IGNEOUS ROCKS, METAMORPHISM, SHRINKAGE AND DISTURBANCE OF THE EARTH'S CRUST.
I HAVE already explained that all rocks are divided into two great classes, those of aqueous and
those of igneous origin; and I have shown how aqueous rocks may generally be known by their
stratification and by the circumstance that a great many of them contain relics of marine and
freshwater life, in the shape of fossil shells, fish-bones, and other kinds of organic remains.
The materials also of which these beds are composed generally show signs of having been in
water, being rounded by the action of the waves of the sea, or by the running waters of rivers.
The other kinds of rocks, termed igneous, occasionally are associated in different localities with
the formations named in the foregoing table. For instance, there are no volcanic rocks in Wales
associated with the Carboniferous and Old Red Sandstone strata, while there are in Scotland, and
true contemporaneous volcanic rocks are intercalated with the Lower Silurian rocks of Wales
and Cumberland, while there are none associated with the equivalent strata in Scotland. Some of
these contemporaneous igneous rocks consist of beds of volcanic ashes, others of old lavas,
others of masses of matter which were intruded among the strata from below. Rocks that have
been melted are known
[Igneous Rocks. 39]
to be igneous by their crystalline, slaggy, scoriaceous, vesicular, or columnar structures, and
also by the effects
1. Dyke with veins.
2. Overflow of basaltic lava.
3. Altered strata at junction.
4. Unaltered sandstone and shale
they have produced on the strata with which they are associated. Shales, sandstones, &c.,
are often hardened, bleached, and even vitrified at the points of junction with greenstone,
basaltic, and felspathic dykes, or old lava beds (fig. 9), and the same kind of
alteration takes place on a
greater scale when large masses of igneous rocks have been intruded among the strata.
Then by comparing volcanic rocks of old date with those of modern origin, we are able to decide
with perfect truth, that rocks which were melted long before the human race appeared upon the
world are yet of truly igneous origin.
Changes of a more general character are especially marked in cases where granite, syenite,
felspar and other porphyries and their allies, are associated with stratified deposits. Their
igneous affinities are known by their crystalline structure, their modes of occurrence, and the
effects they produce on the strata. Granite is composed of crystals of quartz, felspar, and mica;
and syenite, according to old nomenclatures, of quartz, felspar, and hornblende. They often send
veins or dykes into stratified rocks with which they are in contact, as in figs. 10 and 11, and
frequently all along the line of junction, and often at great distances from it, alterations of the
strata of an extreme character (metamorphism) are common. One marked distinction between
granitic and volcanic and ordinary trap rocks is, that though injected veins of granite are
rocks never rose to the surface in a melted state, and overflowed like lava streams. This and
A, vein of granite; B, gneissic contorted mica-schist. The ramifying white spaces are white quartz. Milldam Goatfell, Brodick,
largely crystalline structure, together with peculiarities of crystallisation showing the
presence of moisture, and
1. Granitic mass with injected veins among gneissic rocks.
2. Gneiss, metamorphosed strata.
also the transformations effected on the adjoining strata, prove the granitic rocks to have cooled
and consolidated deep beneath the surface.
A third division, or sub-class, is known as metamorphic rocks. All strata as they assume a solid form become to a certain extent altered; for
originally they were loose sediments of mud, sand, gravel, carbonate of lime, or mixtures of
these. When these were accumulated, bed upon bed, till thousands of feet were piled one upon
[Metamorphic Rocks. 41]
the other, then, by intense and long-continued pressure, heat, and chemical changes that took
place in consequence of infiltrations among the strata themselves, by degrees they became
changed into hard masses, consisting of shale, sandstone, conglomerate, or limestone, as the case
may be. But these have not always remained in the condition in which they were originally
consolidated, for it has often happened that disturbances of a powerful kind took place, and
strata originally flat have been bent into every possible curve.
