THE BASIC SILLS OF THE BASALT-PLATEAUX
We have now followed the distribution of the basalt-plateaux, the
arrangement of their component materials which were erupted at the
surface, and the character of the dyke-fissures and vents from which
these materials were ejected. But there remains to be considered
an extensive series of rocks which display some of the underground
phenomena of the Tertiary volcanoes. The injection of many basaltic
sheets had been clearly enforced by Macculloch. In 1871 I pointed out
that at different horizons in the plateau-basalts, but especially at
their base and among the stratified rocks underneath them, sheets
of basalt and dolerite occur which, though lying parallel with the
stratification of the volcanic series, are not truly bedded, but
intrusive, and therefore younger than the rocks between which they
lie.[309] The non-recognition of their true nature had led to their being
regarded as proofs of volcanic intercalations in the Jurassic series of
Scotland. There is, however, no trace of the true interstratification
of a volcanic band in that series, every apparent example being due
to the way in which intrusive sheets simulate the characters of
contemporaneous flows.
[Footnote 309: _Quart. Jour. Geol. Soc._ xxvii. (1871), p. 296.]
If such sheets had been met with only at one or two localities, we
might regard them as due to some mere local accident of structure in
the overlying crust through which the erupted material had to make its
way. But when we find them everywhere from the cliffs of Antrim to
the far headlands of Skye and the Shiant Isles, and see them reappear
among the Faroe Islands, it is obvious that, like those of Palæozoic
time, they must be due to some general cause, and that they contain the
record of a special period or phase in the building up of the Tertiary
volcanic tablelands. I will first describe some typical examples
of them from different districts, and then discuss their probable
relations with the other portions of the plateaux.
i. ANTRIM
First to be examined, and now most familiar to geologists, are the
remarkable sheets that underlie the plateau of Antrim, and project at
various parts of the picturesque line of coast between Portrush and
Fair Head. From the shore at Portrush, as I have already remarked,
came the evidence that was supposed to prove basalt to be a rock of
aqueous origin, inasmuch as shells were obtained there from what was
believed to be basalt. The long controversy to which this supposed
discovery gave rise is one of the most curious in the history of
geology.[310] It continued even after the illustrious Playfair had shown
that the pretended basalt was in reality highly indurated shale, and
hence that, instead of furnishing proof of the aqueous formation
of basalt, the Portrush sections only contributed another strong
confirmation of the Huttonian theory, which claimed basalt to be a rock
of igneous origin.
[Footnote 310: For an excellent summary of this controversy and an
epitome of the descriptions of the Portrush section, see the _Report
on the Geology of Londonderry_, etc. (_Mem. Geol. Survey_), by J. E.
Portlock (1843), p. 37.]
It is now well known that the rock which yielded the fossils is a
Liassic shale, that it is traversed by several sheets of eruptive rock,
and that by contact-metamorphism it has been changed into a highly
indurated substance, breaking with a splintery, conchoidal fracture,
but still retaining its ammonites and other fossils. The eruptive
material is a coarse, distinctly crystalline dolerite, in some parts of
which the augite, penetrated by lath-shaped crystals of plagioclase, is
remarkably fresh, while the olivine has begun to show the serpentinous
change along its cracks.[311] This rock has been thrust between the
bedding planes of the shales, but also breaks across them, and occurs
in several sheets, though these may all be portions of one subterranean
mass. Some of the sheets are only a few inches thick, and might at
first be mistaken for sedimentary alternations in the shale. But their
mode of weathering soon enables the observer readily to distinguish
them. It is to be noticed that these thin layers of eruptive material
assume a fine grain, and resemble the ordinary dykes of the district.
This closeness of texture, as Griffith long ago pointed out,[312] is
also to be noticed along the marginal portions of the thicker sheets
where they lie upon or are covered by the shales. But away from the
surfaces of contact, the rock assumes a coarser grain, insomuch that
in its thickest mass it presents crystals measuring sometimes an inch
in length, and then externally resembles a gabbro. A more curious
structure is shown in one of these coarsely crystalline portions by the
occurrence of a band a few inches broad which is strongly amygdaloidal,
the cells, sometimes three inches or more in diameter, being filled
with zeolites.[313] The general dip of the shales and of the intrusive
sheets which have been injected between them is towards the east. From
underneath them a thick mass of dolerite rises up to form the long
promontory that here projects northwards from the coast-line, and is
prolonged seawards in the chain of the Skerries.
[Footnote 311: Dr. F. Hatch, Explanation of Sheets 7 and 8, _Geol. Survey
of Ireland_, p. 40.]
[Footnote 312: "Address to Geological Society of Dublin, 1835," p. 13,
_Jour. Geol. Soc. Dublin_, vol. i. The varieties of the Portrush rock
were described by the late Dr. Oldham, in Portlock's _Report on the
Geology of Londonderry_, p. 150; see also the same work for Portlock's
own remarks, p. 97.]
[Footnote 313: For a list of the minerals in this rock, see Oldham, _op.
cit._ p. 151.]
An interesting feature of the Portrush sections is the clear way
in which they exhibit the phenomena of "segregation-veins"--so
characteristic of the thicker and more coarsely crystalline sills.
These veins or seams here differ from the rest of the rock mainly in
the much larger size and more definitely crystalline form of their
component minerals. Though sharply defined, when looked at from a
little distance, they are found on closer inspection to shade into the
surrounding rock by a complete interlacing of crystals. On the shore,
they can be seen to lie, on the whole, parallel with the bedding of
the sheets in which they occur, but without rigidly following it,
since they undulate and even ramify. A good section across their dip
has been exposed in a quarry near the end of the promontory, and shows
that they are considerably less regular than the plan of their outcrop
on the shore would have led us to anticipate. The accompanying drawing
(Fig. 314) represents the veins laid bare on a face of rock nine feet
in length by five feet in height. It will be seen that while there is a
general tendency to conform to the dip-slope, which is here from right
to left, the seams or layers unite into a large rudely-bedded mass,
which sends out processes at different angles. The peculiar aggregation
of minerals which distinguishes such veins is perhaps best seen at Fair
Head, and I reserve for the description of that locality what I have
to say on the subject, only remarking with regard to the Portrush rock
that the felspar shows a disposition to collect in the centre of the
veins with the augite and the other dark minerals at the outer margins.
[Illustration: Fig. 314.--View of "Segregation-Veins" in a dolerite
sill, Portrush, Antrim.]
The contact-metamorphism at this locality is of more historical
interest in connection with the progress of geological theory than of
scientific importance. It consists mainly in an intense induration of
the argillaceous strata. These pass here from their usual condition of
fissile, laminar, dull, dark shales into an exceedingly compact, black,
flinty substance, which in its fracture, colour and hardness reminds
one of Lydian stone. Yet the ammonites and other organic remains have
not been destroyed. They are preserved in pyrites.
[Illustration: Fig. 315.--View of Fair Head, from the east, showing the
main upper sill and a thinner sheet cropping out along the talus slope.]
Of all the examples of Tertiary sills in Britain few are more imposing
than that of the noble range of precipices which form the promontory
of Fair Head. Leaving out of account the minor masses of eruptive rock
which occur underneath it, we find the main sheet to extend along the
coast for nearly four miles, to rise to a height of 636 feet above the
sea, and to attain a maximum thickness of 250 feet. This enormous bed
dies out rapidly both to the east and west, and seems also to thin away
inland. Seen from the north, it stands upon a talus of blocks as a
sheer vertical wall, 250 feet high, and the rude prisms into which it
is divided are continuous from top to bottom (Fig. 315). So regular is
this prismatic structure, and so much does it recall the more minute
columnar grouping of the bedded basalts, that at a little distance we
can hardly realize the true scale of the structure. It is only when
we stand at the base of the cliff or scramble down its one accessible
gully, the "Grey Man's Path," that we appreciate how long and thick
each of the prisms actually is (Fig. 316). It may here be remarked
that this regular prismatic jointing is one of the distinguishing
features of the large sills, and serves to mark them off from the
bedded basalts, even when these have assumed a columnar structure. The
prisms are much larger than the basalt-columns, and never display the
irregular starch-like arrangement so common among the plateau-basalts.
