlet Limestone and swell out so rapidly
that on Craigmaddie Muir and South Hill of Campsie, only some five
miles off, they must form a total thickness of not less than from 600
to 800 feet of ordinary non-volcanic deposits, chiefly thick pebbly
sandstones (Fig. 140). Such local variations not improbably serve to
indicate hollows on the flanks of the plateaux that were filled up with
detritus before the depression and clearing of the water that led to
the deposition of the Hurlet Limestone.
[Illustration: Fig. 139.--Section across the upper surface of the Clyde
volcanic plateau, Burnhead, north-west of Kilsyth.
1. Lavas of the plateau; 2. Tuffs; 3. Hurlet Limestone; 4. Hosie's
Limestone; _f_, Fault.]
[Illustration: Fig. 140.--Section across the upper surface of the Clyde
volcanic plateau at Campsie.
1. Shales, sandstones, cement-stones, etc. ("Ballagan Beds"); 2. Lavas
of the plateau; 3. Thick white sandstone and conglomerate; 4. Hurlet
Limestone; 5. Hosie's Limestone; _f_. Fault.]
[Illustration: Fig. 141.--Section across western edge of the Garleton
plateau.
1. Trachyte lavas of the plateau; 2. Calciferous Sandstones; 3. Hurlet
Limestone.]
I have already remarked that the eruptions of the plateau period lasted
longer in the western than in the eastern parts of the region. In the
Garleton district, where the peculiar viscous trachytic lavas probably
gave rise to a more uneven surface or more prominent cones than was
usual among the andesitic plateaux, the eruptions ceased some time
before the deposition of the Hurlet Limestone. As the area sank, the
successive zones of the Calciferous Sandstones crept over the flanks
of the trachytes, until at last they had completely buried these rocks
before the limestone spread over the area (Fig. 141). In consequence,
probably, of the uneven surface of this plateau, there is here a strong
overlap of the higher part of the Calciferous Sandstones. On the west
side of the volcanic area there can hardly be more than some 200 feet
of strata between the top of the trachytic series and the limestone,
while on the south side there must be greatly more than that thickness.
This structure probably indicates that the Garleton volcanoes became
extinct after having piled up a mass of tuffs and lavas to such a
height that its summits were not submerged until the area had subsided
800 or 1000 feet in the waters, over the floor of which the Calciferous
Sandstones were laid down. Hence, in spite of the proximity of the
lavas to the limestone, there may have been a vast interval of time
between their respective epochs, as has been already suggested with
regard to other plateaux. This subject will be again referred to in
discussing the relative chronology of the plateaux and puys.
In the Berwickshire and Solway districts, the extinction of the
plateau-vents appears to have taken place at a still earlier part of
the Carboniferous period, for there the andesites, while they rest on
the Upper Old Red Sandstone, are covered with at least the higher group
of the Calciferous Sandstones (Fig. 142). The equivalent of the Hurlet
Limestone of Central Scotland must lie many hundred feet above them.
The submergence of the plateaux, and their entombment under the
thick Carboniferous Limestone series, did not mark the close of
volcanic activity in Central Scotland during Carboniferous time. The
plateau-type of eruption ceased and was not repeated, but a new type
arose, to which I would now call the reader's attention.
[Illustration: Fig. 142.--Section across the Solway plateau from
Birrenswark to Kirtlebridge.
1. Upper Silurian strata; 2. Upper Old Red Sandstone; 3. Plateau-lavas;
4. Calciferous Sandstones and Carboniferous Limestone series; 5.
Trias.]
CHAPTER XXVI
THE CARBONIFEROUS PUYS OF SCOTLAND
i. General Character and Distribution of the Puys; ii. Nature of
the Materials Erupted--Lavas Ejected at the Surface--Intrusive
Sheets--Necks and Dykes--Tuffs.
i. GENERAL CHARACTER AND DISTRIBUTION
After the beginning of the Carboniferous Limestone period, when
eruptions of the plateau-type had generally ceased, volcanic activity
showed itself over the area of the British Isles in a different guise
both as regards the nature of its products and the manner and scale
of their discharge. Instead of widely extended lava-sheets and tuffs,
piled above each other sometimes to a thickness of many hundred feet,
and stretching over hundreds of square miles, we have now to study the
records of another phase of volcanism, where scattered groups and rows
of _Puys_, or small volcanic cones, threw out in most instances merely
tuffs, and these often only in trifling quantity, though here and there
their vents also poured forth lavas and gradually piled up volcanic
ridges which, in a few cases, almost rivalled some of the plateaux. The
evidence for these less vigorous manifestations of volcanic activity
is furnished (1) by layers of tuff and sheets of basaltic-lavas
intercalated among the strata that were being deposited at the time
of the eruptions, (2) by necks of tuff, agglomerate, or different
lava-form rocks that mark the positions of the orifices of discharge,
and (3) by sills, bosses, and dykes that indicate the subterranean
efforts of the volcanoes. The comparatively small thickness of the
accumulations usually formed by these vents, their extremely local
character, the numerous distinct horizons on which they appear, and
the intimate way in which they mingle and alternate with the ordinary
Carboniferous strata are features which at once arrest the attention of
the geologist, presenting, as they do, so striking a contrast to those
of the plateaux.
From the clear intercalation of these volcanic materials on successive
platforms of the Carboniferous system, the limits of geological
time within which they were erupted can be fixed with considerable
precision. It may be said that, in a broad sense, they coincided with
the period of the Carboniferous Limestone, and certainly it was during
the deposition of that formation that the eruptions which produced
them reached their greatest vigour and widest extent. Here and there
in Scotland evidence may be found that the phase of the Puys began
during that earlier section of Carboniferous time recorded by the
Calciferous Sandstones. This is markedly the case in Liddesdale and
the neighbouring territory. Over the western part of Midlothian also,
the eastern portion of West Lothian, and the southern margin of Fife,
abundant traces occur of puy-eruptions during the deposition of the
Calciferous Sandstones. Elsewhere in Central Scotland there is no
evidence of the vents having been opened until after the deposition
of the Hurlet Limestone, which, as we have seen, may conveniently be
taken as the base of the Scottish Carboniferous Limestone series. The
volcanoes remained active in West Lothian until near the close of the
time represented by that series; but in Ayrshire they continued in
eruption until the beginning of the accumulation of the Coal-measures.
These western examples of the puy-type are, so far as I am aware, the
latest known in Britain.
Whether or not the earliest puy-eruptions began before the latest
plateau-lavas and tuffs were accumulated is a question that cannot be
readily answered. It will be remembered that in the basin of the Firth
of Forth a thickness of more than 3000 feet of sedimentary strata,
including the Burdiehouse Limestone and numerous oil-shales as well as
thin coal-seams, lies above the red and green marls, shales, sandstones
and cement-stones of the Calciferous Sandstone series. This remarkable
assemblage of strata is absent in the western parts of the country,
where the top of the Clyde volcanic plateau is almost immediately
overlain by the Hurlet Limestone. If we were to judge of the sequence
of events merely from the stratigraphy, as expressed in such sections
as Figs. 137, 138, 139 and 140, we might naturally infer that as no
trace of any break occurs at the top of the Clyde plateau, the tuffs
shading upward there into the limestone series, no important pause in
sedimentation took place, but that the last volcanic eruptions were
soon succeeded by the conditions that led to the deposition of the
widespread encrinite-limestones. If this inference were well founded it
would follow that while the plateau-eruptions in the west lasted till
the time of the Hurlet Limestone, those in the east ceased long before
that time and were succeeded by the puys of Fife and the Lothians.
