During the last twenty years much has been written about the
"pre-Cambrian" rocks of the British Isles. Unfortunately when attention
began to be sedulously given to the study of these ancient formations,
the problems of metamorphism were still a hundred fold more obscure
than they have since become; the aid of the microscope had not been
seriously and systematically adopted for the investigation of the
crystalline schists, and geologists generally were still under the
belief that the broad structure of these schists could be treated like
those of the sedimentary rocks, and be determined by rapid traverses of
the ground. We have now painfully discovered that these older methods
of observation were extremely crude, and that the work performed in
accordance with them is now of little interest or value save as a
historical warning to future generations of geologists. Geological
literature has meanwhile been burdened with numerous contributions
which remain as a permanent incubus on our library shelves.
It may serve a useful purpose at the present time in possibly aiding
those who are engaged in the study of the oldest rocks of North
America, if I place before them, as briefly as possible, the main facts
which in my opinion have now been satisfactorily proved regarding the
corresponding rocks of Britain, and if I indicate at the same time some
of the more probable inferences in those cases where the facts, at
present known, do not warrant a definite conclusion.
It is obvious that in any effort to establish that a group of rocks is
older than the very base of the sedimentary fossiliferous formations,
we must somewhere find that group emerging from under the bottom of
these formations. Until lithological characters are ascertained to be
so distinctive and constant as to be comparable to fossil evidence
for purposes of stratigraphical identification, we should not assume
that detached areas of older rocks rising amid Palæozoic, Secondary or
Tertiary formations are pre-Cambrian. We should, if possible, begin
at the bottom of the Palæozoic systems and work backward, tracing
each successive system or group as these rise from under each other,
until we arrive at what appears to be the oldest traceable within
the region of observation. It is clear that in the present state of
knowledge we have no satisfactory means of identifying such successive
systems in widely separated countries. All that can be attempted in
the meantime is to ascertain the special types in each region, and to
point out their general resemblances or contrasts to those of other
regions. It is better to avoid confusion by refraining from applying
the stratigraphical names adopted for the oldest rocks of one region
to those of another geographically remote, though we may hope that
eventually it may be possible to work out the equivalence of these
local names.
In the British Isles, by much the most important region for the study
of the oldest rocks is to be found in the north-west Highlands of
Scotland. The very basement strata of the Cambrian system are there
traceable for a distance of more than 100 miles, reposing with a
strong unconformability upon all rocks of older date. They consist
of dolomitic shales with _Olenellus_, resting upon a thick group of
quartzites, full of annelid tubes. One of the most remarkable features
of these ancient strata is the persistence of their component bands
or zones which, though sometimes only a few feet thick, can be traced
throughout the whole tract of country just referred to. For the study
of the pre-Cambrian rocks this is an important point, for we can be
quite certain that even where fossil evidence locally fails, the same
basement members of the Cambrian system are persistent and lie directly
upon the pre-Cambrian series.
_Lewisian Gneiss._ Ever since the researches of Murchison and Nicol in
the north-west of Scotland, it has been known that two distinct systems
of rock underlie the quartzites to which I have just alluded. Murchison
regarded the upper of these as of Cambrian age, while he assigned the
unconformable quartzites and limestones above it to the Lower Silurian
period. But the recent discovery of the _Olenellus_ zone intercalated
conformably between the quartzites and the overlying limestones may be
regarded as proving that all the rocks which underlie the quartzites
and are separated from them by a strong unconformability must be
pre-Cambrian. It is thus established beyond any reasonable doubt that
two great pre-Cambrian systems of rock exist in the north-west of
Scotland.
