THE PLATEAUX
Nature and Arrangement of the Rocks: 1. Lavas.--Basalts, Dolerites,
Andesites--Structure of the Lavas in the Field--2. Fragmental
Rocks.--Agglomerates, Conglomerates, and Breccias--Tuffs and their
accompaniments.
We have now to consider the structure and history of those volcanic
masses which, during Tertiary time, were ejected to the surface
within the area of the British Islands, and now remain as extensive
plateaux. Short though the interval has been in a geological sense
since these rocks were erupted, it has been long enough to allow of
very considerable movements of the ground and of enormous denudation,
as will be more fully discussed in Chapters xlviii. and xlix. Hence
the superficial records of Tertiary volcanic action have been reduced
to a series of broken and isolated fragments. I have already stated
that no evidence now remains to show to what extent there were actual
superficial outbursts of volcanic material over much of the dyke-region
of Britain. The subsequent waste of the surface has been so enormous
that various lava-fields may quite possibly have stretched across parts
of England and Scotland, whence they have since been wholly stripped
off, leaving behind them only that wonderful system of dykes from which
their molten materials were supplied.
There can be little doubt, however, that whether or not other Phlegrean
fields extended over portions of the country whence they have since
been worn away, the chief volcanic tract lay in a broad and long hollow
that stretched from the south of Antrim to the Minch. From the southern
to the northern limit of the fragmentary lava-fields that remain in
this depression is a distance of some 250 miles, and the average
breadth of ground within which these lava-fields are preserved may be
taken to range from 20 to 50 miles. If, therefore, the sheets of basalt
and layers of tuff extended over the whole of this strip of country,
they covered a space of some 7000 or 8000 square miles. But they were
not confined to the area of the British Islands. Similar rocks rise
into an extensive plateau in the Faroe Islands, and it may reasonably
be conjectured that the remarkable submarine ridge which extends thence
to the North-west of Scotland, and separates the basin of the Atlantic
from that of the Arctic Ocean, is partly at least of volcanic origin.
Still further north come the extensive Tertiary basaltic plateaux of
Iceland, while others of like aspect and age cover a vast area in
Southern Greenland. Without contending that one continuous belt of
lava-streams stretched from Ireland to Iceland and Greenland, we can
have no doubt that in older Tertiary time the north-west of Europe was
the scene of more widely-extended volcanic activity than had shown
itself at any previous period in the geological history of the whole
continent. The present active vents of Iceland and Jan Mayen are not
improbably the descendants in uninterrupted succession of those that
supplied the materials of the Tertiary basaltic plateaux, the volcanic
fires slowly dying out from south to north. But so continuous and
stupendous has been the work of denudation in these northern regions,
where winds and waves, rain and frost, floe-ice and glaciers reach
their highest level of energy, that the present extensive sheets of
igneous rock can be regarded only as magnificent relics, the grandeur
of which furnishes some measure of the magnitude of the last episode in
the extended volcanic history of Britain.
The long and wide western valley in which the basalt-plateaux of this
country were accumulated seems, from a remote antiquity, to have been
a theatre of considerable geological activity. There are traces of
some such valley or depression even back in the period of the Torridon
Sandstone of the north-west. This formation, as we have seen, was laid
down between the great ridge of the Outer Hebrides and some other land
to the east, of which a few of the higher mountains, once buried under
the sandstone, are now being revealed by denudation between Loch Maree
and Loch Broom, and also in Assynt. The conglomerates and volcanic
rocks of Lorne may represent the site of one of the older water-basins
of this ancient hollow. The Carboniferous rocks, which run through the
North of Ireland, cross into Cantyre, and are found even as far north
as the Sound of Mull, mark how, in later Palæozoic time, the same
strip of country was a region of subsidence and sedimentation. During
the Mesozoic ages, similar operations were continued; the hollow sank
several thousand feet, and Jurassic strata to that depth filled it up.
Before the Cretaceous period, underground movements had disrupted and
irregularly upheaved the Jurassic deposits, and prolonged denudation
had worn them away, so that when the Cretaceous formations came to be
laid down on the once more subsiding depression, they were spread out
with a strong unconformability on everything older than themselves,
resting on many successive horizons of the Jurassic system, and passing
from these over to the submerged hillsides of the crystalline schists.
Yet again, after the accumulation of the Chalk, the sea-floor along
the same line was ridged up into land, and the Chalk, exposed to
denudation, was deeply trenched by valleys, and entirely removed from
wide tracts which it once covered.
It was in this long broad hollow, with its memorials of repeated
subsidences and upheavals, sedimentation and denudation, that the
vigour of subterranean energy at last showed itself in volcanic
outbreaks, and in the gradual piling up of the materials of the
basalt-plateaux. So far as we know, these outbursts were subærial.
At least no trace of any marine deposit has yet been found even at
the base of the pile of volcanic rocks. Sheet after sheet of lava was
poured out, until several thousand feet had accumulated, so as perhaps
to fill up the whole depression, and once more to change entirely the
aspect of the region. But the volcanic period, long and important as
it was in the geological history of the country, came to an end. It,
too, was merely an episode during which denudation still continued
active, and since which subterranean disturbance and superficial
erosion have again transformed the topography. In wandering over
these ancient lava-fields, we see on every hand the most stupendous
evidence of change. They have been dislocated by faults, sometimes with
a displacement of hundreds of feet, and have been hollowed out into
deep and wide valleys and arms of the sea. Their piles of solid rock,
hundreds of feet thick, have been totally stripped off from wide tracts
of ground which were once undoubtedly buried under them. Hence, late
though the volcanic events are in the long history of the land, they
are already separated from us by so vast an interval that there has
been time for cutting down the wide plateaux of basalt into a series of
mere scattered fragments. But the process of land-sculpture has been of
the utmost service to geology, for, by laying bare the inner structure
of these plateaux, it has provided materials of almost unequalled value
and extent for the study of one type of volcanic action.
I. NATURE AND ARRANGEMENT OF THE ROCKS OF THE PLATEAUX
The superficial outbursts of volcanic action during Tertiary time in
Britain are represented by a comparatively small variety of rocks.