There would thus be an overlap of the two phases of volcanic action.
I am inclined to believe, however, that in spite of the superposition
of the Hurlet Limestone almost immediately upon the volcanic rocks
of the Clyde plateau, and the absence of any trace of a break in the
process of sedimentation, a long interval nevertheless elapsed between
the last eruptions and the deposit of that limestone. The Campsie
section (Fig. 140) shows us how rapidly a thick mass of strata can come
in along that horizon. The volcanic ridges may have remained partly
unsubmerged for such time as was required for the subsidence of the
Forth basin and the deposit of the thick Calciferous Sandstone series
there, and their summits may only have finally sunk under the sea
not long before the Hurlet Limestone grew as a continuous floor of
calcareous material over the whole area of central Scotland. In these
circumstances, the puy-eruptions of that basin would be long subsequent
to the eruptions of the Clyde plateau, as they certainly were to those
of the plateaux of Midlothian and the Garleton Hills.
In tracing the geographical distribution of the puy-eruptions we are
first impressed with the force of the evidence for their extremely
local and restricted character (Map IV.). Thus in the area of the basin
of the Firth of Forth, which may be regarded as the typical region in
Britain for the study of this form of Carboniferous volcano, traces of
them are abundant to the west of the line of the Pentland Hills. To
the east of that line, however, not a vestige of puy-eruptions, save a
few sills of uncertain relationship, can be detected, though the same
series of stratigraphical horizons is well developed on both sides
of the Lothian coal-field. Again, to the westward of the Forth basin
over the area of Stirlingshire, Lanarkshire and Renfrewshire lying to
the north of the great volcanic plateau, no record of puy-eruptions
has been noticed. Immediately to the south of that plateau, however,
these eruptions were numerous in the north of Ayrshire. Yet the rest of
the Carboniferous area in that large county has supplied no relics of
these eruptions save at one locality--the Heads of Ayr. Lastly, while
no trace of any younger display of volcanic activity occurs in the
Merse of Berwickshire, east of the plateau series of that district, the
ground immediately to the west abounds in puys, and contains likewise
extensive sheets of tuff and beds of basic lavas connected with these
vents.
Another fact which at once attracts notice in Scotland is the way in
which the puy-vents have generally avoided the areas of the plateaux,
though they sometimes approach them closely. As a rule, it is possible
to distinguish the tuffs and agglomerates which have filled up these
vents from those that mark the sites of the eruptive orifices of the
plateaux. There are, no doubt, some instances, as in Liddesdale, where
puys have appeared on the sites of the older lavas, but these are
exceptional collocations.[442] On the other hand, many examples may be
found where puys have risen in the interspace between the limits of
the eruptions of two plateau-areas. Thus the tract between the Clyde
plateau-eruptions on the west and those of the Garleton Hills on the
east was dotted over with puys. Again, the southern margin of the Clyde
plateau in Ayrshire, from Dalry to Galston is flanked with puys and
long sheets of their lavas and tuffs.[443]
[Footnote 442: A means of definitely placing some of these vents in the
series of puy-eruptions is stated further on, at p. 476.]
[Footnote 443: Reference may again be made here to the remarkable
similarity between the Scottish Carboniferous puy-vents and those of
older Tertiary time in the Swabian Alps so fully described by Professor
Branco in the work already cited p. 46. Denudation in that region has
bared the cones and exposed the structure of the necks which, down to
even minute details, repeat the phenomena of Carboniferous and Permian
time in Scotland.]
ii. NATURE OF THE MATERIALS ERUPTED
A. The Lava-form Rocks
We have now to consider the nature of the materials erupted by the
volcanic activity of the puys. The geologist who passes from the
study of the plateau lavas to those of the puys at once remarks the
prevalent more basic character of the latter. The great majority of
them are basalts, generally olivine-bearing, in the various types
embraced in the table on the following page. The olivine-free dolerites
are generally found as intrusive bosses, sills and dykes. Such more
acid rocks as andesites occur only rarely, and still more seldom are
quartziferous masses met with in some of the bosses.
Dolerites and Basalts.--The great majority of the lava-form rocks
connected with the puys are basic in composition, and belong to
the family of the Dolerites and Basalts. They graduate, on the one
hand, into ultra-basic rocks such as limburgite and picrite, and on
the other, into compounds that approach andesites or trachytes in
composition. A large series of specimens from Central Scotland was
studied a few years ago by Dr. Hatch, who proposed a petrographical
classification of the rocks, and arranged them in a number of types
which he named after localities where they are well developed.[444]
More recently the rocks have again been subjected to microscopic
investigation by my colleague Mr. Watts, who, confirming generally Dr.
Hatch's discriminations, has made some modifications of them. He has
furnished me with a revised classification (p. 418), based on purely
petrographical considerations. The doleritic and basaltic series
may be grouped into two divisions, one with, and the other without,
olivine, and each division may be further separated into a dolerite
group, which presents an ophitic or sub-ophitic structure, and a
basalt-group in which the groundmass is made up of felspar and granular
augite, and possesses the "intersertal structure" of Rosenbusch, or
consists of idiomorphic augite embedded in felspar substance. The term
"sub-ophitic" is employed by Mr. Watts "to imply that the augite grains
are neither very large nor very continuous, optically, and that they
rarely contain entire felspar-crystals imbedded in them, merely the
ends of a group of these crystals as a rule penetrating into them."
[Footnote 444: This classification was given in my Presidential Address
to the Geological Society, 1892, _Quart. Journ. Geol. Soc._ vol.
xlviii. p. 129. See Report of Geological Survey for 1896.]
Transitional forms occur between many of the following types by the
increase or diminution in the relative proportions of the constituents.
Thus it is not easy to draw a line between 2_b_ and 2_c_; the latter
again shades into 2_d_ and 2_e_ by the decrease of the felspar.
Mr. Watts has further observed that the rocks containing no olivine
offer greater difficulties in classification than those in which that
mineral is present. "The very distinction," he remarks, "between
dolerites and basalts is less marked, the types are much less sharply
distinguished, and decomposition and masking of the structure are
more common. While using the term Dolerite for such rocks as have a
sub-ophitic structure, I have extended it to those rocks in which
evidence exists that a great part of the crystallization took place
under intratelluric conditions. Although not quite holocrystalline,
the crystals of felspar, augite and magnetite are large and the
structure coarse-grained, while the groundmass is confined to
comparatively small interstitial patches. In these rocks there is
usually no one dominant porphyritic ingredient."
I. The Olivine-bearing Series
1. _Olivine-Dolerites_
1_a_. Porphyritic elements inconspicuous, olivine being }
the principal, and felspar of secondary importance; } Jedburgh Type.
groundmass sub-ophitic. }
1_b_. Strongly porphyritic; felspar-phenocrysts large; } Kilsyth Type.
olivine smaller; groundmass sub-ophitic. }
1_c_. Porphyritic olivine, but not felspar; groundmass } Gallaston Type.
sub-ophitic. }
2. _Olivine-Basalts_
2_a_. Porphyritic olivine, augite and felspar; groundmass} Lion's Haunch
of felspar-laths imbedded in granules of augite. } Type.