[Illustration: Fig. 316.--View of Fair Head from the shore. (From a
Photograph by Mr. R. Welch.)]
The rock composing this magnificent sheet is a coarsely crystalline,
ophitic, olivine-dolerite.[314] The same diminution of the component
crystals, which is so marked along the margins of the eruptive masses
at Portrush, is strikingly exhibited at the borders of the Fair Head
sill. For about 18 or 20 inches upward from the bottom, where the
bed rests on the black, Carboniferous shales, the dolerite is dark
and finely crystalline, weathering spheroidally in the usual manner.
But immediately above that bottom layer of closer grain, the normal
coarsely crystalline texture rapidly supervenes. A similar closeness of
grain is observable at the surfaces of contact where the sheet splits
up on its western border.
[Footnote 314: Professor Judd has described what he calls a
"glomero-porphyritic structure" in this rock (_Quart. Journ. Geol.
Soc._ xlii. (1886), p. 71).]
Nowhere, so far as I know, can the phenomena of "segregation-veins"
be so instructively studied as along the abundant exposures of this
great sheet. The veins are most conspicuous where the rock occurs
in thickest mass. They vary up to three or four feet in thickness,
and, as at Portrush and elsewhere, lie on the whole parallel to the
upper and under surfaces of the sheet. An erroneous impression may be
conveyed by the term "veins" applied to them. They are quite as much
layers, parallel on the whole with the bedding of the sheet, yet not
adhering rigidly to one plane, but passing across here and there from
one horizon to another. That they are not due to any long subsequent
protrusion of younger material through the main sheet is made manifest
by the thorough interlocking of their component crystals with those
of the body of the rock in which they lie. The material that fills
these veins has obviously been introduced into them while there was
still some freedom of movement among the crystals of the surrounding
rock, which must thus have been still not quite consolidated and
therefore intensely hot. Both crystallized slowly, and in so doing
their component minerals dovetailed with each other. The constituents
of the veins consist of an exceedingly coarse aggregate of crystals, or
rather of crystalline lumps of the same minerals that constitute the
general mass of the rock, the felspar and augite showing the ophitic
intergrowth of the main rock, but on a far larger scale. Some of the
pieces of augite measure two inches or more in diameter. The conditions
under which these veins were produced must have differed in some
essential respects from those that prevailed during the formation of
the fine-grained, highly siliceous veins already described as occurring
in some dykes and sills.
This great Fair Head sill lies upon Carboniferous strata, but that
it is to be classed with the Tertiary volcanic series is, I think,
demonstrated by its relations to the Chalk at its eastern end. It has
there broken through that rock, and converted it for a short distance
into a white, granular marble. But it is at the western side that the
most interesting sections occur to show the truly intrusive nature of
the mass. The rock there splits up into about a dozen sheets, which,
keeping generally parallel with each other, have forced their way
between and partly across the bedding planes of the Carboniferous
shales (Fig. 317). In this way the huge, unbroken mass, 250 feet thick,
subdivides itself and disappears in a few hundred yards, though it
continues a little further inland, and approaches the shore again half
a mile to the south-west. Further evidence of the intrusive nature of
this rock may be observed along the base of the precipice, where at
least one sheet 70 feet thick diverges from the main mass and runs
eastwards between the Carboniferous shales (Fig. 315). At the contact
with the eruptive rock the shales are everywhere much indurated.
[Illustration: Fig. 317.--Section at Farragandoo Cliff, west end of
Fair Head, showing the rapid splitting up and dying out of an Intrusive
Sheet.
_a_, Carboniferous sandstone; _b_, Carboniferous shale; _c_, intrusive
sheet.]
ii. SKYE
All through the Inner Hebrides the base of the basalt-plateaux presents
abundant examples of sills. The general parallelism of these intrusive
sheets to the bedding of the Jurassic strata among which they lie has
been above referred to as having given rise to the erroneous conclusion
that in Skye and elsewhere the basalts are interstratified with
Jurassic rocks, and are consequently of Jurassic age. It was Macculloch
who first described and figured in detail the proofs of their intrusive
nature. His well-known sections in plate xvii. of the illustrations to
his work on the _Western Islands_ have been repeatedly copied, and have
served as typical figures of intrusive igneous rocks.
Nowhere in North-Western Europe can the phenomena of sills be studied
so fully and with such exuberance and variety of detail as in the
island of Skye and its surrounding islets. On the western coast the
greater subsidence of the basaltic plateau has for the most part
submerged the platform of intrusive sheets, though wherever the base
of the bedded lavas is brought up to the surface the accompanying
sills are exposed to view. The east coast of the island has been
classic ground for this part of volcanic geology since it supplied the
materials for Macculloch's descriptions and diagrams. From the mouth of
Loch Sligachan to Rudha Hunish, at the north end of Skye, a series of
sills may be traced, sometimes crowning the cliffs as a columnar mural
escarpment, sometimes burrowing in endless veins and threads through
the Jurassic rocks. The horizontal distance to which this continuous
band of sills extends in Skye is not far short of 30 miles. But it
stretches beyond the limits of the island. It forms the group of islets
which prolongs the geological structure and topographical features of
Trotternish for 4 miles further to the north-west. It reappears 10
miles still further on in the Shiant Isles. Thus its total visible
length is fully 40 miles, or if we include some outlying sills near the
Point of Sleat, to be afterwards described, it extends over a distance
of not less than 60 miles. From the last outlier in Skye to the sills
of the Isle of Eigg is a distance of only 8 miles, thence to those
of Ardnamurchan 17 miles, and to those of the south coast of Mull 25
miles. Thus this platform of intrusive sheets of the Inner Hebrides can
be interruptedly followed for a space of not less than 110 miles.
[Illustration: Fig. 318.--View of the Trotternish Coast, showing the
position of the band of Sills.
The dark band crowning the first slope above sea-level marks a
conspicuous band of sills which towards the right descends to the
beach and is prolonged seaward in the group of islands. The Storr Rock
appears as a slanting obelisk of rock on the hill to the left.]
Though none of the sills in Skye itself attain the dimensions of
the Fair Head sheet, they present a greater variety of rock and of
geological structure than is to be found in Antrim. They are specially
developed at the base of the thick, overlying, basalt-plateau--a
platform on which such a prodigious quantity of eruptive material
has been injected. Part of this material consists of basic rocks in
the form of dykes, veins, or sills; part of it is included in the
intermediate and acid groups, and comprises veins, sheets, and bosses
of granitoid, felsitic, rhyolitic, trachytic, and pitchstone rocks.
One of the peculiarities of the Skye sills is the occurrence among
them of compound examples, where sheets of basic and acid material
have been injected along the same general platform. These will be more
specially referred to in Chapter xlviii. With regard to the basic
sills (dolerites, basalts, etc.), I would remark that while in Western
Scotland the Antrim type of short, thick intrusions, or laccolites,
is also found, the vast majority of the sheets are much thinner, more
persistent, and less easily distinguishable from the bedded basalts.
[Illustration: Fig. 319.--Columnar Sill intrusive in Jurassic Strata
east of Kilmartin, Trotternish, Skye.
[The high ground to the left is a portion of the basalt-plateau to the
north of the well-known Quiraing.]]