These consist almost wholly of basalts, but include a number of less
basic rocks which may be classed as andesites. Many andesitic sheets,
like the andesitic dykes, have been intruded into the basalts, and are
really sills.
Besides the lavas of the basaltic-plateaux there are intercalated
deposits of tuffs and breccias and large masses of agglomerate. A brief
notice of the general petrography of the various constituents of the
plateaux and their mode of occurrence will here be given. The intrusive
bosses which have disrupted the superficial lavas will be discussed in
subsequent chapters.
i. LAVAS
1. _Petrographical Characters_
(_a_) _Basalts and Dolerites._--In external characters these rocks
range from coarsely crystalline varieties, in which the constituent
minerals may be more or less readily detected with the naked eye or a
field-lens, to dense black compounds in which only a few porphyritic
crystals may be megascopically visible. One of their characteristic
features is the presence of the ophitic structure, sometimes only
feebly developed, sometimes showing itself in great perfection.
Many of the rocks are holocrystalline, but usually show more or
less interstitial matter; in others the texture is finer, and the
interstitial matter more developed; in no case, as far as I have
observed, are there any glassy varieties, which are restricted to
the dykes and sills, though in some of the basalts the proportion of
glassy or incompletely devitrified substance is considerable. The
felspars are generally of the characteristic lath-shaped forms, and
are usually quite clear and fresh. The augite resembles that of the
dykes, occurring sometimes in large plates that enclose the felspars,
at other times in a finely granular form. Olivine is frequently
not to be detected, even by green alteration products. Magnetite
is sometimes present in such quantity as to affect the compass of
the field-geologist. Porphyritic varieties occur with large felspar
phenocrysts; but such varieties are, I think, less frequent among the
plateau-rocks than among the dykes. They are well developed in the west
part of the island of Canna, and have been described from the Faroe
islands. Occasionally the plateau lavas are full of enclosed fragments
of other rocks which have been carried up in the ascending magma.
(_b_) _Andesites and Trachytes._--Probably the majority of these rocks
where they occur intercalated between the basalts of the plateaux are,
as already remarked, intrusive sheets rather than true lavas. But
they have also been poured out intermittently among the basalts and
dolerites. The most extensive development of lavas which are readily
distinguishable from the group of plateau-basalts, and must be placed
in the present series, occurs in the island of Mull. These rocks form
part of a group of pale lavas which overlie the main mass of the
plateau-basalts, and cap the mountain Ben More, together with several
of its lofty neighbours. They are interstratified with true ophitic
dolerites, and basalts showing characteristic granular augite. They
are not so heavy as the ordinary plateau-lavas, their specific gravity
ranging from 2·55 to 2·74. Externally they are light grey in colour
and dull in texture, sometimes strongly amygdaloidal, sometimes with a
remarkable platy structure, which, in the process of weathering, causes
them to split up like stratified rocks. In some of their amygdaloidal
varieties the cells are filled with epidote, which also appears in the
fissures, and sometimes even as a constituent of the rock.
Specimens from this "pale group" of Ben More, when examined in thin
slices under the microscope, were found by Dr. Hatch to consist almost
wholly of felspar in minute laths or microlites, but in no instance
sufficiently definite for satisfactory determination. In one of them
he observed that each lath of felspar passed imperceptibly into
those adjacent to it; the double refraction being very weak, and the
twin-striation, if present, not being traceable.[220] More recently my
colleague, Mr. W. W. Watts, has looked at some of the same slides. He
is disposed to class the rocks rather with the trachytes than the
andesites. He remarks that "in the apparent holocrystalline character,
the size and shape of the felspars, the sort of damascened appearance
in polarized light, the finely scattered iron-ores and the presence
of a pale green hornblende, possibly augite, in small, often complex,
grains, these rocks much resemble the Carboniferous trachytes of the
Garlton Hills in Scotland."
[Footnote 220: In the course of my investigations I have had many
hundreds of thin slices cut from the Tertiary volcanic rocks for
microscopic determination. These I have myself studied in so far as
their microscopic structure appeared likely to aid in the investigation
of those larger questions of geological structure in which I was
more especially interested. But for further and more detailed study
I placed them with Dr. Hatch, who submitted to me the results of his
preliminary examination, and where these offered points of geological
import I availed myself of them in the memoir published in 1888 in
the _Transactions of the Royal Society of Edinburgh_. I have retained
most of these citations in their place in the present volume, and have
supplemented them by notes supplied to me from fresh observations by
Mr. Watts and Mr. Harker. Professor Judd, in a series of valuable
papers, has discussed the general petrography of the Tertiary volcanic
rocks (_Quart. Jour. Geog. Soc._ vols. xxxix. xli. xlii. xlvi. xlix.)]
One of the most interesting lavas of the Tertiary volcanic series is
the "pitchstone-porphyry" of the Scuir of Eigg. This rock, the latest
known outflow of lava in any of the volcanic areas of Britain, was
formerly classed with the acid series. Microscopical and chemical
analyses prove it, however, to be of intermediate composition, and
to be referable to the andesites or dacites. It is more particularly
described in Chapter xxxviii.
Professor Judd, collecting the andesitic rocks as a whole (both lavas
and sills), has grouped them into amphibole and mica-andesites, and
pyroxene-andesites.[221] The thick lumpy and non-persistent sheets of
these rocks sometimes found near the centres of protrusion of the
gabbros and granophyres are probably sills.
[Footnote 221: _Quart. Journ. Geol. Soc._ vol. xlvi. (1890), p. 356.
Professor Judd has there described under the name of "propylites"
various members of the volcanic series which he believes to have
undergone alteration from solfataric action. I have not been able to
discover any trace of such action, but I have found that the lavas of
the plateaux assume a peculiar condition where they have been affected
by large intrusive masses of granophyre or gabbro. (See _postea_,
Chapter xlvi.)]
(_c_) _Rhyolites._--In the Antrim plateau a group of rhyolite bosses
occurs, some of which have been claimed as superficial lavas. In
some cases it can be demonstrated that they are intrusive, and in no
instance can they be decisively shown to have escaped in streams at
the surface. It is probable, however, that some of these bosses did
actually communicate with the outer air, for between the lower and
upper group of basalts in this plateau, bands of rhyolitic conglomerate
occur which may indicate the degradation of exposed masses of rhyolite.