} (See Fig. 207.)
2_b_. Porphyritic olivine and augite; groundmass of }
felspar-laths imbedded in granules of augite. More } Craiglockhart
rarely the groundmass is made of idiomorphic augite } Type.
imbedded in felspar-substance. }
2_c_. Porphyritic olivine abundant, augite much less }
common, and felspar very rare or absent; groundmass } Dalmeny Type.
with granular or idiomorphic augite (one of the most }
common types). }
2_d_. Porphyritic olivine more common than augite; }
groundmass of granules of augite set amongst lath-like } Picrite Type.
felspars which are much fewer in number than in 2_c_. }
2_e_. Porphyritic olivine more common than augite; }
groundmass of idiomorphic augite imbedded in } Limburgite
felspathic material which is not abundant. } Type.
2_f_. Porphyritic olivine and felspar, without augite; } Kippie Law Type.
groundmass of granular or idiomorphic augite, with } (For analysis
lath-shaped felspars. } see p. 379).
II. The Non-olivine-bearing Series
3. _Olivine-free Dolerites_
Felspar, augite, magnetite in coarse-grained aggregate usually ophitic
or sub-ophitic; groundmass not plentiful.
3_a_. Groundmass absent Ophitic Type.
3_b_. Groundmass micropegmatitic Ratho Type.
3_c_. Groundmass an unstriated felspar (not orthoclase) }
and occasionally some interstitial altered glass or a } Burntisland
little quartz. } Sill Type.
4. _Doleritic Basalts_
Felspar, augite and magnetite in coarse-grained aggregate; groundmass
rather more plentiful and often in large patches.
4_a_. Felspar and augite, related sub-ophitically where }
together, but augite showing crystalline contours in } Bowden
contact with the groundmass; some interstitial quartz } Hill Type.
and unstriated felspar. }
5. _Basalts_
Finer-grained rocks, generally with a porphyritic ingredient and much
scattered interstitial matter in the groundmass.
5_a_. Porphyritic felspar, and occasionally a little }
augite; groundmass of granular augite, felspar needles } Binny Craig
and magnetite with some interstitial matter. } Type.
5_b_. Porphyritic felspars not conspicuous and small; the }
rock mainly made up of a mesh of fine felspar-laths set } Tholeiite
amongst granular augite, magnetite and base. } Type.
5_c_. Similar to the last but even finer-grained, and } Crypto-
with the base in a cryptocrystalline condition. } crystalline
} Type.
Taking first the superficial lavas, I know of only one locality where
picrite occurs in such a position that it may be included among the
surface outflows. This is the quarry near Blackburn, to the east of
Bathgate, where I originally observed it.[445] The rock occurs there
on the line of the basalt-flows from the Bathgate Hills, and I mapped
it as one of them before the microscope revealed the remarkable
composition of the mass. I still believe it to be a lava like the
"leckstone" described on p. 443, though the other known examples of
this rock in the basin of the Firth of Forth are intrusive sheets.
The rock locally known as "leckstone" or "lakestone" has long been
quarried for the purpose of constructing the soles of bakers' ovens, as
it stands a considerable temperature without cracking. Its microscopic
structure is now well known. As exposed in Blackburn quarry, an
interesting difference is observable between the lower and upper
parts of the sheet. The lower portion is a picrite, with abundant
serpentinized olivine, large crystals of augite, and a considerable
amount of ores. The upper portion, on the other hand, has plagioclase
as its most abundant definite mineral, with a minor quantity of minute
prisms of augite and of iron-ores, and scattered crystals of olivine.
Here, within the compass of a few yards and in one continuous mass of
rock, we have a transition from a variety of olivine-basalt into a
picrite.
[Footnote 445: _Trans. Roy. Soc. Edin._ vol. xxix. (1879) p. 506.]
The great majority of the puy lavas belong to the olivine-bearing
series. A few of them are dolerites, but most are true basalts of the
Dalmeny type, of which typical examples may be seen at the Kirkton
quarries, Bathgate, and in the coast section between Pettycur and
Kinghorn. Occasionally they present transitions towards picrite, as in
the sheet overlying the lowest limestone at Kirkton, and in the lowest
lava of King Alexander's Crag, Burntisland. These puy lavas exhibit
considerable variety of structure as seen in the field. Some are
solid, compact, black rocks, not infrequently columnar and weathering
into spheroidal exfoliating forms. Others are somewhat granular in
texture, acquiring green and brown tints by weathering, often showing
amygdaloidal kernels, and even passing into well-marked amygdaloids.
Many of them exhibit a slaggy structure at their upper and under
surfaces (Figs. 153, 170, 171). These external differences are an
index to the corresponding variations in composition and microscopic
structure enumerated in the foregoing tabular arrangement.
As a rule, the basic rocks which occur intrusively in connection
with the puys, especially where they form a considerable mass, have
assumed a much more coarsely crystalline texture than those of
similar composition which have been poured out at the surface. They
are generally dolerites rather than basalts. But with this obvious
distinction, the two groups have so much in common, that the geologist
who passes from the study of the subterranean phenomena of the Plateaux
to that of the corresponding phenomena of the Puys is at once impressed
with the close relationship between the material which, in the case
of the puys, has consolidated above ground, and that which has been
injected below. There is no such contrast between them, for example, as
that between the basic and intermediate lavas of the plateaux and the
more acid intrusions associated with them.
By far the largest number of the basic sills, bosses and dykes
associated with the puys are somewhat coarsely crystalline dolerites
without olivine. They include, however, olivine-dolerites and basalts,
and even some extremely basic compounds. Of these last, a typical
example is supplied by the now well-known picrite of Inchcolm, in
the Firth of Forth, which occurs as an intrusive sheet among the
Lower Carboniferous Sandstones.[446] In recent years one or two other
picrite-sills have been observed in the same district. An interesting
example has been described from a railway cutting between Edinburgh
and Cramond where the rock invades and alters shales. More detailed
reference to it will be made in the account of the sills connected
with the puys. Another instance of the occurrence of this rock is in
a railway cutting immediately to the west of Burntisland where it has
been intruded among the Calciferous Sandstones below the Burdiehouse
Limestone.
[Footnote 446: _Trans. Roy. Soc. Edin._ vol. xxix. (1879) p. 506.
Teall, _British Petrography_, p. 94.]
Rocks approaching limburgite occur among the sills and bosses which
pierce the Carboniferous Limestone series of Fife between Cowdenbeath
and Inverkeithing. One of these is found at Pitandrew, near Fordel
Castle. Dr. Hatch observed that it consists of "numerous porphyritic
crystals of olivine, with a few grains of augite and an occasional
small lath-shaped crystal of felspar imbedded in a groundmass which is
composed principally of idiomorphic augite microlites, small crystals
of a brown mica, granules of magnetite and prisms of apatite. In
addition, there is a considerable amount of interstitial matter, which
is partly colourless glass, and partly shows a slight reaction between
crossed nicols." Another example of the same type of rock occurs as
a plug or boss in the tuff-vent of the Hill of Beath, and a further
display of the limburgite type is to be seen in Dunearn Hill near
Burntisland.
Although olivine-basalts of the Dalmeny type are most frequently met
with as interstratified lavas, they also occur as bosses and sills. The
typical example from Dalmeny is itself intrusive. Other illustrations
are to be found in the Castle Rock of Edinburgh and in the sheets near
Crossgates and Blairadam in Fife. The presence or absence of olivine,
however, may sometimes be a mere accident of cooling or otherwise.