These two systems differ so entirely from each other that their
respective areas can be defined with minute accuracy. The uppermost
consists chiefly of dull reddish sandstones with conglomerates, and
especially towards their base in Rosshire, some bands of dark grey
shale, the whole having a thickness of at least 8,000 or 10,000 feet,
though as both the base and the top of the series are marked by strong
unconformabilities, the whole original thickness of deposits is nowhere
seen. As these rocks are well developed around Loch Torridon, they
were named by Nicol the Torridon Sandstone--a designation which has
more recently been shortened into "Torridonian." The lower system is
mainly composed of various foliated rocks which may be embraced under
the general term "gneiss." These masses present the usual characters of
the so-called "fundamental complex", "Urgebirge," or "Archæan Series"
of other countries. The contrast between the thoroughly crystalline,
gnarled, ancient-looking gneisses below, and the overlying, nearly
horizontal Torridonian conglomerates, sandstones, and shales, which
are largely made out of their debris, is so striking that every
observer feels persuaded that in any logical system of classification
they can not be both placed in the same division of the geological
record. They are certainly both pre-Cambrian, but they must belong
to widely separated eras, and must have been produced by entirely
different processes. If it is proposed to regard the gneisses as
"Archæan," we must refuse to include the Torridonian strata in the
same section of pre-Cambrian time. But so much uncertainty exists as
to the application of this term Archæan, examples are so multiplying
wherein what was supposed to be the oldest and truly Archæan rock is
found to be intrusive in rocks that were taken to be of much younger
date, and there are such slender grounds for correlating the so-called
Archæan rocks of one country with those of another, that I prefer for
the present, at least, not to use the term at all. Let me very briefly
state some of the main characteristics of the two sharply contrasted
rock-systems of the north-west of Scotland.
The oldest gneiss of that region was originally called "Lewisian" by
Murchison, from its large development in the Island of Lewis, and I
think it would be, for the present at least, an advantage to retain
this geographical appellation. At first this "fundamental gneiss"
was thought to be a comparatively simple formation, and the general
impression probably was that it should be regarded as a metamorphic
mass, produced mainly from the alterations of very ancient stratified
rocks. Its foliation-planes were believed to be those of original
deposit which by terrestrial disturbance had been thrown into numerous
plications and corrugated puckerings. But a detailed study of this
primeval rock has revealed in it a far more complicated structure. The
supposed bedding-planes have been ascertained to have nothing to do
with sedimentary stratification, and the gneiss has been resolved into
a complex series of eruptive rocks, varying from a highly basic to an
acid type, and manifestly belonging to different times of extrusion.
With the exception of one district, to which I shall immediately refer,
no part of the whole region yet examined has revealed to the rigid
scrutiny of my colleagues of the Geological Survey, any trace of rocks
which can be regarded as probably of other than igneous origin. It is
true that our researches have been hitherto confined to the mainland
of Scotland, the large area of the Outer Hebrides, which consists of
similar gneisses, remaining to be explored. It is therefore possible
that indisputable evidence of an ancient sedimentary series through
which the gneiss was originally protruded, may yet be discovered in
the unexplored islands. But taking the gneiss as at present known in
Sutherland and Rosshire, we find it to be generally coarse in texture,
rudely foliated, and passing sometimes into massive types in which
foliation is either faintly developed or entirely absent. Much of this
gneiss is considerably more basic than the more typical rocks to which
the term gneiss was formerly restricted. It consists of plagioclase
felspar with pyroxene, hornblende, and magnetite, sometimes with blue
opalescent quartz, and sometimes with black mica. These predominant
minerals are segregated in different proportions in the different
bands, some bands consisting mainly of pyroxene or hornblende, with
little or no plagioclase, others chiefly of plagioclase, with small
quantities of the ferro-magnesian minerals and quartz, others of
plagioclase and quartz, others of magnetite. This separation of mineral
constituents can hardly be attributed to mere mechanical deformation.
It rather resembles the segregation layers which may be studied in
intrusive sills and other deep-seated masses of eruptive material, and
which are obviously due to a process of separation that went on while
the igneous magma was still in a liquid or viscous condition. At the
same time it is manifest that extensive dynamical changes have affected
the rocks since the appearance of this original banded structure.