The description of these Antrim bosses will be given in Chapter xlvii.,
in connection with the acid eruptive rocks of the Tertiary volcanic
series.
2. _Structure in the Field_
Passing now to the consideration of the lavas as they are built up
into the plateaux, we have to note their distinctive characters as
individual sheets of rock, and their influence on the topography of the
regions in which they occur. Every tourist who has sailed along the
cliffs of Antrim, Mull, Skye, or the Faroe Islands is familiar with the
singular terraced structure of the great volcanic escarpments which
stretch as mural precipices along these picturesque shores. Successive
sheets of lava, either horizontal or only gently inclined, rise above
each other from base to summit of the cliffs as parallel bars of brown
rock with intervening strips of bright green grassy slope.
The geologist who for the first time visits these coast-lines is
impressed by the persistence of the same lithological characters
giving rise to the same topographical features. He soon realises
that the plateaux, so imposingly truncated by the great escarpments
that spring from the edge of the sea, are built up essentially of
dark lavas--basalts and dolerites--and that fragmental volcanic
accompaniments, though here and there well developed, play, on the
whole, a quite insignificant part in the structure and composition
of these thick piles of volcanic material. Closer examination in the
field enables him to ascertain that, regarded as rock-masses, the lavas
include four distinct types:--
1st. Thick, massive, prismatic or rudely-jointed sheets, rather
more coarsely crystalline and obviously more durable than the other
types, inasmuch as they project in tabular ledges and tend to retain
perpendicular faces owing to the falling away of slices of the rock
along lines of vertical joints. Many rocks of this type are undoubtedly
intrusive sheets, and as such will be further referred to in a later
chapter. But the type includes also true superficial lavas which
show the characteristic slaggy or vesicular bands at their upper and
lower surfaces. The mere presence of such bands may not be enough,
indeed, absolutely to establish that the rock possessing them flowed
at the surface as a lava, for they are occasionally, though it must be
confessed rarely, exhibited by true sills. But the rough scoriaceous
top of a lava-stream, and the presence of fragments of this surface
in the overlying tuff, or wrapped round by the next succeeding lava,
sufficiently attest the true superficial outflow of the mass.
2nd. Prismatic or columnar basalts, which, as at the Giant's Causeway
and Staffa, have long attracted notice as one of the most striking
topographical elements of the plateaux. Columnar structures are typical
of the more compact heavy basalts. A considerable variety is observable
in the degree of perfection of their development. Where they are least
definite, the rock is traversed by vertical joints, somewhat more
regular and close-set than those in the dolerites, by the intersection
of which it is separated into rude quadrangular or polygonal columns.
The true columnar structure is shown in two chief forms. (_a_) The rock
is divided into close-fitting parallel, usually six-sided columns; the
number of sides varying, however, from three up to nine. The columns
run the whole thickness of the bed, and vary from 8 or 10 to 40 or even
80 feet in length. They are segmented by cross joints which sometimes,
as at Giant's Causeway, take the ball-and-socket form. Occasionally
they are curved, as at the well-known Clam-shell cave of Staffa.
(_b_) The prisms are much smaller, and diverge in wavy groups crowded
confusedly over each other, but with a general tendency upwards. This
starch-like aggregation may be observed superposed directly upon the
more regular columnar form as at the Giant's Causeway and also at
Staffa. Excellent illustrations of both these types may be seen at many
points along the sea-cliffs of the Inner Hebrides; the western coast
of Skye, the south-west side of Mull, and the cliffs of the island of
Canna may be specially cited.
[Illustration: Fig. 259.--Section of scoriaceous and prismatic Basalt,
Camas Tharbernish, north shore of Canna Island.]
Though generally rather compact, becoming indeed dense, almost vitreous
rocks in some sheets, the columnar basalts are often more or less
cellular throughout, and highly slaggy along their upper and under
surfaces. In some cases, as in that of a prismatic sheet which overlies
the rough scoriaceous lava of Camas Tharbernish, in the island of
Canna, the rows of vesicles are disposed in lines parallel to the under
surface of the sheet (Fig. 259.)
As already remarked with regard to the massive, rudely-jointed sheets,
many of the most perfectly columnar rocks of the plateaux are not
superficial lavas, but intrusive sills, bosses or dykes. Conspicuous
examples of such sills are displayed on the coast of Trotternish in
Skye, and of the bosses and dykes at the eastern end of Canna. To these
further reference will be made in the sequel. It is not always possible
to be certain that columnar sheets which appear to be regularly
intercalated among the undoubted lavas of the volcanic series may not
be really intrusive. In some instances, indeed, we can demonstrate that
they are so, when after continuing perfectly parallel with the lavas
above and below them, they eventually break across them. One of the
most remarkable examples of this feature is supplied by the great sill
of the south-west of Stromö, in the Faroe Islands, of which I shall
give some account in Chapter xlii. (Figs. 312, 328, 329).
3rd. Slaggy or amygdaloidal lavas without any regular jointed
structure, but often with roughly scoriform upper and under layers, and
tending to decay into brown earthy debris. Some of the upper surfaces
of such sheets among the Tertiary basalt-plateaux must have resembled
the so-called "Aa" of the Sandwich Islands. A striking example of the
structure may be noticed at Camas Tharbernish, on the north coast of
the Island of Canna. There the hummocks on the upper surface of a
slaggy basalt measure about 15 feet in breadth, and rise about three
feet above the hollows between them, like a succession of waves (see
Fig. 259). The steam-holes are disposed in a general direction parallel
to the strike of the hummocks.
Great variety obtains in the size and shape of the vesicles. Huge
cavities a foot or more in diameter may occasionally be found, and from
such extremes every gradation may be traced down to minute pore-like
vacuoles that can hardly be made out even with a strong lens. In regard
to the deformation of the vesicles, it is a familiar general rule that
they have been drawn out in the direction of the flow of the original
lava. Occasionally they have become straight, narrow, sometimes
bifurcating pipes, several inches long, and only an eighth of an inch
or so in diameter.[222] A number of such pipes, parallel to each other,
resembles a row of worm-burrows (see Fig. 2).