For long it was the fashion to attribute most of the disturbances that the outer part of the earth
has undergone to the intrusion of igneous rocks. The inclined positions of beds, the contortions
of stratified formations in mountain chains, and even the existence of important faults—in fact,
disturbance of strata generally—were apt to be referred to direct igneous action operating from
below. Granite and its allies, from the time of Hutton, were always, without exception, included
in the ordinary list of igneous rocks, and some writers of deserved reputation still do so. In
connection with this subject, gneiss, and other kinds of metamorphic rocks were, and by some are still,
supposed to be exclusively the effect of the direct intrusion of granite among previously
As a general rule highly metamorphosed rocks occur in regions where the strata have been
greatly disturbed. Such rocks, when the metamorphism is extreme, consist of gneiss, which
may be micaceous, hornblendic, or chloritic; and of mica-schist, chlorite-slate, talc-slate,
hornblende-rock, crystalline limestone, quartz-rock, and a number of others, which it is not
necessary for my present purpose to name. In Scotland, Ireland, Norway, Canada, &c.,
[42 Granite and]
sandstones, and sandstones, as they approach granites, lose their (sometimes fossiliferous)
characters, and become changed into crystalline limestones, serpentine, &c., and quartz rock. In
other cases gradual changes of a different kind are observed in slaty and schistose rocks as they
approach granites. Clay-slates are simply clays consolidated by pressure, often affected by
cleavage, and sometimes chemically altered. Approaching granites ordinary slates often assume
a foliated structure by the development of distinct mineral layers of quartz, felspar, and mica.
This is gneiss. Analyse some kinds of mica-slate, gneiss, and common sandy clay, and their
average composition will not differ more than three clays, three pieces of gneiss, and three bits
of granite often do from each other.
Granite is sometimes merely gneiss still further metamorphosed by heat in the presence of
moisture; and, though this is not the popular notion, I have long held it, and some other
geologists share this opinion. When slate is changed to gneiss, there is no development of
materials which were previously absent, but simply a re-arrangement of its constituents,
according to their chemical affinities, in rudely crystalline layers, which seem in gneiss to
have found facilities for their development in pre-existing planes, whether of bedding or of
cleavage; or, in other words, if the rocks be uncleaved when metamorphism occurs, the foliated
planes show a tendency to coincide with those of bedding; but if intense cleavage has preceded,
the foliation will generally tend to follow the planes of cleavage. Furthermore, in gneissic
rocks, garnets, schorl, staurolite and staurotide, hornblende, and other minerals are frequent
in some localities, especially near and in contact with granite. All the chief materials of these
such as occur in the unaltered rock, and the chemical action (brought into activity by heat and
moisture) which induced their development, may perhaps in some cases have been assisted by
sublimations proceeding from melted matter below. The intensity in many countries of these
metamorphisms, extending over many thousands of square miles (as in Scotland, Norway and
Sweden, and Canada), and through rocks thousands of feet in thickness, proves that it was the
result of a long-continued process, taking place probably in all cases at considerable depths.
The whole has then been upheaved and disturbed, often many times, and after denudation the
gneissic and. the more thoroughly metamorphosed and sometimes intrusive granitic rocks were
at length exposed at the surface.
Some of the metamorphic rocks, which I have to explain, have been highly disturbed, and in the
north occupy about one-half of Scotland. Most of this area includes, and lies north-west of, the
Grampian mountains; and I must endeavour to explain by what processes metamorphism of
rocks has taken place, not in detail, but simply in such a manner as to give a general idea of the
I have already said that typical gneiss consists of irregular lamine of mica, quartz, and felspar,
and it frequently happens that they are bent, or rather minutely folded, in a great number of
convolutions, so small, that in a few yards of gneiss they may sometimes be counted by the
hundred. All these metamorphic rocks and granite, were by the old geologists called Primary or
Primitive strata, and were considered to have been formed in the earliest stages of the world's
history, because in those countries that were first geologically described, they were supposed to
[44 Gneiss, Old Theory.]
at the base of all the ordinary strata. From the peculiarity of the minute contortions in the
gneissic rocks, a theory now known to be erroneous was developed, which was this:
It is frequently found that granite and granitic rocks are intimately associated with gneiss. Thus
we often find masses and veins of granite, with gneiss upon their flanks bent in a number of
small wavy folds or contortions. Granite is a crystalline rock, composed of felspar, quartz, and
mica, and the old theory (so far true) was that the world at one time was in a state of perfect
igneous fusion; but by and by, when it began to cool, the materials arranged themselves as
distinct minerals, according to their different chemical affinities, and consolidated as granite.