There is further evidence that beside the original eruptive masses,
which for want of any means of discriminating their relative dates
of protrusion must in the meantime be regarded as belonging to
one eruptive period, other portions of igneous material have been
subsequently and at successive epochs, after the first mechanical
deformations, injected into the body of the original gneiss. These
consist of dykes of basalt and dolerite, followed by still more basic
peridotites and picrites, and lastly by emanations from a distinctly
acid magma in the form of granites. The oldest or doleritic dykes form
a wonderful feature in the gneiss, from their abundance, persistence
and uniformity of trend in a west-northwest direction. They have no
parallel in British Geology until we reach the crowded dykes of older
Tertiary time.
Throughout this remarkable complex of eruptive material, though its
different portions present many features that may be compared with
those of intrusive bosses and sheets belonging to later geological
periods, there is no trace of any superficial volcanic manifestation.
No tuffs or agglomerates or slaggy lavas have been detected, such
as might serve to indicate the ejection of volcanic materials to
the surface. All the phenomena of the Lewisian gneiss point to the
consolidation of successively protruded portions of eruptive material
at some depth within the crust.
Nevertheless it may yet be possible to show that these deep seated
masses have been injected into rocks of older date and of sedimentary
origin, and that they have communicated with the surface in true
volcanic eruptions. I have already alluded to one limited area where
various rocks exist, distinctly different from the prevalent types in
the Lewisian gneiss. In the area which is traversed by the long valley
of Loch Maree in western Rosshire, there occur clay-slates, fine mica
schists, graphitic schists, and saccharoid limestones. These rocks
remind us of some of the prevalent members of a series of metamorphosed
sediments. The minerals enclosed in the marbles are just such as might
be expected in the metamorphic aureole of a granite boss, piercing
limestone. But the relations of this group of rocks to the ordinary
gneiss of the region are not quite so clear as could be desired, though
they seem to point to these rocks being surrounded by and enclosed
within the gneiss.
The detailed field-work of the officers of the Geological Survey has
made known the remarkable amount of mechanical deformation which the
various rock-masses composing the Lewisian gneiss have undergone.
These rocks have been compressed, crushed, and drawn out, until what
were originally massive crystalline protrusions have been converted
into perfect schists. The dykes of dolerite have been transformed
into hornblende-schists and the granitic pegmatites have been reduced
to a kind of powder which has been rolled out so as to simulate the
flow-structure of a lava. There is evidence that most, if not all, of
this dynamical change was effected long before the deposition of the
Torridonian series, for the latter rests in nearly horizontal sheets,
with a strong unconformability upon the crushed and sheared gneiss.
_Torridon Sandstone._ This group of rocks covers only a limited area
in the north-west of Scotland, but it must once have spread over a
far more extensive region. It reaches a thickness, as I have said, of
8,000 or 10,000 feet, and consists almost wholly of dull, purplish-red
sandstones, often pebbly, and bands of conglomerate. Dark grey shales,
already alluded to as occurring towards the base of the series, are
repeated also in the highest visible portion, and have yielded tracks
of what seem to have been annelids and casts of nail-like bodies which
may have been organic. I have said that the Torridonian deposits
which were classed by Murchison as Cambrian, have been proved by the
discovery of the _Olenellus_ zone in an unconformable position above
them, to be of pre-Cambrian age. Except along the line of disturbance
to which I shall immediately refer, these strata are quite unaltered.
Indeed, in general aspect they look as young as the old red sandstones
with which Hugh Miller identified them. It is at first hard to believe
that such flat undisturbed sandstones are of higher antiquity than
the very oldest Palæozoic strata which are so generally plicated and
cleaved.
The interval of time between the deposition of the Torridon Sandstone
and of the overlying Cambrian formations must have been of enormous
duration, for the unconformability is so violent that the lowest
Cambrian strata, not only transgressively overspread all the
Torridonian horizons, but even lie here and there directly on the
old gneiss, the whole of the intervening thick mass of sandstone
having been there removed by previous denudation. At Durness, in the
north of Sutherland, about 2000 feet of Cambrian (possibly in part
Lower Silurian) strata can be traced, the lower portion consisting
of quartzites, the central and upper parts of various limestones,
sometimes abundantly fossiliferous. Nowhere else in the north of
Scotland can so thick a mass of early Palæozoic rocks be seen.