[Footnote 222: Some examples have been deposited by me in the Museum
of Practical Geology, Jermyn Street, in the case illustrating
rock-structures. The elongation of the vesicles into annelide-like
tubes may also be observed among the stones in the volcanic
agglomerates.]
It may often be noticed that, even where the basalt is most perfectly
prismatic, it presents a cellular and even slaggy structure at the
bottom. The rock that forms the Giant's Causeway, for instance, is
distinctly vesicular, the vesicles being drawn out in a general east
and west direction. The beautiful columnar bed of Staffa is likewise
slaggy and amygdaloidal for a foot or so upwards from its base, and
portions of this lower layer have here and there been caught up and
involved in the more compact material above it. Even the bottom of the
confusedly prismatic bed above the columnar one on that island also
presents a cellular texture. A similar rock at Ardtun, in Mull, passes
upward into a rugged slag and confused mass of basalt blocks, over
which the leaf-beds lie.
Amygdaloidal structure is more or less developed throughout the whole
series of basalts. But it is especially marked in certain abundant
sheets, which, for the sake of distinction, are called amygdaloids.
These beds, which form a considerable proportion of the materials of
every one of the plateaux, are distinguished by the abundance and
large size of their vesicles. In some places, the cavities occupy at
least as much of the rock as the solid matrix in which they lie. They
have generally been filled up with some infiltrated mineral--calcite,
chalcedony, zeolites, etc. The amygdales of the west of Skye and of
Antrim have long been noted for their zeolites. As a consequence of
their cellular texture and the action of infiltrating water upon them,
these amygdaloidal sheets are always more or less decomposed. Their
dull, lumpy, amorphous aspect contrasts well with the sharply-defined
columnar sheets above and below them, and as they crumble down they
are apt to be covered over with vegetation. Hence, on a sea-cliff
or escarpment, the green declivities between the prominent columnar
basalts usually mark the place of such less durable bands.
Exceedingly slag-like lavas are to be seen among the amygdaloids,
immediately preceded and followed by beds of compact black basalt
with few or no vesicles. From the manner in which such rocks yield
to the weather, they often assume a singularly deceptive resemblance
to agglomerates. One of the best examples of this resemblance which
have come under my notice is that of the rock on which stands Dunluce
Castle, on the north coast of Antrim. Huge rounded blocks of a harder
consistency than the rest of the rock project from the surface of
the cliffs, like the bombs of a true volcanic agglomerate, while the
matrix in which they are wrapped has decayed from around them. But an
examination of this matrix will soon convince the observer that it is
strongly amygdaloidal, and that the apparent "bombs" are only harder
and less cellular portions of it. The contrast between the weathering
of the two parts of the rocks seems to have arisen from an original
variety in the relative abundance of steam-cavities. The origin of
such nodular or pillow-like blocks has been already referred to at
pp. 26 and 193. Another singular instance occurs at the foot of the
outlier of Fionn Chro (Fig. 360), in the island of Rum. A conspicuous
band underlying the basalts there might readily be taken for a
basalt-conglomerate. But in this case, also, the apparent matrix is
found to be amygdaloidal, and the rounded blocks are really amygdales,
sometimes a foot in length, filled or lined with quartz, chalcedony, &c.
A somewhat different structure, in which, however, the appearance
of volcanic breccia or agglomerate due to explosion from a vent is
simulated, may be alluded to here. The best instance which I have
observed of it occurs at the south end of Loch-na-Mna, in the island of
Eigg, within a basalt which is remarkable for a streaky flow-structure.
On the weathered faces the streaky layers may be observed to have
been broken up, and their disconnected fragments have been involved
in ordinary basalt wherein this flow-structure is not developed,
while large blocks and irregular masses are wrapped round in a more
decomposing matrix. There can be no doubt that in such cases we see the
effects of the disruption of chilled crusts, and the entanglement of
the broken pieces in the still fluid lava.
It is a common belief that the filling in of the steam-cavities has
taken place long subsequent to the volcanic period, by the slow
percolation of meteoric water through the rock. I believe, however,
that at least in some cases, if not in all, the conversion of the
vesicular lavas into amygdaloids was effected during the volcanic
period. Thus it can be shown that the basalts which have been disrupted
by the gabbros and granophyres were already amygdaloids before these
basic intrusions disturbed them, for the kernels of calcite, zeolite,
etc., have shared in the general metamorphism induced in the enclosing
rock. Again, the blocks of amygdaloid contained in the agglomerates of
the volcanic series are in every respect like the amygdaloidal lavas
of the plateaux. It would thus seem that the infilling of the cavities
with mineral secretions was not merely a long secular process of
infiltration from the cool atmosphere, but was more rapidly completed
by the operation of warmer water, either supplied from volcanic sources
or heated by the still high temperature of the cellular lavas into
which it descended from the surface.[223]
[Footnote 223: Professor J. D. Dana, originally an advocate of
infiltration from above, subsequently supported the view that the
kernels of amygdaloids were filled in by the action of moisture within
the rocks during the time of cooling.--_Amer. Journ. Sci._ ser. 3, vol.
xx. (1880), p. 331. Messrs. Harker and Marr have demonstrated that the
Lower Silurian vesicular lavas of the Lake district had already become
amygdaloids before the uprise of the Shap granite.--_Quart. Journ.
Geol. Soc._ vol. xlix. (1893).]
4th. Banded or stratiform lavas, consisting of successive parallel
layers or bands which weather into projecting ribs and flutings. The
deceptive resemblance to sedimentary rocks thus produced has no doubt
frequently led to these lavas being mistaken for tuffs. As I have
recently found them to be much more plentiful than I had supposed, a
more detailed description of them seems to be required.
The banded character arises from marked distinctions in the texture
of different layers of a lava-sheet. In some cases (_a_) these
distinctions arise from differences in the size of the crystals or in
the disposition of the component minerals of the rock; in others (_b_)
from the varying number and size of the vesicles, which may be large or
abundantly crowded together in some layers, and small or only sparsely
developed in others. The structure thus points to original conditions
of the lava at the time of its emission and may be regarded as, to some
extent, a kind of flow-structure on a large scale.