I have shown that in the same mass of rock at Blackburn a gradation
can be traced from a rock largely composed of altered olivine into
one consisting mainly of felspar with but little olivine, and another
example occurs in the picrite-sill between Edinburgh and Cramond. Dr.
Stecher has ascertained that the marginal portions of the sills in
the basin of the Firth of Forth, which cooled first and rapidly, and
may be taken, therefore, to indicate the mineral composition of the
rock at the time of extrusion, are often rich in olivine, while that
mineral may be hardly or not at all discernible in the main body of the
rock.[447]
[Footnote 447: Dr. Stecher, _Tschermak's Mineralog. Mittheil._ vol. ix.
(1887) p. 193. _Proc. Roy. Soc. Edin._ vol. xv. (1888) p. 162.]
Of the ordinary and characteristic dolerites without olivine which
constitute most of the intrusive masses, the various types enumerated
in the tabular arrangement are abundantly developed in Central
Scotland. Thus the normal ophitic type is displayed by the uppermost
sill of the Burntisland series, and by the rock which forms the plug
of the Binns Hill neck in Linlithgowshire. The Ratho type is well
seen in the large sill at Ratho, likewise in the extensive intrusive
sheets in the west of Linlithgowshire as at Muckraw and Carribber. The
Burntisland sill type is shown by the lower sills of Burntisland and
by some others in the same region, especially by that of Colinswell,
and by another on the shore east from the Poorhouse, near Kinghorn. The
great boss among the Bathgate Hills likewise displays it. The Bowden
Hill type occurs in well-marked development at Bowden Hill, three miles
south-west of Linlithgow, and in the massive sill at St. Margaret's,
west from North Queensferry.
The non-olivine-bearing basalts are found in various bosses and sheets
in the basin of the Firth of Forth. Thus the Binny Craig type occurs
in the prominent and picturesque sill from which it is named, likewise
among the intrusive sheets near Kirkcaldy, in Fife. Sometimes the same
mass of rock displays more than one type of structure, as in the case
of the great Galabraes neck among the Bathgate Hills wherein both the
Tholeiite and Burntisland sill types may be recognized.
Some of the sills in West Lothian, as I pointed out many years ago,
contain bitumen and give off a bituminous odour when freshly broken.
They have been injected into bituminous shales or coal-seams.[448]
[Footnote 448: _Geol. Survey Memoir on Geology of Edinburgh_ (Sheet 32,
Scotland), p. 46.]
2. Andesites.--Rocks referable to this series appear to have been of
rare occurrence among the puy-eruptions. Examples of them containing
as much as 60 per cent of silica occur among the lavas of the Limerick
basin. Some of the necks and what may be sills in the same district
likewise consist of them.
3. Trachytes and Quartz-bearing Rocks.--Acid rocks, as I have already
said, are extremely rare among the puy-eruptions. The only important
examples known to me are those around the Limerick basin, where they
rise apparently in old vents and form conspicuous rounded or conical
hills. These rocks have been examined microscopically by Mr. W. W.
Watts. One of the most interesting varieties, which occurs at the
Standing Stone near Oola, was found by him to show quartz enclosing
ophitically the felspars which, with well-terminated prisms, project
into it. Further west, near Knockaunavoher, another boss occurs with
conspicuous quartz. These rocks have much in common with trachytes but
have a wholly crystalline structure. They will be described in the
account of the Limerick basin.
B. Tuffs
The fragmental rocks connected with the puy-eruptions form a
well-marked group, easily distinguishable, for the most part, from the
tuffs of the plateaux. They vary from exceedingly fine compacted dust
or volcanic mud, through various stages of increasing coarseness of
texture, to basalt-conglomerates and tumultuous agglomerates.
The fragmentary material found in the necks of the puys is generally
an agglomerate of a dull dirty-green colour. The matrix ranges from
a fine compact volcanic mud to a thoroughly granular detritus, and
sometimes shows a spheroidal concentric structure in weathering. In
this matrix the lapilli are distributed with great irregularity and in
constantly varying proportions. They consist in large measure of a pale
yellowish-green, sometimes pale grey, very basic, finely vesicular,
devitrified glass, which is generally much decomposed and cuts easily
with the knife. This highly basic substance is a kind of palagonite.
So minute are its vesicles that under the microscope a thin slice may
present a delicate lace-like network of connected walls, the palagonite
occupying much less space than the vesicles. The material has been a
finely frothed-up pumice.
Besides this generally distributed basic pumice, the stones in the
agglomerate of the necks likewise include fragments of older volcanic
grits or tuffs, blocks of basalt or diabase, as well as pieces of the
Carboniferous strata of the district, especially shale, sandstone and
limestone. Not infrequently also, they comprise angular blocks of
fossil wood.
The materials which fill the necks are generally much coarser than
those that form intercalated beds. But while in numerous cases huge
blocks of basalt and large masses of sandstone, shale, limestone,
ironstone or other strata may be seen wrapped up in a matrix of
coarse basalt-tuff, in not a few instances the material in the
necks may be observed to consist of a tuff quite as fine as that of
the interstratified bands. Such necks appear to mark the sites of
tuff-cones where only fine ashes and lapilli were ejected, and where,
after sometimes a brief and feeble period of activity, the orifice
became extinct.
The bedded tuffs interstratified with the ordinary Carboniferous
strata do not essentially differ in composition from the material of
the necks. They are basalt- (diabase-) tuffs and basalt- (diabase-)
conglomerates, usually dull green in colour and granular in texture,
the lapilli consisting in great measure of various more or less
decayed basalts, but containing the same highly vesicular basic glass
or pumice above referred to. They are mainly to be distinguished
by their conspicuous stratification, and especially by their rapid
alternations of coarser and finer material, by the intercalation of
shales, limestones, sandstones or ironstones in them, and by the
insensible gradations by which they pass both vertically and laterally
into ordinary sediments. Occasional large blocks or bombs, indicating
some paroxysm of explosion, may be observed even among the finer tuffs,
shales and other strata, which round the sides of these masses have had
their layers bent down by the fall of heavy blocks.[449] Many of the
bedded tuffs contain fossils, such as crinoids, corals, brachiopods,
fish-teeth or macerated fragments of land-plants. Coal-seams also are
occasionally interstratified among them.
[Footnote 449: _Ante_, p. 36, and Figs. 15 and 151. See also _Geol.
Mag._ i. (1864), p. 22; _Trans. Roy. Soc. Edin._ vol. xxix. (1879) p.
515.]
Of the finer kinds, the best example is furnished by a remarkable
group of "green and red marls" which lie above a seam of coal (Houston
Coal) in the Calciferous Sandstones of West Lothian.[450] These
strata, which differ much from any of the rocks with which they are
associated, are exceedingly fine in grain, dull sage-green and brownish
or chocolate-red in colour, not well laminated like the shales, but
breaking under the influence of weathering into angular fragments,
sometimes with a conchoidal fracture. They look like indurated mud. Mr.
H. M. Cadell, who has recently re-examined them in connection with a
revision of the Geological Survey Map (Sheet 32) has found them passing
into ordinary granular tuff.
[Footnote 450: Memoir on Sheet 32 _Geol. Surv. Scotland_ (1861), p.