The great globe was thus composed entirely of granite at the surface ; and by and by, as cooling
still progressed, and water, by condensation, attempted to settle on the surface which still
remained intensely heated, the water could not lie upon it, for it was constantly being
evaporated into the atmosphere; but when the cooling became more decided, and consolidation
had fairly been established, then water was able to settle on the surface of the heated granite.
But as yet it could not settle quietly like the present sea: for by reason of strong radiating heat,
all the sea was supposed to be kept in a boiling state, playing upon the granite hills that rose
above its surface. The detritus thus worn from the granite by the waves of this primitive sea
was spread over its bottom; and because the sea was boiling, the sediment did not settle down in
the form of regular layers, but became twisted and contorted in the manner common in gneiss.
All gneiss, therefore, was conceived to be the original primitive stratified rock of the world.
[contortion and Metamorphism. 45]
Subsequent research has shown that this theory will not hold; for this, among other reasons,
that we now know gneissic rocks of almost all ages in the geological scale. Thus in Scotland the
gneissic rocks are of Laurentian and Silurian age; in Devon and Cornwall we have gneiss both of
so-called Devonian and Carboniferous ages. In the Andes there are gneissic rocks of the age of
the Chalk, and in the Alps of the New Red, Liassic, Oolitic, and Cretaceous series; and in 1862 I
saw in the Alps an imperfect gneiss of Eocene date pierced by granite veins, these strata being of
the age of some of the soft and often almost horizontal strata of the London and Hampshire basins.
It is therefore now perfectly well known to geologists that the term Primitive, as applied to
gneiss, is no longer tenable;
and the old theory has been abandoned.
I have stated that regions occupied by metamorphic rocks are apt to be much contorted. There
seems, in fact, to be an intimate connection between excessive disturbance of strata and
metamorphism. But by what means were masses of strata many thousands of feet thick bent and
contorted, and often raised high into the air, so as to produce existing scenic results by
affording matter for air and water to work upon? Not by igneous pressure from below raising
the rocks, for that would stretch instead of crumpling strata, in the manner in which we find
them in the Alps, Norway and the Highlands, or in less degree in Wales and Cumberland; but
rather because of the radiation from the earth of heat into space, gradually producing a
shrinkage of the earth's crust, which, here and there giving way, became crumpled along lines
more or less irregular, producing partial upheavals, even though the absolute bulk of the globe
was diminishing by cooling
[46 Shrinkage and]
(figs. 3, 12, and 57). This, according to the theory long ago proposed by Elie de Beaumont, and
adopted by De la Beche in his 'Researches in Theoretical Geology,' is the origin of mountain
chains. After water took its place on the earth, by such processes land was again and again raised
within the influence of atmospheric disintegration, and rain, rivers, and the sea, acting on it,
were enabled to distribute the materials of sedimentary strata. Such disturbances of strata have
been going on through all known geological time, and I firmly believe are still in progress.
Such shrinkage and crumpling, where it has been most intense and on the greatest scale, is often
(where I know it) accompanied by the appearance of gneissic or other metamorphic rocks, and
often of granite or its allies.
The oldest rock in the British Islands is gneiss, but that originally was doubtless a common
stratified formation of some kind or other. In fact, as far as the history told by the rocks
themselves informs us, we cannot get at their beginning, for all strata have been made from the
waste of rocks that existed before; and therefore the oldest stratified rocks, whether
metamorphosed or not, have a derivative origin.