(_a_) Where the banding is due to differences of crystalline texture,
the constituent felspars, augites, and iron-ores may be seen even with
the naked eye as well-defined minerals along the prominent surfaces
of the harder ribs, while the broader intervening flutings of finer
material show the same minerals in minuter forms. The alternating
layers of coarser and finer crystallization lie, on the whole,
parallel with the upper and under surfaces of the sheets in which they
occur. But they likewise undulate like the streaky lines in ordinary
flow-structure.
Banded structure of this type may be seen well developed in the
lower parts of the basalt-plateaux throughout the Inner Hebrides and
the Faroe Islands. A specimen taken from the west end of the island
of Sanday, near Canna, which showed the structure by a conspicuous
parallel fluting on weathered surfaces, was sliced for microscopical
examination. Mr. Harker has been kind enough to supply me with the
following observations regarding this slice:--
"In the slice [6660][224] the banding becomes less conspicuous under the
microscope. The rock is of basaltic composition, and, with reference
to its micro-structure, might be styled a fine-grained olivine-diabase
or olivine-dolerite in some parts of the slice, an olivine-basalt in
others. It consists of abundant grains of olivine, imperfect octahedra
and shapeless granules of magnetite, little simple or twinned prisms
of labradorite, and a pale brown augite. The last-named mineral is
always the latest product of consolidation, but it varies in habit,
being sometimes in ophitic patches moulded upon or enclosing the other
minerals, sometimes in small granules occupying the interstices between
the felspars and other crystals. The ophitic habit predominates in the
slice, while the granulitic comes in especially along certain bands. If
the former be taken as indicative of tranquil conditions, the latter of
a certain amount of movement in the rock during the latest stages of
its consolidation, the banding, though not strictly a flow-structure,
may be ascribed in some degree to a flowing movement of the nearly
solidified rock. There is, however, more than this merely structural
difference between the several bands. They differ to some extent in the
relative proportions of the minerals, especially of olivine and augite;
which points to a considerable flowing movement at an early stage in a
magma which was initially not homogeneous."
[Footnote 224: The figures within square brackets throughout the
following pages refer to the numbers of the microscopic slides in the
Geological Survey collection, where I have deposited all those prepared
from my specimens.]
(_b_) Where the banding arises from the distribution of the vesicles,
somewhat similar weathered surfaces are produced. In some instances,
while the basalt is throughout finely cellular, interposed bands of
harder, rather finer-grained and less thoroughly vesicular character
serve to give the stratified appearance. Instances may be observed
where the vesicles have been crowded together in certain bands, which
consequently weather out differently from the layers above and below
them. An excellent illustration of this arrangement occurs in the
lowest lava but one of the largest of the three picturesque stacks
known as Macleod's Maidens on the west coast of Skye (Figs. 260, 283,
284 and 287). This lava is thoroughly amygdaloidal, but the vesicles
are specially crowded together in certain parallel bands from an inch
to three or four inches thick. Some of these layers lie close to each
other, while elsewhere there may be a band of more close-grained, less
vesicular material between them. But the most singular feature of the
rock is to be seen in the shape and position of the vesicles that are
crowded together in the cellular bands. Instead of being drawn out into
flattened forms in the general direction of banding, they are placed
together at high angles. Each layer remains parallel to the general
bedding, but its vesicles are steeply inclined in one direction, which
was doubtless that of the flow of the still unconsolidated lava.[225]
Weathering along these bands, the lava might easily be mistaken at a
little distance for a tuff or other stratified intercalation.
[Footnote 225: This elongation of vesicles, more or less perpendicular to
the general bedding, may be noticed sometimes even in sills, as will be
shown in a later Chapter.]
[Illustration:
Fig. 260.--Banded amygdaloidal basalt showing layers of elongated
and steeply inclined vesicles, Macleod's Maidens, Skye.
]
Banded lavas possessing the characters now described are of frequent
occurrence among the Inner Hebrides. Many striking examples of them may
be seen along the west coast of Skye. Still more abundant in Faroe,
they form one of the most conspicuous features in the geology of that
group of islands. Along the whole of its western seaboard, on island
after island, they are particularly prominent in the lower parts of
the precipices, while the upper parts consist largely of amorphous or
prismatic sheets. So much do they resemble stratified rocks that it was
not until I had landed at various points that I could satisfy myself
that they are really banded lavas.[226]
[Footnote 226: For recent contributions to the Geology of the Faroe
Islands, see Prof. James Geikie, _Trans. Roy. Soc. Edin._ vol. xxx.
(1880), p. 217, where the banding of the basalts is noticed; Prof. A.
Helland, _Dansk. Geografisk. Tidskr._ (1881); R. Bréon, _Notes pour
servir à l'étude de la Géologie de l'Islande et des Isles Faeroe_
(1884); Mr. J. Lomas, _Proc. Geol. Soc. Liverpool_, vol. vii. (1895),
p. 292. Various writers have treated of the petrography of Faroe,
particularly A. Osann, _Neues Jahrb._ (1884), vol. i. p. 45, and M.
Bréon in the volume here cited.]
5th. Ordinary flow-structure, save in these banded lavas, is rather
rare among the plateaux. It may, however, be occasionally observed,
where there is no distinct banding. On a weathered surface it appears
in fine, widely parallel streaks, which are sometimes wavy, puckered
and broken up, as in rhyolites and felsites, while the porphyritic
felspars are arranged with their long axes in the direction of flow.
A good example of these characters may be seen on the summit of the
Dùn Can--the remarkable truncated cone which forms the highest point
on the Island of Raasay. The rock is a black olivine-basalt, partly
amygdaloidal, with zeolites filling up the cavities, and its flow-lines
are prominent on the weathered faces where they lie parallel to the
general bedding of the lavas. Another illustration may be observed
in the basalt already cited from Loch-na-Mna, in the island of Eigg,
where the rock presents in places a remarkable streaky structure
which, though hardly visible on a fresh fracture, reveals itself on a
weathered face in thin nearly parallel ribs coincident in direction
with the upper and under surfaces of the mass.