Elsewhere the limestones have been in large measure replaced by a
complex group of schistose rocks which rest upon the Cambrian strata,
and like them dip, generally at gentle angles, towards the east. It
was the opinion of Murchison, and was commonly admitted by geologists,
that these overlying schists represented a thick group of sediments,
which, originally deposited continuously after the limestones, had
been subsequently altered into their present condition by regional
metamorphism. They were variously named the "Eastern schists," the
"younger gneiss," the "gneissose and quartzose flagstones." Nicol,
who at first shared the general opinion regarding them, afterwards
maintained that they did not belong to a later formation than the
limestones, but were really only the old gneiss, brought up again from
beneath by enormous dislocations and over-thrusts. We now know from the
labors of Professor Lapworth and the officers of the Geological Survey,
that Murchison and Nicol had each seized on an essential part of the
problem, but that both of them had missed the true solution. Murchison
was in error in regarding his younger gneiss as a continuous sequence
of altered sedimentary rocks conformably resting on the Cambrian (or
to use his terminology, Lower-Silurian) formations. But he sagaciously
observed the coincidence of dip and strike between the schists and
sedimentary rocks below them and inferred that this coincidence,
traceable for many leagues, proved that the metamorphism which had
given these schists their structure must have taken place after the
deposition of the Durness limestones. Nicol, on the other hand, with
great insight recognized that there was no continuous sequence above
those limestones, but that masses of the old gneiss had been thrust
over them by gigantic faults. But he failed to see that no mere faults
would account for the coincidence between the structural lines just
referred to in the Cambrian strata, and in the overlying schists, and
that the general tectonic structures and lithological characters of the
eastern schists differed in many respects from those of the Lewisian
gneiss.
The problems in tectonic geology presented by the complicated
structures of the northwest of Scotland have been ably worked out
by the officers of the Geological Survey, to whose report in the
_Quarterly Journal of the Geological Society_ for 1888, I would
refer for full details. It has been shown that, besides stupendous
dislocations and horizontal displacements, the rocks have been cut into
innumerable slices which have been driven over each other from the
eastward, while at the same time there has been such a general shearing
of the whole region that for many hundreds of square miles the original
rock-structures have been entirely effaced, and have been replaced
by new divisional planes, which, when they approach the underlying
Cambrian strata, are roughly parallel with the bedding planes of these
strata.
In this region, therefore, we have striking proofs of a stupendous
post-Cambrian regional metamorphism. But there is still much
uncertainty regarding the geological age of the rocks which have been
affected by it. There can be no doubt that large masses of the old
gneiss, torn up from below, have been thrust bodily westward for many
miles, and are now seen with their dykes and pegmatites resting on the
Durness limestones and quartzites. It is equally certain that in other
districts huge slices of the Torridon sandstones have been similarly
treated. But where all trace of original structure has disappeared, we
have, as yet, no means of definitely determining from what formation
the present eastern schists have been produced. The ordinary gneissose
and quartzose flagstones do not appear to me to be such rocks as could
ever be manufactured by any chemical or mechanical process out of the
average type of Lewisian gneiss. I have long held the belief that
they were originally sediments, but whether they represent altered
Torridon Sandstone, or some clastic formations which may have followed
the Durness limestones, but which have been everywhere and entirely
metamorphosed, remains for future discovery. For my present purpose,
it is sufficient to observe that, in the meantime, as we can not be
sure of the origin of most of the rocks, which, between the West Coast
and the line of the Great Glen, have been subjected to a gigantic
post-Cambrian regional metamorphism, it seems safest to exclude them
from an enumeration of the pre-Cambrian rocks of Britain.