42. The stratigraphical position of these "Houston Marls," as they are
locally called, is indicated in Fig. 155.]
Palagonitic-tuff is of frequent occurrence. It is met with in the
Firth of Forth district,[451] and Mr. Watts has detected fragments of
palagonite among the tuffs of the Limerick basin.
[Footnote 451: _Trans. Roy. Soc. Edin._ vol. xxix. (1819) p. 515.]
CHAPTER XXVII
GEOLOGICAL STRUCTURE OF THE CARBONIFEROUS PUYS OF SCOTLAND
1. Vents: Relation of the Necks to the Rocks through which they
rise--Evidence of the probable Subærial Character of some of
the Cones or Puys of Tuff--Entombment of the Volcanic Cones and
their Relation to the Superficial Ejections. 2. Bedded Tuffs and
Lavas--Effects of Subsequent Dislocations. 3. Sills, Bosses and
Dykes.
The puy-type of volcanic hill differs widely in one respect from
those which we have hitherto been considering. In the earlier epochs
of volcanism within the British area, it is the masses of material
discharged from the vent, rather than the vents themselves which arrest
attention. Indeed, so copiously have these masses been erupted that the
vents are often buried, or their positions have been rendered doubtful,
by the uprise in and around them of sills and bosses of molten rock.
But among the Carboniferous puys the vent is often the only record
that remains of the volcanic activity. In some cases we know that it
never ejected any igneous material to the surface. In others, though
it may be filled with volcanic agglomerate or tuff, there is no record
of any shower of such detritus having been discharged from it. In yet
a third class of examples, we see that lava rose in the vent, but no
evidence remains as to whether or not it ever flowed out above ground.
Other cases occur where beds of lava or of tuff, or of both together,
have been intercalated in a group of strata, but with no trace now
visible of the vent from which they came. The most complete chronicle,
preserving at once a record of the outflow of lava, of the showering
forth of ashes and bombs, and of the necks that mark the vents of
eruption, is only to be found in some of the districts.
I shall therefore, in the present instance, reverse the order of
arrangement followed in the previous chapters, and treat first of the
vents, then of the materials emitted from them, and lastly of the sills
and dykes.
i. VENTS
A large number of vents rise through the Carboniferous rocks of
Scotland. Some of these are not associated with any interbedded
volcanic material, so that their geological age cannot be more
precisely defined than by saying that they must be later than the
particular formations which they pierce. Some of them, as I shall
endeavour to show, are in all probability of Permian age. But many,
from their position with reference to the nearest intercalated lavas
and tuffs, are to be regarded as almost certainly belonging to the
Carboniferous period. Those which are immediately surrounded by sheets
of lava and tuff, similar in character to the materials in the vents
themselves, may without hesitation be connected with these sheets as
marking the orifices of discharge.
The vents of the puys are in general much less than those of the
plateaux. Their smallest examples measure only a few yards in diameter,
their largest seldom much exceed half a mile.[452]
[Footnote 452: The following measurements of necks belonging to the
puy-eruptions in different parts of Scotland are taken from the 6-inch
field-maps of the Geological Survey:--Saline Hill, Fife, 6000 × 4000
feet; Binn of Burntisland, 3500 × 1500; Hill of Beath, Fife, 2900 ×
1550; Binns Hill, Linlithgowshire, 4800 × 2200; Tor Hill, Ecclesmachan,
Linlithgowshire, 1900 × 1000 (Fig. 155); Great Moor, near Maiden Pap,
Roxburghshire, 2600 × 2400; Tinnis Hill, Liddesdale, 1500 × 1000; Roan
Fell, Liddesdale, 300 × 200; Hadsgarth Burn, Liddesdale, 250 × 200;
Dalbate Burn, 250 × 120. In some cases, especially in those of the
larger necks, it is probable that the tuff belongs to more than one
funnel. Thus the Binn of Burntisland almost certainly includes two
necks, a smaller one to the west and a much larger one to the east.
Saline Hill may also conceal more than one vent. But in the continuous
mass of tuff at the surface it is at present impossible to determine
precisely the number and boundaries of the several orifices.]
The dislocations of the Carboniferous system are probably on the whole
later than its volcanic phenomena. It is at least certain that the
lavas and tuffs of the puys have been extensively faulted, like the
surrounding sedimentary strata, and the vents seldom show any apparent
relation to faults. It may sometimes be observed, however, that the
vents are arranged in lines suggestive of fissures underneath. A
remarkable instance of the linear distribution is furnished by the
chain of necks which extends from the Vale of the Tweed at Melrose
south-westwards across the watershed and down Liddesdale. The most
notable part of this line lies among the uplands to the east of the
Old Mosspaul Inn at the head of the Ewes Water. A string of masses
of agglomerate has there solidified in a fissure among the Silurian
greywackes and shales, running in a north-easterly direction for
several miles. The largest connected mass of agglomerate is 4700 feet
long, and from 350 to 600 feet broad (see No. 1 in Fig. 22). That
this curious vent, or connected line of vents along a great fissure,
belongs to the puy-eruptions of Liddesdale is shown by the abundant
fragments of yellow sandstone and cement-stone which occur in the
agglomerate.[453]
[Footnote 453: These facts were ascertained by Mr. Peach in mapping the
ground for the Geological Survey. See Sheet 17, Scotland.]
Most frequently the vents are distributed irregularly in groups. As
examples of this arrangement I may cite those of the west of Fife, of
West Lothian and of the north of Ayrshire.
A convenient classification of the vents may be made by dividing them
into four groups according to the nature of the material that now
fills them: 1st, Necks of non-volcanic debris; 2nd, Necks of tuff and
agglomerate; 3rd, Necks of similar materials, but with a central plug
of basalt; 4th, Bosses of basalt or other lava, without agglomerate or
tuff.
1. _Necks of Non-volcanic Debris._--In a few instances the orifices
of eruption have been filled up entirely with non-volcanic debris.
They have served as funnels for the discharge of explosive vapours
only, without the expulsion of any solid volcanic materials. At
least no trace of fragmentary lavas is met with in them, nor are
any beds of tuff or lava intercalated among the surrounding strata.
Some interesting examples of this kind were laid bare in the open
ironstone-workings near Carluke in Lanarkshire. They were circular in
ground-plan, descended vertically into the strata, and were somewhat
wider at the top of the quarry than at the bottom. They were filled
with angular pieces of Carboniferous sandstone, shale, limestone,
ironstone and other rocks, these materials being rudely arranged with
a dip towards the centre of the neck, where the blocks were largest
in size. Though no fragments of igneous rocks were observed among the
debris, a few string-like veins of "white trap," or altered basalt,
were seen to traverse the agglomerate here and there. The necks and
the strata surrounding them were highly impregnated with pyrites and
sulphate of lime.[454]
[Footnote 454: Prof. Jas. Geikie, _Mem. Geol. Surv. Scotland_,
Explanation of Sheet 23, p. 39.]
[Illustration: Fig. 143.--Section of volcanic vent at East Grange,
Perthshire coal-field, constructed by Mr. B. N. Peach from the rocks
exposed in a railway-cutting, and from plans of ironstone- and
coal-pits.
1. Three feet coal; 2. Ontake coal; 3. Upper and Lower Black-band
Ironstones; 4. Index Limestone; 5. Gas Coal and Janet Peat Coal; 6.