In describing the sills of Skye I shall take first those of the eastern
and then those of the western side of the island. Along the east coast,
from Loch Sligachan to the most northerly headlands and islets the
sills play a notable part in the scenery, inasmuch as they cap the
great sea-cliff of Trotternish and run as a line of ridges parallel
to the trend of the coast, while the plateau-basalts rise above them
further inland as a lofty escarpment, which includes the picturesque
landslips of the Storr Rock and Quiraing (Figs. 318, 319). Beneath
the thick sills, the Jurassic sandstones form a range of pale yellow
precipices, along which many thinner sheets of eruptive material have
been intruded. As Macculloch well showed, many of these sheets, if seen
only at one point, might readily be taken for regularly interstratified
beds, but perhaps only a few yards distant they may be found to break
across the strata and to resume their course on a different level.
The sills of this Trotternish coast may be distinguished even at some
distance from the bedded basalts by the regular prismatic jointing,
already referred to, and by their frequently greater thickness, while
on closer inspection they are characterized by their much coarser
texture. They are generally somewhat largely crystalline ophitic
dolerites, gabbros or diabases, and exhibit the persistent uniformity
of composition and structure so characteristic of intrusive sheets and
dykes. These characters are well exhibited in the Kilt Rock, a columnar
sill capping the cliffs to the south of Loch Staffin (Fig. 319).
These massive sills are prolonged in a series of picturesque flat
tabular islets beyond the most northerly headlands of Skye. They
probably continue northwards under the sea at least 12 miles further,
for sills of the same type rise there in the singularly striking group
of the Shiant Isles (Fig. 320). These lonely islets, extending in
an east and west direction for about three miles, display in great
perfection most of the chief characters of the Skye sills. They are
especially noteworthy for including the thickest intrusive sheet and
the noblest columnar cliff in the whole of the Tertiary volcanic series
of Britain. The larger of the two chief islands consists of two masses
of rock connected by a strip of shingle-beach, and having a united
length from north to south of about two miles. The northern half, or
Garbh Eilean, presents towards the north a sheer precipice 500 feet
high. This magnificent face of rock consists of one single sill, but as
its original upper limit has been removed by denudation and its base,
where it is thickest, is concealed under the sea, the sill may exceed
500 feet in thickness. The rock has the usual prismatic structure,
which imparts to it an impressive appearance of regularity. The columns
retain their individuality to a great height, and though none of them
perhaps can be followed from base to crest of the cliff, many of them
are evidently at least 300 or 400 feet long.
Macculloch, who gave the first geological description of the Shiant
Isles, showed the intrusive nature of the igneous rocks, and described
the remarkable globular or botryoidal structure of the Jurassic
shales between which they have been injected.[315] Professor Heddle has
published a brief account of the geology of the islands.[316] Professor
Judd visited the group and brought away a series of specimens of
their eruptive rocks, which he found to include basic and ultra-basic
varieties.[317]
[Footnote 315: _Western Islands_, vol. i. p. 441.]
[Footnote 316: _Trans. Norfolk Nat. Hist. Soc._ vol. iii. (1880) p. 61.]
[Footnote 317: _Quart. Journ. Geol. Soc._ vol. xxxiv. (1878) p. 677, and
xli. (1885) p. 393. My description in the text is the result of three
successive visits to the islands.]
[Illustration: Fig. 320.--View of the northern precipice (500 feet
high) of the largest of the Shiant Isles.
(From a Photograph by Colonel Evans.)]
In Garbh Eilean, where the thickest mass of erupted material presents
itself, at least three sills may be observed. Some low reefs that
run parallel with the northern coast of the island consist of coarse
ophitic gabbro in two or more sheets which have been intruded between
the Jurassic shales. Above these strata comes the great columnar sill,
its base gradually sinking towards the west until it passes under the
sea, and the vertical columns then plunge abruptly into the water.
The rock of which this massive sill consists is another large-grained
gabbro or dolerite, with an ophitic structure. Owing to the form of
the ground it cannot be so satisfactorily examined as the neighbouring
island of Eilean Mhuire, which, though less lofty and rather smaller
than Garbh Eilean, affords a succession of admirable and easily
examined sections along its precipitous shores.
Professor Judd found that while the rocks are mainly ophitic gabbros
and dolerites, they include such highly basic compounds as dunite.
An examination of the Eilean Mhuire cliffs enables the observer to
ascertain that the sills display considerable variety in texture and
in the character and arrangement of their component minerals. They
are marked by a persistent, more or less distinct disposition in rude
beds, and these again often display a banding of their constituents
in lines parallel with the general bedding. Some of these bands are
largely felspathic, and are thus paler in colour. Others, where the
ferro-magnesian minerals and ores are more specially aggregated, are
dark in colour. In some layers the long black prisms of augite are
ranged in a general parallelism with the banding.
A specimen selected as typical of the ordinary coarse-grained amorphous
rock was sliced and placed in Mr. Harker's for microscopic examination,
and he has supplied the following observations regarding it: "The
gabbro from Eilean Mhuire [7110] is a crystalline rock showing to the
eye lustrous black augites, half an inch long, and (predominating)
felspar. The microscope reveals, in addition, irregular grains of black
iron-ore and little hexagonal prisms of apatite. No olivine is to be
detected. As regards structure, the augite has tended to crystallise
out in advance of the felspar, but this relation is not constant.
"The augite is of a light-brown tint in slices, and has an unusual
kind of pleochroism. The colour for vibrations parallel to the [Greek:
b]-axis is of the purplish-brown tone seen in some soda-bearing
augites; parallel to [Greek: g] and [Greek: a] it has a yellow or
citron tint. The colour and pleochroism are more marked in the
interior of a crystal than towards the margin, but some crystals
pass at the margin into a slightly pleochroic, pale-green, recalling
ægerine-augite. The felspar tends to build elongated crystals. It is a
rather finely lamellated labradorite, sometimes showing pericline- as
well as albite-lamellae."
Another specimen from one of the black bands in the same island, with a
linear arrangement of its component minerals, is thus described by the
same petrographer: "This rock [7111] is of darker appearance than the
preceding, and contains abundant black iron-ore, besides some pyrites.
It also differs in having a marked parallel disposition of its crystals.
"Except for the greater prominence of large irregular grains of
iron-ore, this rock under the microscope closely resembles the last
described, the parallel structure not being conspicuous in the slice.
The augite has the peculiar colour and pleochroism already noted, and
the felspar is of the same kind as before."
I did not succeed in finding in place any bands of dunite, but this
basic material probably occurs at the base of some of the sills where
it has segregated from the rest of the mass, like the picrite at the
bottom of the Bathgate diabase.
The amount of contact-metamorphism effected even by such thick sills as
those of Trotternish and Shiant is much less than might be expected.
It seldom goes beyond a mere induration of the strata for a few yards,
often only for a few inches from the surface of junction. In the Shiant
Isles, however, the shales between the sills have undergone a more
remarkable alteration. They have not only been greatly indurated, but
have acquired the globular or botryoidal structure so fully described
by Macculloch. The spheroidal aggregates vary from not more than a
line to more than half an inch in diameter, and appear on the surface
as dark, irregularly grouped, pea-like aggregates. This structure is
perhaps best developed immediately under the thick sill on the west
side of Eilean Mhuire.
The massive sills are not the only evidence of the injection of igneous
material on the Shiant Isles. The sill, or more probably group of
sills, forming Eilean Mhuire is traversed by a number of sheets of
basalt varying from only two or three inches to 20 feet in thickness.
These black fine-grained rocks invariably present chilled selvages next
the coarse gabbro, and though they have been on the whole injected
parallel to the general bedding or banding, they here and there break
across it as veins. The most important of these later intrusions forms
a columnar sill on the eastern side of the island, and can be followed
for several hundred yards. It consists of a dark finely crystalline
olivine-basalt, which towards the margin assumes a dense black texture.