I must now briefly endeavour to give an idea of the theory of metamorphism. The simplest kind
is of that nature mentioned in Chapter I. namely, when melted matter has been forced through or
overflows a stratified rock, and remaining for a time in a melted state, an alteration of the
stratified rock in immediate contact with it takes place. Thus sandstone may, by that process,
become converted into quartz-rock, which is no longer hewable, like ordinary sandstone, but
breaks with a hard and splintery fracture. Here then rocks
have been changed in character for a short distance from the agent that has been employed in
effecting that minor kind of metamorphism (figs. 4 and 9).
On a much larger scale, the phenomena we meet with in a truly metamorphic
region are as follows. In the midst of a tract of mica-schist, gneiss, or
other altered rocks, a boss of granite (or one of its allies) rises, like
those for instance of Dartmoor and Cornwall or of the north end of the Island
of Arran. At a distance from the granite the beds may consist, perhaps, of
unaltered shale, or of slate, sandstone, and limestone. As we approach the
granite, the limestones become crystalline, and often lose all traces of
their fossils; the sandstones harden and pass into quartz-rocks, and the
shales or slates, or sandy beds and shales, lose their ordinary bedded texture,
and pass by degrees into mica-schist, or perhaps gneiss, in which we find
rudely alternating laminæ of quartz, felspar, and mica, often arranged
in gnarled or wavy lines (foliation, figs. 10 and 11). As we approach the
granite still more closely, we find possibly that, in addition to the layers
of mica, quartz, and felspar, distinct crystals, such as garnets, staurolites,
schorl, &c, are developed near the points of contact, both in the gneissic
rock and in the granite itself.
It is not necessary for my argument that I should describe these minerals. It is sufficient at
present to state the fact that such minerals are developed under these circumstances, and this is
due to the influence of metamorphism.
Furthermore in some cases, as in the Lauretian
rocks of Canada, great thicknesses of interstratified
gneiss are so crystalline that, when a hand specimen or even a small part of the country is
[48 Analyses of Rocks.]
might seem to be truly granitic; but when the detailed geology of the country has been worked
out, they are found to follow all the great anticlinal and synclinal folds of metamorphosed strata
that have also in a minor way been intensely contorted. The same is the case in parts of the Alps.
I have already stated that if we chemically analyse a series of specimens of clays, shales, and
slates, often more or less sandy, together with various gneissic rocks and granites, it is
remarkable how closely the quantities of their ultimate constituents, in many cases, approach
to each other. They are never identical, while yet the resemblance is close, as close indeed as it
may be in two specimens of the same kind of sandy shale or slate. In all of them silica would
form by far the largest proportion, say from 60 to 70 per cent.; alumina would come next, and
then other substances, such as lime, soda, potash, iron, &c., would be found in smaller varying
proportions; and what I now wish to express is, that the distinct minerals developed in the
gneiss, such as felspar, mica, garnets, &c., were not new substances introduced into the rock,
by contact with granite, or by any other process, but were all developed under the influence of
metamorphism from materials that previously existed in the strata before their metamorphism
began, aided by hydrothermal action due to the presence of heated alkaline waters deep beneath
the surface of the earth. Through some process, in which heat played a large part, the rock
having been softened, and water—present in most rocks underground—having been diffused
throughout the mass and heated, chemical action was set up, and the substances that composed
the shale or slate, often mingled with silicious sandy material, were enabled more or less to re-arrange
[Mountain Chains. 49]
themselves according to their chemical affinities, and distinct mineral materials were
developed in layers from elements that were in the original rock.
I have stated that to produce this kind of metamorphism, heat aided by water is necessary, so as
to allow of internal movements in the rocks by the softening of their materials, without which I
do not see how complete re-arrangement of matter accompanied by crystallisation could take
place; and though it has always been easy to form theories on the subject, yet so little is known
with precision about the interior of the earth beyond a few thousand feet in depth, that how to
obtain the required heat is a difficulty.