Calmy Limestone; 7. Neck.]
A vent of the same nature, but on a much larger scale, has been mapped
by Mr. Peach in the south of Perthshire, near East Grange, where
it rises through the higher coal-bearing part of the Carboniferous
Limestone series (Fig. 143). It has been encountered in the mining
of coal and ironstone, and its cross-section has been ascertained in
the underground workings which have been carried up to its margin. It
measures 1500 feet in diameter from east to west and 2000 feet from
north to south. It does not appear ever to have emitted any ashes
or lava. Mr. Peach found it filled with dark sandy crumbling clays,
full of fragments of sandstone, shale and coal. These sediments are
arranged in layers that dip in the same general direction as the strata
surrounding the vent. They contain abundant calcareous nodules of all
sizes from that of a hazel-nut up to concretions 18 feet in diameter.
The clays likewise include many of the common shells and crinoids of
the Carboniferous Limestone sea, and the same fossils are enclosed in
the nodules. A remarkable feature in this vent is the occurrence of
abundant vertical rents, which have been filled partly with the same
material that forms the nodules, and partly with sandstone.
The formation of the neck took place after the deposition of the Index
Limestone, and probably about the time of the accumulation of the next
limestone, which lies immediately to the west somewhat higher in the
series. It would appear that the eruption which produced this funnel
gave forth only gaseous explosions, and occurred on the sea-floor;
that the low crater-walls were washed down to such an extent that the
sea entered and carried some of its characteristic organisms into the
lagoon or _maar_ within; further, that as the silt gathered inside,
successive subsidences occurred, whereby the sediment was rent by
cracks into which sand and calcareous mud were washed from above.[455]
[Footnote 455: The vent is shown in Sheet 39, _Geol. Surv. Scotland_.]
Many necks occur wherein non-volcanic materials, though not forming
the whole of the agglomerate, make up by far the larger part, with
only a slight admixture of volcanic tuff between them. Among the
Burntisland necks of Fife, for instance, abundant fragments of the
well-marked cyprid limestone and shale may be observed, while at Niddry
in Linlithgowshire blocks several yards in length, and consisting of
different layers of shale and cement-stone still adhering to each
other, may be seen imbedded at all angles in the tuff.
Where only the debris of non-volcanic rocks occupies a vent, we may
infer that the volcanic action was limited to the explosion of steam,
whereby the rocks were dislocated, and an orifice communicating with
the surface was drilled through them, and that while no true volcanic
rock in such a case appeared, the pipe was filled up to perhaps not
far from the surface by the falling back of the shattered detritus. A
little greater intensity or farther prolongation of the volcanic action
would bring the column of lava up the funnel, and allow its upper part
to be blown out as dust and lapilli; while still more vigorous activity
would be marked by the rise of the lava into rents of the cone or
its actual outflow at the surface. Every gradation in this scale of
progress may be detected among the Carboniferous volcanoes of the basin
of the Firth of Forth.
2. _Necks of Tuff and Agglomerate._--The majority of the necks
connected with the puys consist of tuff or agglomerate. Externally they
generally appear as smooth rounded grassy hills that rise disconnected
from other eminences. In some districts their materials consist of
a greenish granular often stratified tuff, enclosing rounded balls
of various basic lavas and pieces of sandstone, shale, limestone or
other strata through which they have been drilled. This is their usual
character in the Forth region. But in some cases, the tuff becomes a
coarse agglomerate, made up partly of large blocks of basalt and other
volcanic rocks and partly of the sedimentary strata around them, of
which large masses, many cubic yards in bulk, may be seen. Among the
enclosed fragments it is not unusual to find pieces of older stratified
tuff. These resemble in general petrographical character parts of the
tuff among which they are imbedded. Sometimes they have been
[Illustration: Fig. 144.--View of the Binn of Burntisland--a volcanic
neck of agglomerate. (This illustration and Figs. 145, 152, 153,
164, 166 and 168 are from photographs taken by Mr. Robert Lunn
for the Geological Survey.)] derived from previous tuffs which,
interstratified among the sedimentary strata, had been broken up by
the opening of a new vent. But probably in most cases they should be
regarded as portions of the volcanic debris which, having solidified
inside the crater, was blown out in fragments by subsequent explosions.
In a modern volcano a considerable amount of stratified tuff may be
formed inside the crater. The ashes and stones thrown out during a
period of activity fall not only on the outer slopes of the cone,
but on the steep inner declivities of the crater, where they arrange
themselves in beds that dip at high angles towards the crater
bottom. This feature is well seen in some of the extinct cones in
the Neapolitan district. In some of the Scottish puys the tuff is
stratified and has tumbled down into a highly inclined or vertical
position (Fig. 145).
As a good illustration of the variety and relative proportions of the
ejected blocks in the green tuff of the puy-vents, I may cite the
following table of percentages which I took many years ago in the tuff
which rises through the Cement-stone group on the beach at the Heads of
Ayr.
Diabase and basalt 57 per cent.
Older tuff 3 "
Andesite (probably from Old Red Sandstone
volcanic series of the neighbourhood) 14 "
Limestone (cement-stone, etc.) 20 "
Shale 3 "
Sandstone 2 "
Fossil wood 1 "
----
100
While many examples might be cited where no molten rock of any kind has
risen in the vents, or where at least all the visible materials are of
a fragmentary character, yet small veins and dykes of basalt have not
infrequently been injected into the tuff or agglomerate. These seldom
run far, and usually present a more or less tortuous course. Thus, on
the south front of the Binn of Burntisland (Figs. 166, 168) a number of
basalt-dykes, which vary in breadth from five or six feet to scarcely
so many inches, bifurcate and rapidly disappear in the tuff, one of
them ascending tortuously to near the top of the cliff. They at once
recall the appearance of the well-known dykes in the great crater wall
of Somma.
Though not by any means the largest or most perfect of the vents in the
basin of the Firth of Forth, the Binn of Burntisland, of which a view
is given in Fig. 144, may be cited in illustration of their general
characters. It presents in detail some of the most strikingly volcanic
aspects of scenery anywhere to be seen in that region. Consisting of a
dull green granular volcanic tuff, it rises abruptly out of the Lower
Carboniferous formations to a height of 631 feet above the sea. The
southern edge of this neck has been so extensively denuded, that it
presents steep craggy slopes and rugged precipices, which descend from
the very summit of the cone to the plain below--a vertical distance of
nearly 500 feet. Here and there the action of atmospheric waste has
hollowed out huge crater-like chasms in the crumbling tuff. Standing
in one of these, the geologist can realize what must have been the
aspect of the interior of these ancient Carboniferous volcanic cones.
The scene at once reminds him of the crater-walls of a modern or not
long extinct volcano. The dull-green rudely stratified tuff rises
around in verdureless crumbling sheets of naked rock, roughened by the
innumerable blocks of lava, which form so conspicuous an element in
the composition of the mass. The ribs or veins of columnar basalt run
up the declivities as black shattered walls. The frosts and rains of
many centuries have restored to the tuff its original loose gravelly
character. It disintegrates rapidly, and rolls down the slopes in long
grey lines of volcanic sand, precisely as it no doubt did at the time
of its ejection, when it fell on the outer and inner declivities of
the original cone. Some of these features may be partly realized from
Fig. 145, which represents a portion of the south front of the hill.