Under the microscope Mr. Harker found a thin slice of this rock to
be "an olivine-basalt of semi-ophitic, semi-granulitic structure
[7112]. The olivine is mostly fresh, but part of it is converted
into a yellowish-brown pseudomorph like iddingsite. Magnetite occurs
chiefly in imperfect octohedra. The felspar is in little lath-shaped
sections, many of which are finely striated, and give extinction-angles
indicating a labradorite. The augite, light brown in the slice, never
has crystal-boundaries, and often enwraps the felspars."
The narrow veins are composed of a much closer-grained basalt in which
a few scattered felspars are visible. Mr. Harker remarks, with regard
to a thin slice of one of these rocks [7113], that "the microscope
shows this, too, to be an olivine-basalt. The porphyritic felspars
are twinned on the Carlsbad and albite laws. Olivine and pseudomorphs
after it are well represented. Magnetite is only sparingly present.
The general mass of the rock consists of very small striated prisms of
labradorite, granules of augite, and interstitial matter which must be
partly glassy."
This is perhaps the most striking of all the examples known to me where
an older sill has been split open to receive a subsequent injection of
molten material. The Eilean Mhuire gabbro must be at least 200 feet
thick, and it not impossibly passes under the still thicker pile of
Garbh Eilean. Yet it has been horizontally ruptured near its base, and
into the rent thus produced another mass of molten matter has been
thrust. This subject will be again referred to in connection with
another remarkable example on the west coast of Skye.
[Illustration: Fig. 321.--Section of thin Intrusive Sheets and Veins in
carbonaceous shales lying among the Plateau-basalts, cliffs north of
Ach na Hannait, between Portree Bay and Lock Sligachan.]
In contrast to such enormous thicknesses of intrusive material as those
of Trotternish and the Shiant Isles, instances may be culled from the
same belt of sills where the molten rock has been injected in thin
leaves and mere threads into the Jurassic sandstones and shales, or
into the shales and coals intercalated among the plateau-basalts. Thus,
on the cliff immediately to the north of Ach na Hannait, between Loch
Sligachan and Portree Bay, the section may be seen which is represented
in Fig. 321. At the base lies a vesicular dolerite with a slaggy upper
surface (_a_). Next comes a zone of sedimentary material about five
or six feet thick, the lower portion consisting of an impure coal,
which passes towards the right hand into brown and grey carbonaceous
shale with plant-remains (_b_). This coaly layer has been already
alluded to as probably lying on the same horizon with the coal of
Portree (p. 288). Traced northward, it is found to have a bed of fine
tuff beneath it, and sometimes a volcanic breccia or conglomerate.
It fills up rents in the underlying slaggy lava, and was undoubtedly
deposited upon the cooled surface of that rock. Immediately above this
lower band the black carbonaceous shale which follows has been invaded
by an extraordinary number of thin cakes or sills and also by veins
or threads of basalt. For a thickness of two or three feet the band
(_d_) consists mainly of these intrusions, which, in the form of a
fine grey basalt, vary from less than an inch to three or four inches
in thickness. They are separated by thin partings of coaly shale,
and as they tend to break up into detached nodule-like portions,
especially towards the right hand of the section, they might, on casual
inspection, be easily mistaken for nodules in the dark shales. Somewhat
later in the time of intrusion are veins of basalt which, as at _c_,
break across the nodular sills, and sometimes expand into thicker beds
(_c´_).
I have never seen such a congeries of minute sills among the Tertiary
basalt-plateaux as that here exhibited. In a space of about three feet
of vertical height there must be more than a dozen of roughly parallel
leaves of intrusive rock. Veins (_e_) run up from the chief band of
eruptive material into the overlying finely vesicular basalt (_f_). The
dyke (_g_) is probably the youngest rock in the section.
The more general and extensive submergence of the base of the
basalt-plateau on the west side of Skye has for the most part carried
the platform of sills below sea-level, so that it is only exceptionally
where, owing to local irregularities, that base has been brought up to
the air, that the intrusive sheets show themselves. Yet the persistence
of the platform on that side is indicated by its extension even as far
as the southern promontory of the island.
The Trotternish type of sill extends down the west coast under the
headlands of Duirinish. Thus at the mouth of Dunvegan Loch, where the
underlying Jurassic platform has been ridged up above the surface of
the sea, it has carried with it the marked sill which forms the islets
of Mingay and Clett that lie as a protecting breakwater across the
entrance of the inlet. The intrusive rock rests on shell-limestones
full of oysters (_Ostrea hebridica_), and referable to the Loch
Staffin group of the Great Oolite Series. This sill, when observed
from a little distance, presents the usual regularly prismatic or
columnar structure so well developed among the Trotternish examples,
but on a closer view shows this structure less distinctly. It is an
olivine-dolerite of medium and fine texture, which in thin slices
displays under the microscope a distinctly ophitic structure, the
abundant light-brown augite enclosing the striated felspars. Its
lowest portion, from three to seven or eight inches upward from the
bottom, is much closer-textured than the rest of the rock and is finely
amygdaloidal. Its vesicles are in many cases drawn out to a length of
three or four inches, and the zeolites which now fill them look like
parallel annelid tubes or stems of _Lithostrotion_. It is noteworthy
also that the elongation of the vesicles has sometimes taken place at
a right angle to the surface of contact with the underlying strata.
But the most remarkable feature in this sill is the surface which it
presents to the oyster-beds on which it rests. The fine-grained dark
dolerite has there assumed the aspect of a sheet of iron-slag, with a
smooth or wrinkled, twisted, ropy surface, which displays fine curving
flow-lines. No one looking at a detached specimen of this surface would
be ready to admit that it could possibly have come from anything but
a true lava-stream that flowed out at the surface. The contours of a
viscous lava are here precisely reproduced on the under surface of a
massive sill.
A little further south, the promontory of Eist, forming the western
breakwater of Moonen Bay, consists of an important sill or group of
sills which has insinuated itself among shales, shell-limestones, and
shaly sandstones, full of _Ostrea hebridica_, _Cyrena aurata_, etc.,
and belonging to the Loch Staffin group of the Great Oolite Series.
The shore-cliff below the waterfall affords the section given in Fig.
322, illustrating the manner in which a thick intrusive sheet may
sometimes give off thin veins from its mass. The rock attains on the
Eist promontory a thickness of probably at least 100 feet, where it
is thickest and undivided. But the two main sheets, or branches of
one great sheet, on this peninsula have probably a united depth of
more than 300 feet. Landwards the rock splits up and encloses cakes
of the Jurassic strata. It possesses the usual prismatic structure
and doleritic composition. In Moonen Bay, as shown in Fig. 322, it
presents a banded structure, marked especially by an alternation of
lines of amygdales and layers of more compact and solid dolerite,
with occasional enclosed cakes of baked shale or sandstone. Its upper
surface is somewhat uneven, and from it are given off narrow, wavy,
ribbon-like veins (_d_), from less than an inch to three inches or
more in width, which keep in a general sense parallel to the top of
the sill, but at a distance of a few inches or feet from it. The sill
becomes as usual fine-grained towards the contact, the shales and
sandstones being indurated and the limestone marmorized.
[Illustration: Fig. 322.--Upper part of Sill, Moonen Bay, Waternish,
Skye, showing the divergence of veins.
_a_, false-bedded shaly sandstone; _b_, shell-limestone; _c_, dolerite
sill; _d_, veins proceeding from the sill.
Length of section about five yards.]