From astronomical considerations it is believed by many persons that the earth has been
condensed from a nebulous fluid, and passing into an intensely heated melted condition, by
radiation into space at length cooled so far, that consolidation commenced at the surface, and by
degrees that surface has gradually been thickening and overlies a melted nucleus within.
As the earth cooled and consequently gradually shrunk in size, the hardened crust, in its efforts
to accommodate itself to the diminishing bulk of the cooling mass within, became in places
crumpled again and again. Hence the upheaval of mountain chains and disturbances of different
dates, which have affected strata of almost all geological ages.1
Reasoning on these disturbances, we know that strata which were originally deposited
horizontally have often
1 This theory is not universally received, and has been variously developed by different authors,
but it would be quite beyond my present purpose to discuss the subject in detail, and, as far as I
know, the hypothesis proposed by Elie de Beaumont seems best to explain the phenomena
exhibited by the outside of the earth.
[50 Internal Heat of the Earth]
descended thousands of feet
towards the centre of the earth, by gradual sinking of the seabottom, and
the simultaneous piling up of newer strata upon them. The layer that is formed
today beneath the water forms the actual sea-bottom; but neither the land
nor the seabottom are steady. The land is in places slowly descending beneath
the sea, and sea-bottoms are themselves descending also. It has frequently
happened, therefore, that for a long period a steady descent over a given
area has taken place, and simultaneously with this many thousands of feet
of strata have by degrees accumulated bed upon bed, as for example in the
Pacific Ocean in the region of modern atolls and barrier coral reefs.
As we descend into the earth the temperature rises, whence, in the main, the theory of central
heat has been derived. In our latitude heat increases about 1° for every sixty feet, and the
temperature therefore, at so great a depth as 30,000 feet, to which it could be shown some
strata have sunk, may at present be about 500°. Furthermore, strata that were deposited
horizontally have been frequently disturbed and thrown into rapid contortions, or into great
sweeping curves ; and by this means especially, strata which once were at the surface have
often been thrown twenty, thirty, or forty thousand feet downwards, and therefore more within
the influence of internal heat, as, for instance, in the bed marked * fig. 12, which may be
supposed to represent a large tract of country. I do not wish it to be understood that the globe is
entirely filled with melted matter-that is a question still in doubt; but were this book specially
devoted to general questions of theoretical geology, I think I could prove, that the heat in the
interior of the globe in places sometimes
[and Metamorphism. 51]
apparently capriciously eats its way towards the surface by the hydrothermal fusion or alteration of
parts of the earth's crust, in a manner not immediately connected with the more superficial
phenomena of volcanic action
—and for this, among other reasons, it may happen that strata which are contorted, have in
places been brought within the direct and powerful influence of great internal heat. Under some
such circumstances, we can easily understand how stratified rocks may have been so highly
heated that they were actually softened; and most rocks being moist (because water that falls
upon the surface often percolates to unknown depths), chemical actions were set going,
resulting in a rearrangement of the substances which composed the sedimentary rock. Thus
certain strata, essentially composed of silica and silicates of alumina, and alkalies such as soda
and potash, may have become changed into crystalline gneiss.
This theory of re-arrangement leads me to another question—connected with, but not quite
essential to my argument, as far as relates to physical geography—viz., What is the origin of
granite, which in most manuals is only classed as an igneous rock? For my part, with some
other geologists, I believe that in one sense it is an igneous rock—that is to say, much of it has
often been completely fused. But in another sense
[52 Origin of Granite.]
it is often a metamorphic rock,
because it is sometimes impossible to draw any definite line between gneiss
and granite, for they pass into each other by insensible gradations. About
halfway up the Matterhorn in the Alps, among the largely-contorted beds,
a thick stratum occurs, one end of which is true gneiss, on the western side
of the mountain, which striking towards the eastern cliff, gradually gets
more and more crystalline till at length it passes into true granite. On
the largest scale, both in Canada and in the Alps, I have frequently seen
varieties of gneissic rocks regularly interbedded with less altered strata,
the gneiss being so crystalline, that in a hand specimen it is impossible
to distinguish it from some granitic rocks, and even on a large scale the
uneducated eye will constantly mistake them for granites. Another very important
circumstance is that granite and its allies frequently occupy the spaces
that ought to be filled with gneiss or other rocks, were it not that they
have been entirely fused and changed into granite. I therefore believe that
many of the granite rocks I have seen, are simply the result of the extreme
of metamorphism brought about by great heat with presence of water.