_Dalradian._ East of the line of Great Glen, which cuts the Scottish
Highlands in two, another group of crystalline schistose rocks is
largely developed. It consists mainly of what were undoubtedly
originally sedimentary deposits, though they are now found in the form
of quartzites, phyllites, graphitic schists, mica-schists, marbles,
and various other foliated masses. With them are associated numerous
eruptive rocks, both acid and basic, sometimes still massive and
easily recognizable as intrusive, sometimes more or less distinctly
foliated and passing into different gneisses, hornblende-schists,
chloritic-schists, etc. Though it is not always possible in such a
series of metamorphic rocks to be certain of any real chronological
order of succession, those of the Highland tracts have now been mapped
in detail over so wide an area, that we are probably justified in
believing that a definite sequence can be established among them.
These masses must be many thousand feet thick. Their succession and
association of materials are so unlike those of any of the known older
Palæozoic rocks of Britain, that they can hardly be the metamorphosed
equivalents of any strata which can be recognized in an unaltered
condition in these islands. Some traces of annelid casts have been
found in the quartzites, but otherwise the whole series has remained
entirely barren of organic remains.
What then is the age of this important series? I must confess that in
the meantime I can give no satisfactory answer to this question. I
have proposed, for the sake of distinction and convenient reference,
to call these rocks "Dalradian." Murchison supposed them to be a
continuation of his Durness quartzites, limestones, and "younger
gneiss." His belief may still prove to be in some measure well founded.
But at present we have no means of deciding whether the quartzites and
limestones of the Central Highlands are the more altered equivalents of
the undoubtedly Cambrian strata of the north-west. It is possible that
in the vast mass of metamorphosed rocks constituting the wide stretch
of country from the northern headlands of Aberdeen to the south-western
promontories of Argyllshire, there may be portions of the old Lewisian
gneiss, tracts of highly altered Torridon sandstone, belts of true
counterparts of the Cambrian quartzites and limestones of Durness,
and, what should not be forgotten, considerable portions of some later
sedimentary series which may have followed these limestones, but
which, by the great dislocations already referred to, have disappeared
from the north-west of Scotland. We are gradually learning more of
these rocks, as the detailed mapping of them by the Geological Survey
advances, and when the ground on either side of the Great Glen is
surveyed, it may be possible to speak with more certainty regarding
their true geological relations.
A glance at a geological map of the British Isles will show that the
metamorphic rocks of the south-western Highlands of Scotland are
prolonged into the north of Ireland, where they spread over a region
many hundred square miles in extent. They retain there the same general
character and present the same difficult problems as to their true
stratigraphical relations. Quite recently, however, a new light seems
to have arisen upon these Irish rocks. My colleagues on the Irish
Branch of the Geological Survey have detected several detached areas
of coarse gneisses, which in many respects resemble parts of the
Lewisian gneiss of north-west Scotland. In some cases these areas lie
amidst or close to "Dalradian" rocks, but with that obstinacy, which
so tries the patience of the field-geologist, they have persistently
refused to disclose their true original position with regard to these.
Some fault, thrust-plane, tract of boulder-clay or stretch of bog
is sure to intervene along the very junction-line where the desired
sections might have been looked for. There can be little doubt that
a strong unconformability exists between them. A close examination of
the ridge of old gneiss in Tyrone and Fermanagh showed me that though
the actual basement-beds of this Dalradian series could not be seen
resting on the coarse gneiss, the lithological character, and tectonic
arrangement of this series are only explicable on the supposition of a
complete discordance between it and the gneiss. As these two groups of
rock have never been found in close proximity in Scotland, and as the
determination of the true age of the Dalradian series is a question of
such great stratigraphical importance in the general mapping of the
United Kingdom, I requested Mr. A. McHenry, of the Geological Survey of
Ireland, to continue the tracing of the mutual boundaries of the old
gneiss of the Ox Mountains and the Dalradian series in County Mayo.
He informs me that he has found in that series a conglomerate full
of blocks of the old gneiss, and resting in one locality apparently
unconformably upon it. If this observation is confirmed it will finally
set at rest the relative position of the coarse massive gneiss and
some portion, at least, of the Dalradian series. Of course there is
no absolute proof that the coarse gneisses of Ireland are really the
equivalents of the Lewisian masses which they so closely resemble. But
there is a strong presumption in favor of their identity.