The next uprise of the base of the basalt-plateau on the west side of
Skye lies about 25 miles to the south-east, where it emerges from the
sea in the Sound of Soa (Fig. 323). A vast volcanic pile has there
been heaped up on the Torridon sandstone, the whole of the thick
Jurassic series, which is found in force only three miles distant in
Strathaird, having been removed by denudation from this area before the
beginning of the Tertiary volcanic period. The plateau-basalts rests on
the upturned edges of the Torridonian sandstones and shales, and are
accompanied as usual by their underlying network of intrusive rocks. It
is hardly possible to exaggerate the wild confusion of sills, dykes and
veins which have been injected among the rocks, at and on both sides of
the unconformability. Endless sheets of basalt and dolerite have forced
their way between the bedded basalts and the sandstones, while across
the whole rise vast numbers of dykes and veins. Narrow, black, wavy
ribbons of basic material cross many of these veins, while the later
north-west dykes cut sharply through everything older than themselves.
As a natural section for the study of the phenomena of intrusion in
many of their most characteristic phases, I know no locality equal
to the northern coast-line of the Sound of Soa, unless it be the
cliffs of Ardnamurchan. But the Skye cliffs, though less imposing than
those of the great Argyllshire headland, have this advantage, that
instead of being exposed to the full roll of the open Atlantic, they
form the margin of a comparatively sheltered strait, and can thus be
conveniently examined.
[Illustration: Fig. 323.--Section of the base of the Basalt-plateau
with sill and dykes, Sound of Soa, Skye.
_a_ _a_, Torridon Sandstone; _b_, Bedded basalts; _c_, Sill; _d_ _d_,
Dykes.]
Following still the western seaboard of Skye, we meet with other
striking examples of sills at a distance of some eight miles in a
straight line eastward where, between Lochs Slapin and Eishort, the
prominent headland of Suisnish juts out into the sea. This promontory
has long been known to geologists from the section of it given by
Macculloch as an instance of the connection between overlying rocks
and dykes. I have already alluded to it in that relation, and refer
to it again as an example of one of the thicker intrusive sheets of
the Inner Hebrides. Denudation has here also proceeded so far that
the whole of the volcanic plateau has been stripped off, only some of
the underlying sills being left, together with the platform of older
rocks between which and the vanished basalts they were injected.
Most of these sills consist of granophyres belonging to the acid
group of rocks to be afterwards described. But basic sheets occur not
infrequently interposed between the granophyres and the subjacent Lias,
and sometimes even intercalated in the former rock. Though at first
sight it might be thought that these sills had insinuated themselves
after the eruption of the granophyre, and there are instances where
this cannot be shown not to be the case, I have obtained so many proofs
of the invasion of the basic by the acid rock that I have no doubt the
former is, as a general rule, the older of the two.
The Suisnish headland exhibits the structure represented in Fig.
249. For about 300 feet above the sea-level the steep grassy slope
shows outcrops of the dark, sandy shales and yellowish brown, shaly
sandstones of the Lias which form the range of cliffs to the eastward.
These gently inclined strata are cut through by many vertical
basalt-dykes, some of which intersect each other, but among which
by far the largest is the mass shown in the figure. This broad dyke
consists of a dolerite or gabbro the largely crystalline texture of
which marks it off at once from the others, which are of the usual
dark, heavy, fine-grained type, with an occasional less basic and
porphyritic variety. Traced up from the sea-margin, the dyke loses
itself in a talus of blocks from the cliff above, so that its actual
junction with the mural front of the sill cannot be seen. But that it
joins that mass, with which it agrees in petrographical characters,
hardly admits of question. The cliff consists of a thick sheet of
coarsely crystalline dolerite or gabbro (_d_ in Fig. 249), which in
its general aspect at once recalls the rock of Fair Head. It varies
considerably in texture, some parts of the mass are exceedingly coarse,
like the Skye gabbros, and present a fibrous structure in their augite
resembling that of the diallage in these rocks; other portions assume
the compactness of basalt. A specimen of medium grain under the
microscope shows the typical ophitic structure so generally found among
the dolerites both of the plateaux and of the intrusive sheets. This
sill must be about 200 feet thick, and like the rock at Fair Head is
traversed from top to bottom by joints that divide it into prisms. It
appears to bifurcate eastward, one portion running with a tolerably
uniform thickness of a few feet as a prominent band at the top of
the shales and sandstones, the other slanting upwards and gradually
thinning away in the granophyre.
Towards its base, near the contact with the underlying shales, the rock
as usual becomes finer grained, and the thin band just referred to
resembles in texture one of the wider basalt-dykes. Westwards the rock
can be followed round the top of the grassy slopes formed by the decay
of the shales. Though concealed by intervals of moorland and peat, it
is visible in the stream sections, and I think must be continuous, as
a band only a few yards thick, round the northern side of the hills as
far as Beinn Bhuidhe, where a similar sill makes a prominent crag. Its
total area measures a mile and a quarter in length by half a mile in
breadth. The granophyre which overlies it forms part of an interesting
series of sheets which I have traced all the way from Suisnish to the
braes above Skulamus.
Whether or not the whole sheet of basic rock is continuous, and whether
it all proceeded from the great Suisnish dyke, cannot be confidently
decided until the ground is mapped in detail, though from the great
thickness of the sill at the dyke, its attenuation outwards from that
centre and its uniformity of petrographical character, I am disposed
to answer affirmatively. There is no other probable vent to be seen
in the neighbourhood, unless a massive dyke that runs from Loch Fada
north-westwards into Glen Boreraig can be so regarded.
Not far from the extreme southern point of Skye a singularly
interesting example of a sill remains as a detached survival of the
basaltic plateau and its accompaniments. In his map of Skye, Macculloch
showed the position of this outlier, which he classed with the
general "trap" formation of the island. The locality was visited by
Professor Judd, who regarded the intrusive rock as a "phonolite"[318]
In 1894, during an excursion with my colleague Mr. C. T. Clough, I had
an opportunity of examining the rocks and collecting notes for the
following account of them.
[Footnote 318: _Quart. Jour. Geol. Soc._ vol. xxxiv. (1878) p. 692.]
At Rudh' an Iasgaich, about two miles from the Point of Sleat, a small
outlier of conglomerate lies on the edges of the Torridon Sandstone.
This deposit has been correctly identified by Professor Judd with
the similar strata which, in Skye and elsewhere on the west coast of
Scotland, underlie the Liassic series. It is here about 10 or 12 feet
thick, reddish and yellowish in colour, and distinctly calcareous. Its
component pebbles consist largely of Cambrian (Durness) limestone,
quartzite, and Torridon Sandstone--rocks which all occur _in situ_ in
Sleat. It may be compared with the limestone conglomerates of Strath
and those which underlie the Lias at Heast on Loch Eishort.[319] That
here, as elsewhere in this region, the basement conglomerate was
followed by the rest of the Lias and Oolites may be inferred with some
confidence from the copious development of the Jurassic series a few
miles off, both to north and south. But the whole of this overlying
succession of formations has here been swept away, and, but for the
protection afforded by the eruptive rocks of Rudh' an Iasgaich, the
conglomerate would likewise have disappeared.
[Footnote 319: _Op. cit._ vol. xiv. (1857), p. 9; vol. xliv. (1888), p.
71.]
[Illustration: Fig. 324.--Section of Dolerite Sill cut by another sill,
both being traversed by dykes, Rudh' an Iasgaich, western side of
Sleat, Skye.]
Above the conglomeratic band lies a sheet of intrusive rock, which in
one place has apparently cut it out, so as to rest directly upon the
Torridon Sandstone (_a_, Fig. 324). The decay of the softer detrital
rock underneath has caused the sill to break off in slices, which have
left behind them a bold mural escarpment (_b_ _b_).
The rock of this sill is a rather coarsely crystalline porphyritic
olivine-dolerite, which towards the north attains a thickness of about
70 feet. It exhibits the usual prismatic jointing, but less perfectly
than some of the Trotternish sills already referred to. Besides these
vertical joints, it is also traversed by a system of horizontal
divisional planes which, though somewhat irregular in their course,
run, in a general sense, parallel to the upper and under surfaces of
the sill.