Great variety is to be found in the thickness of different sheets of
lava in the plateaux. Some of them are not more than 6 or 8 feet;
others reach to 80 or 100 feet, and sometimes, though rarely, to even
greater dimensions. In Antrim, the average thickness of the flows is
probably from 15 to 20 feet.[227] In the fine coast-sections at the
Giant's Causeway, however, some bands may be seen far in excess of that
measurement. The bed that forms the Causeway, for instance, is about
60 or 70 feet thick, and seems to become even thicker further east.
Along the great escarpment, 700 feet high, which rises from the shores
of Gribon, on the west coast of Mull, there are twenty separate beds,
which give an average of 35 feet for the thickness of each flow. On
the great range of sea-precipices along the west coast of Skye, which
present the most stupendous section of the basalts anywhere to be
seen within the limits of the British Islands, the average thickness
of the beds can be conveniently measured. At the Talisker cliffs some
of the flows are not more than 6 or 8 feet; others are 30 or 40 feet.
The chief precipice, 957 feet high (Fig. 286), contains at least 18 or
20 separate lava-sheets, which thus average of from 47 to 53 feet in
thickness. In the cliffs that form the seaward margin of the tableland
of Macleod's Tables (Fig. 283) fourteen successive beds of basalt can
be counted in a vertical section of 400 feet, which is equal to an
average thickness of about 28 feet. But some of the basalts are only
about 6 feet thick, while others are 50 or 60. The Hoe of Duirinish,
759 feet high, is composed of about sixteen distinct beds, which
thus have a mean thickness of 46 feet. The average thickness of the
successive flows on Dunvegan Head, which is 1000 feet high and contains
at least twenty-five separate sheets, is about 40 feet. Still further
north, the cliffs, 800 feet high, comprise sixteen successive flows,
which have thus an average of 50 feet each. Among the Faroe Islands the
average thickness of the basalt-sheets seems to be nearly the same as
in Britain. Thus in the magnificent ranges of precipices of Kalsö, Kunö
and Borö, forty or more sheets may be counted in the vast walls of rock
some 2000 feet high, giving a mean of about 50 feet.
[Footnote 227: See Explanation of Sheet 20, Geol. Survey, Ireland, p. 11.]
Each bed appears, on a cursory inspection, to retain its average
thickness, and to be continuous for a long distance. But I believe that
this persistence is in great measure deceptive. We can seldom follow
the same bed with absolutely unbroken continuity for more than a mile
or two. Even in the most favourable conditions, such as are afforded by
a bare sea-cliff on which every sheet can be seen, there occur small
faults, gullies where the rocks are for the time concealed, slopes of
debris, and other failures of continuity; while the rocks are generally
so like each other, that on the further side of any such interruption,
it is not always possible to make sure that we are still tracing the
same bed of basalt which we may have been previously following. On
the other hand, a careful examination of one of these great natural
sections will usually supply us with proofs that, while the bedded
character may continue well marked, the individual sheets die out, and
are replaced by others of similar character. Cases may not infrequently
be observed where the basalt of one sheet abruptly wedges out, and is
replaced by that of another. Where both are of the same variety of
rock, it requires close inspection to make out the difference between
them; but where one is a green, dull, earthy, amorphous amygdaloid,
and the other is a compact, black, prismatic basalt, the contrast
between the two beds can be recognized from a distance (Fig. 261). In
the basaltic cliffs of the west coast of Skye, the really lenticular
character of the flows can be well seen. I may especially cite the
great headland south of Talisker Bay, already referred to, where, in
the pile of nearly horizontal sheets, two beds may be seen to die out,
one towards the north, the other towards the south. Further north,
in the cliff of the Hoe of Duirinish, a similar structure presents
itself. Along the coast-cliffs of Mull, Morven and Canna the same
fact is clearly displayed. Thus on the west side of the Sound of Mull
the slopes above Fishnish Bay show a group of basalts, which die out
southward, and are overlapped by a younger group that has been poured
over their ends. Such sections are best seen in the evening, when the
grass-covered lavas show their successive sheets by their respective
shadows, their individuality being lost in the full light of day. A
more striking example occurs beyond the west end of Glen More in Mull,
where one series of basalts has been tilted up, probably during some
volcanic episode, and has had a younger series banked up against its
edges.
[Illustration: Fig. 261.--Termination of Basalt-beds, Carsaig, Mull.]
In Antrim also, remarkable evidence is presented of the rapid
attenuation not of single beds only, but of a whole series of basalts.
Thus, at Ballycastle, the group of lavas known as the Lower Basalts,
which underlie the well-known horizon of iron-ore, are at least 350
feet thick. But, as we trace them westwards, bed after bed thins out
until, a little to the west of Ballintoy, in a distance of only about
6 miles, the whole depth of the group has diminished to somewhere
about 40 feet. A decrease of more than 300 feet in six miles or 50
feet per mile points to considerable inequalities in the accumulation
of the lavas. If the next series of flows came from another vent and
accumulated against such a gentle slope, it would be marked by a slight
unconformability. Structures of this kind are much rarer than we should
expect them to be, considering the great extent to which the plateaux
have been dissected and laid open in cliff-sections.
The basalt-plateau of the Faroe Islands exhibits with remarkable
clearness the lenticular character of the basalt-sheets, and a number
of examples will be cited in the description of that region to be
given in Chapter xxxix. In these northern climes vegetation spreads
less widely over rock and slope than it does in the milder air of the
Inner Hebrides. Hence the escarpments sweep in precipices of almost
bare rock from the level of the sea up to the serrated crests of the
islands, some 2000 feet in height. Each individual bed of basalt can
thus be followed continuously along the fjords, and its variation
or disappearance can be readily observed. Coasting along these vast
natural sections, we readily perceive that, as among the Western Isles,
the successive sheets of basalt have proceeded from no one common
centre of eruption. They die out now towards one quarter, now towards
another, yet everywhere retain the universal regularity and gentle
inclinations of the whole volcanic series.
ii. FRAGMENTAL ROCKS
While the plateaux are built up mainly of successive flows of basaltic
lavas, they include various intercalations of fragmental materials,
which, though of trifling thickness, are of great interest and
importance in regard to the light which they cast on the history of the
different regions during the volcanic period. I shall enumerate the
chief varieties of these rocks here, and afterwards give fuller details
regarding their stratigraphical relations and mode of occurrence in
connection with the succession of beds in each of the plateaux.