One reason why it has been inferred that granite is not a common igneous rock is that,
enveloping the crystals of felspar and mica, there is generally a quantity of free silica, not
always crystallised in definite forms like the two other materials. Silica being far less easily
fusible than felspar, it seems clear that had all the substances that form granite been merely
fused like common lavas, the silica ought on partial cooling to have crystallised first, whereas
the felspar and mica have crystallised first, and the silica not used in the formation of these
minerals wraps them round often in
[Fluid Cavities in Quartz. 53]
an amorphous form. Therefore
it is said that it was probably held in partial solution in hot water, even
after crystallisation by segregation of the other minerals had begun. This
theory, now held by several distinguished physical and chemical geologists,
seems to me to be sound, especially as it agrees exceedingly well with the
metamorphic theory as applied to gneiss—granite being sometimes simply the
result of the extreme of metamorphism. In other words, when the metamorphism
has been so great that all traces of the semi-crystalline laminated structure
has disappeared, a more perfect crystallisation has taken place, and the
result is a granitic mass without any minor lamination in it. Even then,
however, certain planes often remain, strongly suggestive of original stratification,
and even of planes of oblique stratification or false-bedding.
These general results are not founded on mere conjectures. In a memoir by Mr. H. C. Sorby, 'On
the Microscopical Structure of Crystals, indicating the Origin of Minerals and Rocks,' among
other important points, he describes numerous small cavities in the quartz of granites, which
are partly filled with water 'holding in solution the chlorides of potassium and sodium, the
sulphates of potash, soda, and lime, sometimes one and sometimes the other salt predominating.'
These 'fluid cavities' sometimes make up about five per cent. of the volume of the quartz, and he
concludes that 'the fluid was not an accidental ingredient due to the percolation of water to a
fused mass naturally containing none, but a genuine constituent of the rock when melted.'
Reasoning on the underground temperatures necessary to expand the liquid so as to fill the
cavities, by an elaborate process of argument he arrives at the approximate result, that 'the
pressures under which granites were most probably
formed' indicate depths from the surface varying from 15,100 to 65,500 feet. From certain
passages it is evident that Mr. Sorby considers that gneiss and granite were formed
approximately under similar circumstances. I quote this thoroughly philosophical memoir, that
the reader may be less startled with the statement, that gneiss and some granites were formed
by the metamorphosis of strata at depths counted by many thousands of feet, and also to give
strength to the assertion, that under such circumstances water was present.1
If the above views be correct, though many granites having been completely fused have been
injected among strata, and are thus to be classed as intrusive rocks, yet in the main, so far from
the intrusion of granite having produced many mountains by mere upheaval, both gneiss and
granite would rather seem to be often the results of the forces that formed certain mountain
chains. Possibly this result was connected with the contraction of the earth's crust and the heat
produced by the intense lateral pressure that, with much movement of parts, produced the
contortion of vast masses of strata, parts of which, now exposed by denudation, were then deep
underground, and already acted on by the internal heat of the earth in a degree proportionate to
1 See 'Journal of the Geological Society,' vol. xiv., 1858. Sorby.
2 See Report, Brit. Assoc.
1866, p. 47: 'Address to the Geological Section,' Ramsay. Also an elaborate memoir by Mr.
Robert Mallet, 'On Volcanic Energy, &c.,1 Trans. Royal Soc., vol. clxiii. p. 147.