It seems to have been along this transverse series of joints that a
second sill (_c_), five or six feet thick, has been injected. The
material of this younger intrusion is a black, finely crystalline
dolerite or basalt, with rudely prismatic jointing. Its most striking
feature, besides its regularity of position and persistency for several
hundred yards as a platform along the shore, is the basalt-glass which
marks both its under and upper surfaces of contact, and which is here
developed upon a scale to which I have not met with an equal among the
Tertiary sills of this country.
The selvage of glass appears as a black tar-like layer, varying from
a mere film to two or three inches in thickness. It is found not only
on the upper and under surfaces, but descends along abrupt step-like
interruptions of the upper surface, a foot or more in height, as if the
sill had been broken by a series of subsidences. The apparent fracture,
however, is probably due to the irregularities of the passage forced
for itself by the molten rock as it passed from one line of horizontal
joint to another through the heart of the older sheet.
The exposed surface of black glass on the top of the younger sill
exhibits long parallel lines, probably marking flow-structure, which
are made conspicuous by a pale yellow ferruginous weathered crust.
Portions of the larger intrusive sheet have been broken off and
involved in the later rock. The younger sill disappears to the north,
and is not found in the cliff of Rudha Chàrn nan Cearc, where the
thick sill, lying once more on the band of conglomerate, forms a fine
escarpment above the shore. Dykes of fine-grained basalt (_d_ _d_) with
compact chilled margins rise through both sills, together with veins
which pursue a wavy upward path like strips of black ribbon.
This example, and that of the Shiant Isles already described, cannot
but impress the observer with the prodigious force with which the
material of the sills was injected. In these instances solid sheets
of intrusive rock have subsequently been rent open, doubtless under a
superincumbent pressure of many hundreds of feet of the terrestrial
crust, and a new injection of molten magma has made its way into
the rents thus caused. In each case, the position of the rents was
obviously determined by structural lines in the older sills, but we are
lost in astonishment at the energy required to split open, even along
these lines, such solid crystalline masses as the thick sills, and to
overcome the superincumbent pressure of so deep a pile of rock.
The isolation of a relic of the Tertiary sills on the west side of
the promontory of Sleat presents some interesting problems to the
mind of the geologist. The locality lies about midway between the
basalt-plateau of Strathaird and that of Eigg, and some eight or nine
miles in a direct line from either. The basalts cannot be proved to
have once stretched continuously between Eigg and Strathaird, and to
have covered this part of Sleat; but the position of the Sleat sills
makes it probable that this continuation did formerly exist. The
denudation of the West of Scotland since early Tertiary time has been
so stupendous that I am prepared for almost any seemingly incredible
evidence of its effects. There can hardly be any doubt, however, that
the sills here described belong to the great platform of intrusive
sheets, and that they were injected under a pile of Secondary strata,
if not also of Tertiary basalts, which has here been entirely removed.
Reference may be made, in conclusion, to a not infrequent feature of
the Skye sills. Like the dykes, they are often double or multiple,
molten material having been successively injected along the same plane.
The example just cited from the west side of Sleat illustrates one
type of such compound sills. More frequently, however, the subsequent
injections have been made along the floor or roof of the first sheet.
Mr. Harker has found numerous cases of this structure in the Strath
district. They are recognizable even from a distance by their terraced
contours when seen in profile. They often vary considerably in
thickness owing to the dying out or coming-in of their separate bands;
while, on the other hand, single sills tend to maintain a uniform
thickness for long distances, or taper away gradually. The compound
arrangement of the basic sills is well brought out where acid material
has been injected between the sheets, as will be more fully described
in Chapter xlviii.
iii. EIGG, ARDNAMURCHAN
The phenomena of the coasts of Skye are repeated on the east side of
Raasay, in Eigg, and still more magnificently along the south coast
of Mull. A single example is here given (Fig. 325) from the east side
of Eigg. Over the Jurassic sandstones (_a_ _a_) a sill of basalt (1)
four to six feet thick has been injected between the stratification,
and another (2) two to four feet thick has forced its way across the
middle of one of the bedded basalts (_b_ _b_) in which it bifurcates,
and above which comes the thick series of lavas of the plateau (_c_,
_d_). In one of the streamlets, which exposes a section of the Jurassic
strata below the volcanic escarpment, more than twenty intrusive sheets
may be counted among the shales and limestones. They are sometimes not
six inches thick, and seldom exceed six or eight feet.
[Illustration: Fig. 325.--Section to show Bedded and Intrusive Sheets,
Eigg.]
I will conclude this account of the Tertiary basic sills of Britain
by referring to one further district in the West of Scotland,
where they are well displayed on bare hillslopes and also along a
picturesque sea-coast. In the promontory of Ardnamurchan in the west of
Argyleshire, one of the most conspicuous eminences, known as Ben Hiant,
affords a striking mass of intrusive material, which, extending along
a rugged shore for three-quarters of a mile, mounts thence inland in a
series of rocky knolls, and in rather less than a mile culminates in a
summit, 1729 feet above sea-level.[320] The rocks which cover this large
space are disposed in numerous rude beds, which have a seaward dip of
perhaps 15° to 20°, and are sometimes distinctly prismatic, the prisms
being not infrequently grouped in fan-shape. They are evidently due
to successive intrusions. Although generally coarsely crystalline in
texture, they include also intermediate and fine-grained sheets. They
are never, so far as I have been able to discover, amygdaloidal,[321]
nor do they present the ordinary external characters of the beds of
the plateaux, though here and there they appear to have caught up
portions of the plateau-series. They distinctly overlie the bedded
basalts on their eastern and southern margins; but westwards they
appear to lie transgressively across the edges of these rocks, while
to the north-west and north they rest on quartzites and schists and on
Jurassic limestones. An outlier from the main mass forms the prominent
hill of Sròn Mhor, and can be seen distinctly overlying the bedded
basalts as well as the neck of agglomerate already described (Fig. 302).
[Footnote 320: This locality has been described by Professor Judd
(_Quart. Jour. Geol. Soc._ xxx. (1874), p. 261; and xlvi. (1890), p.
373).]
[Footnote 321: As amygdaloidal structure is occasionally to be found
among both dykes and sills its presence in the Ben Hiant rocks would
not be inconsistent with their intrusive origin.]
The prevalent rocks of Ben Hiant are well crystallized, ophitic
olivine-dolerites and gabbros. A specimen taken from the shore on the
west side of the mass was found by Dr. Hatch to present under the
microscope its augite in large plates, which enclose narrow laths and
needles of plagioclase felspar as well as grains of olivine. All the
felspars are in lath-shapes, sometimes extremely long and narrow. The
iron-ore likewise assumes an ophitic character, enclosing rectangular
portions of felspar. Dr. Hatch observed in another specimen, taken
from the south-east side of the hill, "a curious intermixture of two
different structures. Scattered portions which show the usual ophitic
structure, their felspar and augite occurring in large crystals,
are, so to speak, imbedded in a groundmass which presents rather
a basaltic type, its felspar, augite, and magnetite, in long thin
needles, microlites, and other skeleton forms, being enclosed in a
dark devitrified base." A third specimen, selected from one of the
columnar sheets near the top of Ben Hiant, is "a fine-grained dolerite
(or gabbro) showing little ophitic structure, the augite occurring in
roundish grains, and only slightly intergrown with the felspars, which
are more or less lath-shaped. The rock contains a considerable quantity
of black iron-ore in irregular grains and some dirty-green viridite."
Still another variety of structure occurs in a specimen which I broke
from one of the shore crags on the south-west side of the hill.