Sections of this neck are given in Figs. 149 and 159.
(3) _Necks of Tuff or Agglomerate with a Central Plug of Basalt or
other Lava._--It has often happened that, after the explosions in a
vent have begun to decrease in vigour, or have at last ceased, lava
has risen in the chimney and finally sealed it up. In such cases the
main mass of the rock may consist of tuff or agglomerate, which the
enfeebled volcanic activity has been unable to expel from the orifice,
while a plug of basalt, dolerite, or even more basic material, of much
smaller dimensions, may have risen up the pipe in the centre or towards
one side. Binns Hill, West Lothian, the Beath and Saline Hills of Fife,
and Tinnis Hill in Liddesdale are good examples of this structure. (See
Figs. 26, 148, 149 and 174).
(4) _Necks of Basalt, Dolerite, etc._--In other cases no fragmental
material is present in the vent, or possibly traces of it may be seen
here and there adhering to the walls of the funnel, the prevailing rock
being some form of lava. Necks of this kind are much less frequent
in the puy- than in the plateau-type. But examples may be found in
several districts. The most striking with which I am acquainted are
those which form so picturesque a group of isolated cones around the
volcanic basin of Limerick, to be afterwards described (Figs. 195,
196). The vents there have been filled by the uprise of much more acid
rocks than the lavas of the basin, for, as I have already stated, they
include even quartziferous trachytes. In the basin of the Firth of
Forth some prominent bosses of basalt probably mark the sites of former
vents, such as Dunearn Hill in Fife, the Castle Rock of Edinburgh, and
Galabraes Hill near Bathgate. Some striking vents which occur in the
Jedburgh district, in the debateable land between the plateau series
on the east and the puy-series on the west, show the nearly complete
usurpation of the funnel by basalt, but with portions of the tuff still
remaining visible.
_Relation of the Necks to the Rocks through which they rise._--A
remarkable feature among the Carboniferous and Permian vents of central
Scotland is presented by the effect which has been produced on the
strata immediately surrounding them. In the interior of the country
this effect is often
[Illustration: Fig. 145.--View of part of the cliffs of vertical
agglomerate, Binn of Burntisland.] concealed by herbage, but where
the rocks have been laid bare by the sea it may be most instructively
studied. In such shore-sections, a singular change of dip is often
observable among the strata round the edge of a vent. No matter what
may be the normal inclination at the locality, the beds are bent
sharply down towards the wall of the neck, and are frequently placed
on end. This structure (shown in Figs. 24, 143, 147, 148 and 149)
is precisely the reverse of what might have been anticipated, and
can hardly be due to upward volcanic explosions. It is frequently
associated with considerable metamorphism in the disturbed strata.
Shales are converted into porcellanite or various jaspery rocks,
according to their composition. Sandstones pass into quartzite, with
its characteristic lustrous fracture. It is common to find vents
surrounded with a ring of this contact-metamorphism, which, from the
hardness and frequently vertical or highly inclined bedding of its
strata, stands up prominently on the beach (as in Figs. 126 and 210),
and serves to mark the position of the necks from a distance.
I have not been able to find an altogether satisfactory explanation of
this inward dip of the strata around vents. Taking it in connection
with the metamorphism, I am inclined to believe that it arose after
the close of the long-continued volcanic action which had hardened
the rocks around the volcanic pipe, and as the result of some kind of
subsidence within the vent. The outpouring of so much tuff and lava
as escaped from many of the volcanoes would doubtless often be apt to
produce cavities underneath them, and on the decay of volcanic energy
there might be a tendency in the solid or cavernous column filling up
the funnel, to settle down by mere gravitation. So firmly, however, did
much of it cohere to the sides of the pipe, that if it sank at all, it
could hardly fail to drag down a portion of these sides. So general
is this evidence of downward movement in all the volcanic districts
of Scotland where the necks have been adequately exposed, that the
structure may be regarded as normal to these volcanic vents. It has
been observed among the shore-sections of the volcanoes of the Auckland
district, New Zealand. Mr. C. Heaphy, in an interesting paper upon
that district, gives a drawing of a crater and lava-stream abutting on
the edge of a cliff where the strata bend down towards the point of
eruption, as in the numerous cases in Scotland.[456]
[Footnote 456: _Quart. Journ. Geol. Soc._ 1860, vol. xvi. p. 245.]
_Evidence for the probable subærial Character of some of the Cones or
Puys of Tuff._--From the stratigraphical data furnished by the basin
of the Firth of Forth, it is certain that this region, during a great
part of the Carboniferous period, existed as a wide shallow lagoon,
sometimes overspread with sea-water deep enough to allow of the growth
of corals, crinoids, and brachiopods; at other times, shoaled to such
an extent with sand and mud as to be covered with wide jungles of a
lepidodendroid and calamitoid vegetation. As volcanic action went on
interruptedly during a vast section of that period, the vents, though
generally submarine, may occasionally have been subærial. Indeed, we
may suppose that the same vent might begin as a subaqueous orifice
and continue to eject volcanic materials, until, as these rose above
the level of the water, the vent became subærial. An instance of a
submarine vent has been cited from the Perthshire coal-field (p. 426).
Among the evidence which may be collected to show that some
Carboniferous volcanoes probably rose as insular cones of tuff above
the surrounding waters, the structure of the tuff in many necks may be
cited, for it suggests subærial rather than subaqueous stratification.
The way in which the stones, large and small, are grouped together
in lenticular seams may be paralleled on the slopes of many a modern
volcano. Another indication of this mode of origin is supplied by
the traces of wood to be met with in some of the tuff-necks. The
vents of Fife and Linlithgowshire contain these traces sometimes in
great abundance. The specimens are always angular fragments, and
are frequently encrusted with calcite.[457] Sometimes they present
the glossy fracture and clear ligneous structure shown by sticks of
well-made wood charcoal. In a neck at St. Magdalen's, near Linlithgow,
the wood fragments occur as numerous black chips. So far as can be
ascertained from the slices already prepared for the microscope, the
wood is always coniferous. These woody fragments seldom occur in the
interstratified tuffs or in the associated strata where _Stigmaria_,
_Lepidodendron_, etc., are common. They are specially characteristic of
the necks and adjacent tuffs. The parent trees may have grown on the
volcanic cones, which as dry insular spots would support a different
vegetation from the club-mosses and reeds of the surrounding swamps. As
the fragments occur in the tuffs which, on the grounds already stated,
may be held to have been deposited within the crater, they seem to
point to intervals of volcanic quiescence, when the dormant or extinct
craters were filled with a terrestrial flora, as Vesuvius was between
the years 1500 and 1631, when no eruptions took place. Some of the
cones, such as Saline Hill and the Binn of Burntisland, may have risen
several hundred feet above the water. Clothed with dark pine woods,
they would form a notable feature in the otherwise monotonous scenery
of central Scotland during the Carboniferous period.
[Footnote 457: The largest I have observed is a portion of a stem about
two feet long and six inches broad, in the (Permian?) neck below St.
Monan's church.]