In England and Wales many detached areas of rock have been claimed as
pre-Cambrian, and successive formations have been classified among
them. I have already dealt in part with this question, and without
attempting here to review the voluminous literature of the subject, I
will content myself with stating briefly what seems to me to have been
established on good evidence.
There can not, I think, be now any doubt that small tracts of gneiss,
quite comparable in lithological character to portions of the Lewisian
rocks of the north-west of Scotland, rise to the surface in a few
places in England and Wales. In the heart of Anglesey, for example,
a tract of such rocks presents some striking external or scenic
resemblance to the characteristic types of ground where the oldest
gneiss forms the surface in Scotland and the west of Ireland. In the
Malvern Hills another small knob of somewhat similar material is
obviously far more ancient than the Cambrian rocks of that locality.
There may possibly be still some further exposures of similar rocks in
the south of England, as for instance in southern Cornwall. In Anglesey
a series of schists, quartzites and limestones has been included by
Mr. J. F. Blake with the coarse gneiss above referred to, and a thick
higher group of slates in what he terms the "Monian" system. These
schists, quartzites and limestones present a close resemblance to the
Dalradian series of Scotland and Ireland, and the quartzites, like
those of the Highlands, contain worm-burrows. The coarse gneiss, as
I have said, may be compared in general character with parts of the
Lewisian rocks, so that we seem to have here, as in Ireland, two groups
of schistose rocks, and both of these must be much older than the
unaltered Cambrian strata which lie above them.
Along the eastern borders of Wales, there is an interrupted ridge of
igneous rocks which were originally supposed to have broken through the
older Palæozoic formations, but which now, owing mainly to the labors
of Dr. Callaway and Professor Lapworth, are shown to be older than
the base of the Cambrian system. These rocks consist of spherulitic
and perlitic felsites, with volcanic breccias and tuffs. They are
undoubtedly older than the _Olenellus_ zone. Though the evidence is
not quite satisfactory, they may not impossibly lie at the base of
a vast mass of sedimentary rocks forming the ridge of the Longmynd.
In that case the whole of the Longmynd succession with the volcanic
group at its base must be pre-Cambrian and lie unconformably below the
_Olenellus_ zone. Dr. Callaway has proposed the name "_Uriconian_"
for this volcanic group, while the sedimentary series has been termed
"_Longmyndian_." On the supposition that the unconformability is
established, there would here be a vast mass of stratified and partly
erupted material forming a pre-Cambrian formation. Whether in that case
any portion of this English series is the equivalent of the Torridonian
rocks of Scotland remains to be determined. The northwestern part of
the Longmynd ridge is made of red sandstones and conglomerates, which
certainly resemble the Torridonian rocks of Ross and Sutherland.
At the base of the Cambrian rocks in Wales, Dr. Hicks has described a
marked volcanic series under the name of "Pebidian," which he claims as
pre-Cambrian, alleging that it is separated from the Cambrian system
by an unconformability, and a band of conglomerates. I have carefully
studied the evidence on this ground, and have come to the conclusion
that there is no unconformability at the line in question, but that the
ordinary Cambrian strata graduate downwards into the volcanic group
and can not be disjoined from it. I therefore regard the so-called
"Pebidian" as merely marking the duration of a volcanic period in early
Cambrian time.
It will thus be seen that according to my view the unmistakably
pre-Cambrian rocks of Britain consist of, first and oldest, the
Lewisian gneiss; second, the Torridonian sandstones and conglomerates.
The Uriconian and Longmyndian formations may prove to be in part or
in whole equivalents of the Torridonian. The Dalradian rocks have not
yet had their position determined. They may possibly mark a distinct
pre-Cambrian series, but it seems quite as probable that they are only
a metamorphic complex in which Archæan, Torridonian and Cambrian, or
even Lower Silurian rocks are included.
=Sir Archibald Geikie=,
Director-General of the Geological Survey of Great Britain and Ireland.