Under the microscope, Dr. Hatch found in it a beautiful aggregate of
"skeleton crystals and microlites of plagioclase, with here and there
a rectangular crystal, long slender microlites of augite, and short
serrated microlites of magnetite, the whole being confusedly imbedded
in a dark glassy base powdered over with a fine magnetite dust."[322] A
sill of pitchstone lies among the bedded basalts on the east side of
the hill.
[Footnote 322: Professor Judd has called the rocks of Beinn Hiant
augite-andesites, and has given descriptions and figures of their
structure, and analyses of their chemical composition (_op. cit._).]
From a number of specimens collected by me during a second visit to
this district in the summer of 1896, I selected some for microscopic
examination and submitted them to Mr. Harker, who has furnished me with
the following descriptions of them: "The sill at the north end of Camas
na Cloiche, Ben Hiant [7114] is an olivine-gabbro of medium grain and
fresh appearance. Olivine, fresh or partly serpentinized, is plentiful.
The felspar is a labradorite with Carlsbad- and albite- (rarely
pericline-) twinning, and some of it has zonary banding. It is for the
most part in crystals giving rectangular sections, but there are some
of allotriomorphic form. Magnetite occurs chiefly in shapeless grains
of later crystallization than the felspar, but sometimes presenting
crystal-faces to the augite. The augite is light-brown in the slice,
without any true diallage-structure, and tends to enwrap the earlier
minerals in ophitic patches.
"The sill south of Uamh na Creadha, on the west side of Ben
Hiant [7115], is a rock of different type, having porphyritic
crystals of felspar, up to an inch or more in length, in a rather
finely-crystalline groundmass. The microscope shows it to be a dolerite
of granulitic structure, the main mass of the rock consisting of little
striated labradorite-crystals, grains of pyroxene, and rather abundant
crystal-grains of magnetite. The pyroxene seems to be chiefly augite,
but hypersthene is also present, and builds rather larger and more
idiomorphic crystals with characteristic pleochroism."
In rambling over this Ardnamurchan district I have often been reminded
of the great intrusive sheets of Fair Head. One of the features in
which the rocks of the two localities resemble each other is their
tendency to assume a coarsely crystalline texture. In some parts of
Ben Hiant the individual crystals reach an inch or more in length.
These more largely crystalline portions, however, do not form distinct
bands so much as patches in the midst of the general mass; at least I
have not noticed any examples of such veins of segregation as are so
prominent in Antrim.
No one familiar with the well-marked distinctions between the lavas of
the plateaux and the sills which traverse them can hesitate in which
series to place the rocks of Ben Hiant. Since, however, these rocks
have been claimed by Professor Judd as the superficial lava-currents of
a volcano which broke out after the time of the plateau-basalts, like
the Scuir of Eigg, some further details in regard to the geological
structure of the district, which would otherwise be superfluous, may
here be given.
The number of sills and dykes in Ardnamurchan is astonishingly great.
There must be hundreds of them visible, and perhaps as many more
concealed under superficial coverings. They are well exposed on the
shore traversing the Jurassic strata and the schists. The sills become
especially large and abundant in the direction of Ben Hiant, which has
evidently been the principal centre from which their materials were
injected. The rocks composing these sills are quite similar to those
of Ben Hiant, save that, as they occur in thinner sheets than in that
mountain, they do not attain the same coarseness of texture which the
more massive beds there display. They generally possess fine-grained
chilled selvages along their upper and under surfaces.
[Illustration: Fig. 326.--Ground-plan of Sills at Ben Hiant,
Ardnamurchan.
_a_ _a_, crystalline schists; _b_ _b_, necks of volcanic agglomerate;
_c_ _c_, numerous thin sills; D, massive sill of Beinn na h-Urchrach;
E, north side of Ben Hiant; F, sill proceeding from the series forming
Ben Hiant and joining that of Beinn na h-Urchrach. The arrows mark the
dip.]
These abundant sills may be traced up into the mass of Ben Hiant from
which they have issued, and of the individual sheets of which they are
a continuation. One of the most striking and easily-followed examples
of this connection is to be seen on the north side of the mountain.
A thick sheet in the middle of Ben Hiant descends from among its
contiguous sheets and, as a prominent rib, runs down the scree-slope
into the valley below, where it forms a prominent feature. Crossing
the streamlet in the middle of the valley, where a section has been
cut through its upper surface, it gradually bends round towards the
north-east, mounts the side of Beinn na h-Urchrach until it reaches the
crest of the ridge and joins the other sills of which this eminence is
built up. The route of this band of rock will be understood from the
annexed ground-plan (Fig. 326).
That this prolongation of one of the thick beds of Ben Hiant is in no
respect a superficial lava-stream but a true sill, is proved not only
by its escarpment and dip-slope, but by its actually passing under and
indurating the schistose grits, as may be seen in the stream-section.
Again Beinn na h-Urchrach, which is mapped by Professor Judd as a
northern expansion of Ben Hiant, is likewise not a lava but a true
sill. Not only does it dip northwards at an angle of about 20°, having
the schists immediately below its crest on the one side and descending
with a long dip-slope on the other, but dwindling down rapidly from
a thickness of 100 or 200 feet in the centre to no more than a few
feet in a south-westerly direction, it there passes under schistose
grits like those on which it lies. The strata that adhere to its upper
surface are as usual indurated.
A section drawn across this attenuated development of the Beinn na
h-Urchrach sill and that from Ben Hiant shows the structure represented
in the accompanying diagram (Fig. 327), which simply gives the facts
as exposed on the ground. The lower sill is that which issues from the
main body of Ben Hiant, massive at first but diminishing in thickness
as it recedes from its source.
Again, among the sheets which descend from the northern face of the
summit of Ben Hiant and strike into the Jurassic outlier below,
intensely indurated shale may be seen lying between two of the
dolerites, which are unquestionably sills that have been injected into
the Jurassic series.
The ridge of Ben Hiant is thus found to consist of a thick and complex
series of sills, some of which are not traceable beyond the side of the
mountain, while others can be followed outwards among the surrounding
rocks. The specially marked dyke-like sills diverge from the main mass
and run for some distance north-eastward, one of them, fully a mile
long, descending among the schists into the valley and ascending into
the basalt-plateau on the opposite side.[323]
[Footnote 323: The sills of Ben Hiant descend on the south-west side into
the sea, and can be examined along the slopes and the beach, where
Professor Judd has mapped a continuous platform of agglomerate. The
broad hollow between that mountain and Beinn na h-Urchrach, over which
he has spread his "augite-andesite lavas," appears to be underlain
mainly by the crystalline schists through which sills from Ben Hiant
have been injected. The northern eminence, which he has united with Ben
Hiant, is entirely separate and, as above shown, is an obvious sill.]
[Illustration: Fig. 327.--Section of two Sills in schistose grits, west
end of Beinn na h-Urchrach, Ardnamurchan.
_a_ _a_, crystalline schists; _b_, neck of volcanic agglomerate; _c_,
small sill; D, massive sill of Beinn na h-Urchrach; F, sill proceeding
from the series forming Ben Hiant and joining that of Beinn na
h-Urchrach.]
On the south-east side of the mountain where the bedded basalts can
be traced close up to the intrusive dolerites, they are found to
present the usual dull indurated aspect so characteristic of contact
alteration among these rocks. There cannot therefore be any doubt that
Ben Hiant never was itself a volcano. Its rocks are characteristically
those of subterranean intrusions. They seem to have been injected from
a line of fissure or from several such lines, running in a general
north-easterly direction, at some late part of the volcanic period. The
group of agglomerate necks of older date shows that already the ground
underneath had been drilled by a number of distinct volcanic funnels,
and discloses a weak part in the terrestrial crust.
iv. FAROE ISLES
In the Faroe Islands the actual base of the volcanic series is
nowhere visible. Hence, the great lower platform of intrusive sheets
being there concealed, this feature of the basalt-plateaux is less
conspicuous than it is in the Inner Hebrides. A number of sills,
however, have been noticed by previous observers,[324] and I have
observed others on the sides of Stromö, Kalsö, Kunö and other islands.