(_a_) _Volcanic Agglomerates._--In the tumultuous unstratified masses
of fragmentary materials which fill eruptive vents in and around the
plateaux, the stones, which vary in size up to blocks several feet in
diameter, consist for the most part of basalts, often highly slaggy and
scoriaceous. They include also fragments of different acid eruptive
rocks (generally felsitic or rhyolitic in texture), with pieces of
the non-volcanic rocks through which the volcanic pipes have been
drilled. The paste is granular, dirty-green or brown in colour, and
seems generally to consist chiefly of comminuted basalt. As in the
Carboniferous and Permian necks, the Tertiary agglomerates contain
abundant detritus of a basic minutely cellular pumice.
(_b_) _Volcanic Conglomerates and Breccias in beds intercalated
between the flows of Basalt._--These are of at least three kinds.
(_a_) Basalt-conglomerates, composed mainly of rounded and subangular
blocks of basalt (or allied basic lava), sometimes a yard or more
in diameter, not unfrequently in the form of pieces of rough slag
or even of true bombs, imbedded in a granular matrix of comminuted
basalt-debris. In some cases, the stones form by far the most abundant
constituents of the rock, which then resembles some of the coarse
agglomerates just described. Perhaps the most remarkable accumulations
of this kind are those intercalated among the basalts in the islands
of Canna and Sanday, of which a detailed account will be given in
Chapter xxxviii. These conglomerates, besides their volcanic materials,
contain rounded blocks of Torridon sandstone and other rocks, which
must have been carried from the east by some tolerably powerful river
that flowed across the basalt-plains during the volcanic period.
Again, on the east side of Mull, the slaggy basalts of Beinn Chreagach
Mhor are occasionally separated by volcanic conglomerates. As a rule,
however, such intercalations are seldom more than a few feet or yards
in thickness. Their coarseness and repetition on successive horizons
indicate that they probably accumulated in the near neighbourhood of
one or more small vents, from which discharges of fragmentary materials
took place at the beginning or at the close of an outflow of lava, and
that the stones were sometimes swept away from the cones and rolled
about by streams before being buried under the succeeding lava-sheets.
More commonly the dirty-green or dark-brown granular matrix exceeds in
bulk the stones embedded in it. It has obviously been derived mainly
from the trituration of already cooled basalt--masses, and probably
also from explosions of the still molten rock in the vents. A striking
illustration of this type of rock may be seen on the south side of
Portree Harbour, where a mass of dark-green basalt-conglomerate, with
a coaly layer above it, lies near the base of the bedded basalts, and
attains at one part of its course a thickness of about 200 feet. This
rock will be again referred to in connection with the vent from which
its materials were probably derived. As in the case of the agglomerates
of the vents, pieces of older acid lavas, and still more of the
non-volcanic rocks that underlie the plateaux, are found in the bedded
conglomerates and breccias. In Antrim and Mull, for instance, fragments
of flint and chalk are of common occurrence. A characteristic example
of this kind of rock forms the platform of the columnar bed out of
which Fingal's Cave, Staffa, has been excavated (Fig. 266_a_).
([Greek: beta]) Felsitic Breccia.--This variety, though of rare
occurrence, is to be seen in a number of localities in the island
of Mull. It is composed in great measure of angular fragments of
close-grained flinty felsitic or rhyolitic rocks, sometimes showing
beautiful flow-structure, together with pieces of quartzite and
amygdaloidal basalt, the dull dirty-green matrix appearing to be made
up chiefly of basalt-dust.
([Greek: gamma]) Rhyolitic Conglomerate.--Between the upper and lower
group of basalts in the Antrim plateau there occur bands of a pale
fawn-coloured conglomerate largely made up of more or less rounded
fragments of rhyolite, like some of the varieties of the rock which
occur in place on the plateau. The rhyolitic debris is often mixed with
pebbles of basalt. Sometimes it becomes so fine as to pass into pale
clays.
([Greek: delta]) Breccias of non-volcanic materials.--These, the most
exceptional of all the fragmentary intercalations in the plateaux,
consist almost wholly of angular blocks of rocks which are known
to underlie the basalts, but with a variable admixture of basalt
fragments. They are due to volcanic explosions which shattered the
subjacent older crust of rocks, and discharged fragments of these from
the vents or allowed them to be borne upwards on an ascending column
of lava. Pieces of the non-volcanic platform are of common occurrence
among the fragmentary accumulations, especially in the lower parts of
the plateaux basalts. But I have never seen so remarkable an example
of a breccia of this kind as that which occurs near the summit of
Sgurr Dearg, in the south-east of Mull. The bedded basalt encloses a
lenticular band of exceedingly coarse breccia, consisting mainly of
angular pieces of quartzite, with fragments of amygdaloidal basalt.
In the midst of the breccia lies a huge mass or cake of erupted
mica-schist, at least 100 yards long by 30 yards wide, as measured
across the strike up the slope of the hill. To the west, owing to
the thinning out of the breccia, this piece of schist comes to lie
between two beds of basalt. A little higher up, other smaller but still
large blocks of similar schist are involved in the basalt, as shown
in Fig. 262. As the huge cake of mica-schist plunges into the hill,
its whole dimensions cannot be seen; but there are visible, at least,
15,000 cubic yards, which must weigh more than 30,000 tons. Blocks of
quartzite of less dimensions occur in the basalts on Loch Spelve, in
the same district. There can be no doubt, I think, that these enormous
fragments were torn off from the underlying crystalline schists which
form the framework of the Western Highlands, and were floated upward
in an ascending flow of molten basalt. Had the largest mass occurred
at or near the base of the volcanic series, its size and position
would have been less remarkable. But it lies more than 2000 feet up
in the basalts, and hence must have been borne upward for more than
that height. A similar but less striking breccia occurs on the south
coast of the same island, near Carsaig, made up chiefly of pieces of
quartzite and quartz.[228]
[Footnote 228: This is noticed by Mr. Starkie Gardner, _Quart. Journ.