_Entombment of the Volcanic Cones and their relation to the bedded
Lavas and Tuffs._--From the facts above detailed, it is evident that
in most cases the necks represent, as it were, the mere denuded stumps
of the volcanoes. As the puys took their rise in areas which, on the
whole, were undergoing a movement of subsidence, they were eventually
submerged and buried under sedimentary accumulations. Their loose
ashes would be apt to be washed down and strewn over the sea-bottom,
so that only the lower and inner part of a cone might remain. We can
hardly hope to discover any of the actual craters among these volcanic
relics. The cones having been submerged and buried under many hundred
feet of sediment, their present position at the surface is due to
subsequent elevation and prolonged denudation. It is obvious that there
must still be many buried cones which the progress of denudation has
not yet reached. Some of these have been revealed in the course of
mining operations. Valuable seams of coal, ironstone and oil-shale
in the Scottish Carboniferous Limestone and Calciferous Sandstone
series are extensively worked, and in the underground operations many
illustrations of former volcanic action have been met with. The most
remarkable instances of the discovery of buried volcanoes have occurred
in the Dalry coal-field in the north of Ayrshire. In one pit-shaft
about a mile and a half to the south-west of the village of Dalry, a
thickness of 115 fathoms of tuff was passed through, and in another
pit 90 fathoms of similar tuff were sunk into before the position of
the black-band ironstone of that mineral field was reached by driving
levels through the tuff into the sedimentary strata outside of it.
Only a short distance from these thick piles of tuff, their place is
entirely taken up by the ordinary sedimentary strata of the district.
The working-plans of the mines show the tuff to occur in irregular
patches and strips, between which the ironstone is workable. From these
data we perceive that the shafts have in some cases been sunk directly
upon the tops of puys of tuff, which were, in one case, nearly 700
feet, and in another instance, 540 high[458] (Fig. 146).
[Footnote 458: Explanation of Sheet 22, _Geol. Surv. of Scotland_, p.
16.]
[Illustration:
Fig. 146.--Diagram of buried volcanic cone near Dalry, Ayrshire.
Constructed from information obtained in mining operations.
1. Hurlet Limestone. 2. Clayband Ironstone. 3. Black-band
Ironstone. 4. Borestone Coal. 5. Wee Coal. 6. Highfield
Limestone. 7 and 8. Thin Limestones. 9. Linn Limestone. 10.
Volcanic neck and cone of tuff.
]
It is obvious that from the condition of a completely buried and
concealed cone every stage may be expected to occur up to the deeply
worn-down neck representing merely the stump of the volcanic column.
The subjoined diagram (Fig. 147) may serve to illustrate this process
of gradual re-emergence.
[Illustration: Fig. 147.--Diagram to illustrate how Volcanic Necks may
be concealed and exposed.
1, Neck, still buried under the succeeding sedimentary accumulations;
2, Neck uncovered and denuded.]
When, in the progress of denudation, a volcanic cone began to show
itself from under the cover of removed strata, it would still for a
time maintain its connection with the sheets of tuff or of lava which,
when active, it had erupted. A number of examples of this structure may
be observed in the basin of the Firth of Forth, where the degradation
of the surface has not yet proceeded so far as to isolate the column
of agglomerate or tuff from the sheets of tuff that were strewn around
the old volcano. In such cases, the actual limits of the vent are still
more or less concealed, or at least no sharp line can be drawn between
the vent and its ejections. As an illustration of this connection of a
volcanic pipe with the materials ejected from it over the surrounding
country I would cite Saline Hill in the west of Fife. That eminence
rises to a height of 1178 feet above the sea, out of a band of tuff
which can be traced across the country for fully three miles. Numerous
sections in the water-courses show that this tuff is regularly
interbedded in the Carboniferous Limestone series, so that the relative
geological date of its eruption can be precisely fixed. On the south of
Saline Hill, coal and ironstone, worked under the tuff, prove that this
portion of the mass belongs to the general sheet of loose ashes and
dust, extending outwards from the original cone over the floor of the
sheet of water in which the Carboniferous Limestone series of strata
was being deposited. But the central portion of the hill is occupied
by one or more volcanic pipes. A section across the eminence from
north-west to south-east would probably show the structure represented
in Fig. 148. Immediately to the east of the Saline Hill lies another
eminence, known as the Knock Hill, which marks the site of another
eruptive vent. A coal-seam (the Little Parrot or Gas Coal) is worked
along its southern base, and is found to plunge down steeply towards
the volcanic rocks. This seam, however, is not the same as that worked
under the Saline Hill, but lies some 600 feet below it. Probably the
whole of the Knock Hill occupies the place of a former vent.
[Illustration: Fig. 148.--Section across the Saline Hills, Fife.
The thick parallel black lines mark the position of seams of coal
and ironstone, some of which are worked under Saline Hill. T,
Tuff of the necks; _t_, Tuff at a little distance from the cone,
interstratified with the ordinary sedimentary beds; B, Basalt.
The larger eminence is Saline Hill, the lower is Knock Hill.
]
A further stage of decay and denudation brings before us the entire
severance of the volcanic column from the materials that were ejected
from it. An excellent example of this isolation of the neck in the
midst of surrounding masses of tuff and lava which proceeded from
it is presented by the Binn of Burntisland, to which I have already
alluded. A section across that eminence gives the geological structure
represented in Fig. 149. The dip of the rocks away from the volcanic
pipe at this locality has been produced long after the volcanic
phenomena had ceased. The arch here shown is really the prolongation
and final disappearance of the great anticlinal fold of which the
Pentland Hills form the axis on the opposite side of the Firth. But
if we restore the rocks to a horizontal, or approximately horizontal
position, we find the Binn of Burntisland rising among them in one or
more necks, which doubtless mark centres of volcanic activity in that
district. A series of smaller neck-like eminences runs for two miles
westward.
Striking as the forms of many of the necks are, and much as their
present conical forms resemble those of active and extinct volcanoes,
the evidence of extensive denudation proves that these contours are
not the original outlines of the Carboniferous vents, but are in every
case the result of prolonged waste. What we now see is a section of the
volcanic chimney, and the conical form is due to the way in which the
materials filling the chimney have yielded to the forces of denudation.
[Illustration: Fig. 149.--Section across the Binn of Burntisland, in an
East and West direction.
1, Sandstones; 2, Limestone (Burdiehouse); 3, Shales, etc.; _b_, _b_,
Interstratified basalts; _t_ _t_, Bedded tuff, etc.; T, Tuff of the
great neck of Burntisland; B, Basalt veins.]
ii. BEDDED TUFFS AND LAVAS
During at least the earlier part of the period of the puys, in some
districts or from certain vents, such as those of East Fife, Western
Midlothian, Eastern Linlithgowshire, Northern Ayrshire, Heads of Ayr
and Lower Eskdale, only fine tuff seems to have been thrown out, which
we now find intercalated among the surrounding strata. These eruptions,
neither so vigorous nor so long-continued as those of the plateaux,
never gave forth such thick and widespread sheets of fragmentary
materials as those associated with the plateaux in East Lothian and the
north-east of Ayrshire. A single discharge of ashes seems in many cases
to have been the sole achievement of one of those little volcanoes;
at least only one thin band of tuff may be discoverable to mark its
activity.
The tuff of these solitary bands is seldom coarse in texture. It
usually consists of the ordinary dull green paste, with dust and
lapilli of basic pumice. The local variations in the tuffs of the
puys generally arise mainly from differences in the composition,
size and numbers of the included ejected blocks. Generally the most
abundant stones are pieces of different diabases, or basalts; then come
fragments from the surrounding Carboniferous strata, from older tuffs
and rarely from rocks of much deeper-seated origin.