IGNEOUS ROCKS
INTRODUCTION[58]
Igneous rocks, those varied masses that have consolidated from a state
of fusion, attracted attention in the eighteenth century through their
active appearance in volcanoes. James Hutton in 1785 showed that the
crystalline granite of the Scottish highlands "had been made to invade
that country in a fluid state." More than a hundred years, however,
elapsed before geologists on the continent of Europe were willing to
connect superficial lavas with the materials exposed by denudation in
consolidated cauldrons of the crust.
It is interesting therefore to note that G. P. Scrope in 1825 treated
of granite, without apology or hesitation, in a work entitled
"Considerations on Volcanoes." So far from separating deep-seated from
superficial products, Scrope wrote of the molten magma in the crust as
"the general subterranean bed of lava." He conceived this fundamental
magma, "the original or mother-rock," to be capable of consolidating
as ordinary granite. Successive meltings and physical modifications of
this granite gave rise, in his view, to all the other igneous rocks.
Scrope laid no stress, however, on chemical variations within the
magma, but urged that the transitions observable between different
types of igneous material established a community of origin.
The connexion between lavas and highly crystalline deep-seated rocks,
so simply accepted by Scrope, was worked out some fifty years later
by J. W. Judd for areas in Hungary and in the Inner Hebrides. The
features displayed in thin sections under the microscope were used by
Judd, in a series of papers, to substantiate his views; but in France
and Germany these features became the source of subtle distinctions
between the igneous rocks of Cainozoic and pre-Cainozoic days. The
lavas, in which some glassy matter could be traced, were said to
be typically post-Cretaceous, and essentially different from those
earlier types in which glass was replaced by finely crystalline matter;
while the coarsely crystalline igneous rocks were uniformly regarded
as pre-Cainozoic. Glassy rocks, such as pitchstone, interbedded
contemporaneously in Permian or Devonian strata, were described as
"vitreous porphyries," while those known to be of post-Cretaceous date
might be styled andesites, trachytes, or rhyolites. Luckily common
sense has recently triumphed in this matter, and the relative scarcity
of glassy types of igneous rocks in early geological formations has
been recognised as due to the readiness with which glass undergoes
secondary crystallisation. The discussion has ended by showing that we
have no evidence of world-wide changes in the types of material erupted
during geological time.
At the present day, attention has been focused on the processes
that go on in subterranean cauldrons, in the hope of explaining the
differences between one type of extruded rock and another. Doctrines
of descent have been elaborated, and one of the most subtle systems
of classification[59] has been based upon characters that the igneous
rock might have possessed, had circumstances not imparted others
to it during the process of consolidation. The principle of this
classification is, however, obviously correct, if we wish to trace back
a rock bearing certain characters at the present day to the molten
source from which it came.
CHARACTERS OF IGNEOUS ROCKS
The characters of igneous rocks vary considerably according as they
have consolidated under atmospheric pressure only, or under that of
superincumbent rocks. We must remember also that submarine lavas have
to sustain a pressure of an extra atmosphere for every thirty feet of
depth, or 400 atmospheres at 2000 fathoms, and that such rocks have
a claim to be regarded as deep-seated. The gases that igneous rocks
contain, probably as essential features of the molten magma, and at a
temperature above their critical points, escape to a large extent near
or at the surface of the earth. The bubbles raised in lava, whereby it
is rendered _scoriaceous_, and the clouds of vapour rising from cooling
lava-flows and from the throat of a volcano in eruption, are sufficient
evidences of this process. The extremely liquid lavas of Kilauea in
Hawaii, which emit very little vapour, are notable as exceptions. In
the case of masses that cool underground, the retention of gases, and
ultimately of liquids, until a very late stage of consolidation retards
crystallisation until temperatures are reached lower than those at
which it starts in surface-flows. As A. Harker points out[60], "the
loss of these substances, by raising the melting-points in the magma,
may be the immediate cause of crystallisation, quite as much as any
actual cooling."
The formation of crystals in lavas is rapid, and the average crystals
are therefore small, and often felted together in a mesh, the
interstices of which are filled by residual glass.
Slowness of cooling is the really important factor that affects the
size of crystals, that is, the coarseness of grain, in igneous rocks.
Pressure may promote crystallisation, by raising the melting-points of
minerals; but, after a certain maximum effect in this direction, it
is quite possible that an increase of pressure may actually lower the
melting-points, and cause one or other mineral to remain in solution
in the magma. It is not clear how pressure can affect the size of any
constituent, except by bringing about conditions under which it can go
on growing, while other constituents remain in solution, or do not grow
so fast.
Such conditions may arise from the aid given by pressure to
the retention of what French geologists have called _agents
minéralisateurs_. Several familiar minerals, for instance albite,
orthoclase, and quartz, require the presence of water for their
formation. Volatile substances, not utilised in the ultimate product,
no doubt similarly assist the formation of many rock-forming minerals.
Occasionally, moreover, as in the development of the micas and certain
of the silicates known as zeolites, some proportion of hydrogen is
retained by minerals thus crystallising from the magma. Micas appear
to require the presence of fluorine for their development. J. P.
Iddings[61], however, lays stress in this case on the chemical activity
of hydrogen at high temperatures.