In the lofty precipices of the Haraldsfjord, many of the massive
light-coloured prismatic sheets are intrusive, for though they preserve
their parallelism with the bedded sheets for considerable distances,
they may be seen sometimes to break across these, as is strikingly
shown in one of the great corries on the east side of Kunö.
[Footnote 324: See in particular Prof. James Geikie and Mr. Lomas, in the
papers already cited on p. 191.]
[Illustration: Fig. 328.--Sill traversing bedded Basalts, cliffs of
Stromö, at entrance of Vaagöfjord.
The caves and notches shown at the bottom of the precipice mark the
position of the vents represented in Figs. 311, 312, 313, 314.]
One of the most remarkable sills in the Faroe Islands is probably that
which forms so prominent an object on the western cliffs of Stromö, at
the entrance into the Vaagöfjord (Figs. 328, 329). It is prismatic in
structure, and where it runs along the face of the cliffs, parallel
to the bedded basalts among which it has been intruded, presents the
familiar characters of such sheets. The precipice of which it forms
a part is that which rises above the row of volcanic vents already
described. But it there begins to ascend the cliffs obliquely across
the basalts until it reaches the crest of the great wall of volcanic
rock at a height of probably about 1000 feet above the waves. From the
crest of the precipice the upward course of the sill is continued into
the interior of the island. It pursues its way as a line of bold crag
along the ridges of the plateau, gradually ascending till it forms the
summit of one of the most prominent hills in the district (Fig. 329).
Some further idea of the enormous energy with which the sills were
injected may be formed from this example, where the eruptive materials
followed neither the line of bedding nor a vertical fissure, but took
an oblique course through the plateau-basalts for a vertical distance
of probably more than 1500 feet.
V. GENERAL DEDUCTIONS REGARDING THE TERTIARY BASIC SILLS
If we consider the facts which have now been adduced regarding
the position and structure of the sills, we are led, I think, to
regard these masses as certainly belonging to the history of the
basalt-plateaux, but, on the whole, to a comparatively late part of
it. They consist of essentially the same materials as the lavas that
form these plateaux, though with the differences of structure which the
conditions of their production would lead us naturally to expect. Where
they occur in thick masses, which must obviously have cooled much more
slowly at some depth beneath the surface than the comparatively thin
sheets could do that were poured out above ground, they have assumed
a far more largely crystalline texture than that of the superficial
lavas. As a rule, we may say that the thicker the sill the coarser is
its texture, while the thinnest sheets are the most close-grained.
Sills are especially abundant about the base of the basaltic-plateaux.
We may examine miles of the central and higher parts of the escarpments
without detecting a single example of them, but if the escarpment is
cut down to the base we seldom need to search far to find them.
[Illustration: Fig. 329.--View of the same Sill seen from the channel
opposite the island of Kolter.]
That the efforts of the internal magma to establish an outlet
towards the surface were accompanied by powerful disturbances of the
terrestrial crust is shown by the abundant dykes which traverse all the
volcanic districts from Antrim to Iceland, and some of which ascend
even to the very highest remaining lavas of the basalt-plateaux. The
parallel fissures filled by these dykes prove that even after the
accumulation of more than 3000 feet of basalt-sheets, the movements
continued to be so powerful as to disrupt these vast piles of volcanic
material. But undoubtedly the highest parts of the plateau-basalts are
less cut by dykes than the lower parts. There would no doubt come a
time when the dislocations would more seldom reach the surface, when
dykes would not be formed so abundantly or up to such a high level, and
when the volcanic energies would more and more sparingly result in the
opening of new vents or in the discharge of fresh eruptions from old
ones.
It appears to me most probable that the injection of the sills was
connected with the same terrestrial disturbances that produced the
dykes which traverse the plateaux. Besides being dislocated by parallel
fissures, the earth's crust in North-Western Europe seems to have been
ruptured internally along lines more or less at right angles to the
vertical fissures. The deep accumulation of bedded basalts presented
an increasing obstacle to the ascent of the magma to the surface.
Unable to gain ample enough egress through such vertical fissures as
might be formed in the volcanic pile, the molten rock would find its
lines of least resistance along the planes of the strata and the lower
basalt-beds, either by the aid of terrestrial ruptures there, or in
virtue of its own energy. On these horizons, accordingly, the sills
occur in extraordinary profusion throughout the volcanic regions. They
are no doubt of all ages in the progress of the building up of the
volcanic plateaux, but I am disposed to believe that a large number
of them may belong to the very latest period of the uprise of basalt
within the area of Britain.
One of the most suggestive features of the abundant Tertiary sills
lies in the evidence they furnish of the enormous energy concerned
in the ascent and intrusion of volcanic material. The infilling of
dykes or the outpouring of successive streams of lava at the surface
hardly appeals to our imagination so strikingly as the proof that the
sills have been impelled into their places with a vigour which, even
when guided and aided by gigantic terrestrial ruptures, was capable of
overcoming the vertical pressure of hundreds, or even thousands of feet
of overlying rock. Had these intrusive sheets been mere thin layers,
their horizontal extent and persistence would still have excited our
astonishment, but when we find them sometimes several hundred feet
thick, and to extend in a continuous series for horizontal distances
of 50 miles or more, we are lost in wonder at the prodigious expansive
strength of the volcanic forces. Again, the intrusions have not always
taken place between the bedding-planes of the stratified or igneous
rocks, but, as we have seen, solid sheets of already deeply buried
lavas have sometimes been split open and the intrusive material has
forced itself between the disrupted portions. Such subterranean proofs
of the vigour of volcanic energy teach some of the most impressive
lessons in the chronicles of volcanic action in the British Isles.
* * * * *
In closing this history of the accumulation of the great Tertiary
volcanic plateaux of North-Western Europe, I would remark that as the
result of prolonged eruptions from innumerable vents, the depression
that extended from the south of Antrim to the Minch was gradually
filled up with successive sheets of basalt to a depth of more than 3000
feet. A succession of lava-fields stretched from the North of Ireland
across the West of Scotland, and perhaps even to the Faroe Islands,
Iceland and Greenland. That the lava spread round the base of the
Highland mountains and ran up the Highland glens, much as the sea now
does, is made clear from the position of the outliers of it which have
been left perched on the ridges of Morven and Ardnamurchan. So far as
can now be surmised, these wide Phlegræan fields were only varied by
occasional volcanic cones scattered over their surface, marking some
of the last vents from which streams of basalt had flowed. But the
volcanic energy was still far from exhaustion. After the accumulation
of such a deep and far-extended sheet of lava, those underground
movements which produced the fissures that served as channels for the
uprise of the earliest dykes continued to show their vigour. The pile
of bedded lavas was rent open by innumerable long parallel fissures
in the prevalent north-westerly direction, up which basic lavas rose
to form dykes, while vast numbers of sills were injected underneath.
Whether the outflow of basalt at the surface had wholly ceased when
the last of these dykes were injected into the plateaux cannot be
told. Nor is there any evidence whether it had ended before the next
great episode of the volcanic history--the extravasation of the gabbro
bosses. All that we can affirm with certainty is, that the formation
of north-west fissures and the uprise of basalt in them were again
repeated, for we find north-west dykes traversing even the crests of
the later eruptive masses of basic and acid rocks. It is difficult to
suppose that none of these latest dykes communicated with the surface,
and gave rise to cones with the outpouring of basalt and the ejection
of dust and stones. But of such later outflows of basic material over
the surface of the plateaux no undoubted trace has yet been recognised.