Geol. Soc._ xliii. (1887), p. 283, note.]
Some remarkable agglomerates, near Forkhill, Armagh, probably belonging
to the Tertiary volcanic series, will be described in the account
of the Irish acid rocks (Chapter xlvii.). They consist entirely of
non-volcanic stones and dust and are traceable for some miles along the
line of a fissure. Where they have been discharged through granite they
consist entirely of the detritus of that rock, but where they have been
erupted in the Silurian area they consist of fragments of grits and
shales. They seem to have been produced by æriform discharges, without
the uprise of any volcanic magma, though eventually andesite and
rhyolite ascended the fissure and became full of granitic and Silurian
fragments.
Some remarkable necks filled almost entirely with fragments of Torridon
Sandstone have been observed in the west of Applecross, Ross-shire, and
some curious plug-like masses of breccia, also made up of fragments of
Torridonian strata, occur in the island of Raasay. These examples will
be more particularly described on later pages (pp. 292, 293).
(_c_) _Tuffs._--The tuffs intercalated in the basalt-plateaux generally
consist essentially of basic materials, derived from the destruction
of different varieties of basalts, though also containing occasional
fragments of older felsitic rocks, as well as pieces of chalk, flint,
quartz, and other non-volcanic materials. They are generally dull,
dirty-green in colour, but become red, lilac, brown, and yellow,
according to the amount and state of combination and oxidation of their
ferruginous constituents. They usually contain abundant fragments
of amygdaloidal and other basalts. As a rule, they are distinctly
stratified, and occur in bands from a few inches to 50 feet or more
in thickness. The matrix being soft and much decomposed, these bands
crumble away under the action of the weather, and contribute to the
abruptness of the basalt-escarpments that overlie them.
[Illustration: Fig. 262.--Breccia and Blocks of mica-schist, quartzite,
etc., lying between bedded Basalts, Isle of Mull.
_a a_, Bedded basalts; _b_, Breccia; _d_, Basic dyke.]
In the group of strata between the two series of basalts in Antrim,
some of the tuffs consist chiefly of rhyolitic detritus, both glassy
and lithoid.
Where the tuffs become fine-grained and free from imbedded stones,
they pass into variously-coloured clays. Among these are the "bauxite"
and "lithomarge" of Antrim, probably derived from pale rhyolitic tuffs
and conglomerates (p. 204). Associated with these deposits in the same
district, is a pisolitic hæmatite, which has been proved to occur over
a considerable area on the same horizon. Many of the clays are highly
ferruginous. The red streaks that intervene between successive sheets
of basalt are of this nature (bole, plinthite, etc.). The source of the
iron-oxide is doubtless to be traced to the decomposition of the basic
lavas during the volcanic period.
(_d_) There occur also grey and black clays and shales, of ordinary
sedimentary materials, containing leaves of terrestrial plants
(leaf-beds), with occasional wing-cases of beetles, sometimes
associated with impure limestones, but more frequently with sandstones
and indurated gravels or conglomerates containing pieces of fossil
wood. These intercalated bands undoubtedly indicate the action of
running water, sometimes even of river-floods, and the accumulation of
sediment in hollows of the exposed flows of basalt at intervals during
the piling up of the successive lava-sheets that form the plateaux.
The alternation of fluviatile gravels with volcanic tuffs, fluviatile
conglomerates, and lava-streams, is admirably displayed in the island
of Canna, as will be narrated in detail in Chapter xxxviii.
The vegetable matter has in some places gathered into lenticular seams
of lignite, and even occasionally of black glossy coal. Amber also has
been found in the lignite. Where the vegetation has been exposed to the
action of intrusive dykes or sheets, it has sometimes passed into the
state of graphite.
The remarkable terrestrial flora found in the leaf-beds, and in
association with the lignites, was first made known by the descriptions
of Edward Forbes already referred to, and has subsequently been studied
and described by Heer, W. H. Baily, and Mr. Starkie Gardner.[229] It
was regarded by Forbes as of Miocene age, and this view has generally
been adopted by geologists. Mr. Starkie Gardner, however, contends
that it indicates a much wider range of geological time. He believes
that a succession of floras may be recognised, the oldest belonging
to an early part of the Eocene period. Terrestrial plants, it must be
admitted, are not always a reliable test of geological age, and I am
not yet satisfied that in this instance they afford evidence of such
a chronological sequence as Mr. Gardner claims, though I am convinced
that the Tertiary volcanic period was long enough to have allowed
of the development of considerable changes in the character of the
vegetation.
[Footnote 229: On this subject consult Duke of Argyll, _Quart. Journ.
Geol. Soc._ vol. vii. (1851), p. 89; E. Forbes, _Ibid._ p. 103; W. H.
Baily, _op. cit._ xxv. (1869), pp. 162, 357; _Brit. Assoc. Rep._ (1879)
p. 162; (1880) p. 107; (1881) p. 151; (1884) p. 209; Mr. J. Starkie
Gardner, _Palæontographical Society_, vols. xxxviii. xxxix. In the last
of Mr. Baily's papers he notices that "the Rev. Dr. Grainger found a
portion of a fish (_Percidæ_, possibly _Lates_)." The discovery of the
remains of a fresh-water fish is an important additional testimony to
the terrestrial conditions under which the lavas were erupted. The
genus _Lates_ now inhabits the Nile and the Ganges.]
For the purpose of the present volume, however, the precise stage
in the geological record, which this flora indicates, is of less
consequence than the broad fact that the plants prove beyond all
question that the basalts among which they lie were erupted on land
during the older part of the long succession of Tertiary periods. Their
value in this respect cannot be overestimated. Stratigraphical evidence
shows that the eruptions must be later than the Upper Chalk; but the
imbedded plants definitely limit them to the earlier half of Tertiary
time.