Igneous rocks, unless cooled with singular rapidity, thus contain
crystals of various kinds. In lavas, these may form the globular
aggregates known as _spherulites_[62], or may accumulate as a compact
ground of minute grains and needles, not quite resolvable with the
microscope. In many rocks of slightly coarser grain, a compact
_lithoidal_ or stony texture is set up, which the microscope resolves
into an aggregate of crystalline rods or granules. Such compact rocks
are often styled _felsitic_. In other types, as in ordinary granite,
the constituent minerals are easily distinguished with the naked eye.
The order in which these constituents have developed is sometimes clear
from the inclusion of one mineral in another. When two substances
are dissolved in one another, there is a certain proportion between
them, varying with the substances, which allows them to crystallise at
the same time, instead of in succession. This _eutectic proportion_,
when attained by two mineral substances in a magma, brings about a
complete interlocking of their crystals, as is seen in the quartz and
alkali-felspar of the rock known as "graphic granite." The order of
crystallisation of minerals from an ordinary non-eutectic magma is
profoundly affected by the proportions in which their constituents are
present in the mass.
The minerals, when they have separated out, are found to be mostly
silicates. A few oxides, such as rutile, magnetite, and ilmenite,
may occur, the two latter being especially common where iron is an
important constituent of the rock. But almost all igneous rocks consist
largely of one or more species of felspar, silica being here combined
with alumina, potash, soda, and lime. Free silica may remain, and
separates as quartz, or rarely as tridymite. Pale mica occurs in many
rocks of deep-seated origin. In contrast with these light-coloured
minerals, iron, magnesium, and part of the calcium, appear in another
series of silicates, usually dark in colour, and this series may
be broadly styled "ferromagnesian." The pyroxenes, of which augite
is the type, the amphiboles, of which hornblende is the type, dark
mica (mostly biotite), and olivine, are the ordinary ferromagnesian
minerals.
Broadly, then, igneous rocks divide themselves by texture into (i)
those which are completely crystalline, and in which the minerals are
distinctly visible; (ii) those which are completely crystalline, but in
which the crystals are so small as to give rise to a compact lithoidal
ground-mass; and (iii) those in which some glass is present. The third
group may appear lithoidal, or in other cases actually glassy, to the
unaided eye.
This mode of division is justified from a natural history point of
view. The first group includes rocks that have consolidated slowly
underground. The second includes rocks cooled more quickly, on the
margins of magma-basins, or as offshoots from them, filling cracks in
the surrounding rocks, and producing wall-like masses known as _dykes_.
The third group appears mostly in dykes and lava-flows.
Where a dyke has intruded among heated rocks and undergoes no sudden
chilling, it may become coarsely crystalline, even though comparatively
small. Some dykes exhibit a chilled margin of glass along their
bounding surfaces, and are none the less completely crystalline at
the centre, where cooling has been slow. No structure is peculiar to
dyke-rocks, nor can a class be established for such rocks on chemical
or mineralogical grounds, even though a few special types of igneous
rock may at present be known only among these minor intrusive bodies.
[Illustration: Fig. 14. Side of a Volcanic Cone. Ash-layer of 1906 on
the west flank of Vesuvius. Cliffs of the exploded crater of Monte
Somma behind.]
The fine-grained layers of _volcanic dust_, commonly spoken of as
_ash_, and the coarser _tuffs_, in which lumps of scoriaceous lava are
clearly visible, bridge the gap between sedimentary and igneous rocks.
The dust, during a great eruption, is distributed by wind over hundreds
of square miles of country. The tuffs, deposited nearer the orifice of
the volcano, vary in coarseness from day to day, and exhibit marked
stratification. Ash-beds and tuffs may be laid out in lakes or in the
sea, and their layers may then include organic remains. Waves may round
their particles on the shore, and may sift them till only a coarse
volcanic sand remains.
After an eruption, the newly deposited ash and tuff usually form
obvious layers on the surface of the country. Landslips on the side
of the volcanic cone may reveal sections of the new coating and of
previously stratified material (Fig. 14). In certain districts,
sedimentary and other rocks torn off from below form a large part of
the fragmental deposits of volcanic action. The characteristic volcanic
cone is itself due to the greater accumulation of tuffs and ashes near
the vent (Fig. 15).
The loose tuffs formed of scoriæ allow water to percolate easily
through them, and a cone of fairly coarse material resists the weather
well. The remarkable freshness of the extinct "cinder-cones" of
Auvergne was thus long ago explained by Lyell. Surfaces of ash, on the
other hand, are easily washed down by rain in the form of dangerous
mud-flows, which spread across the lowlands, and give rise to compact
clays, shrinking as they dry.
[Illustration: Fig. 15. Tuff-Cone with Tuff-Beds at the base. Puy de la
Vache, Puy-de-Dôme, France.]
_Lava-flows_ are masses of molten rock that have welled out from the
vent, without being torn to pieces by the explosion of the gases that
they contained. The rapidity of their flow depends on their chemical
composition, on the amount of gases present, and on the temperature
at which they are extruded. The more highly siliceous lavas, for a
given temperature, are more viscous than those towards the basaltic
end of the series, which contain only about 48 per cent. of silica. A
lava of considerable fluidity will consolidate in somewhat thin sheets
with smooth and ropy surfaces. A less fluid type will become markedly
scoriaceous, where the vapours endeavour to escape from it; the rugged
crust formed on its upper cooling surface will be broken up by the
continued movement of the more liquid mass below, and the blocks thus
formed may become rolled over the advancing front of the flow and
entombed in the portion that has not yet consolidated.
The surface of ordinary lava-flows remains rough for centuries, and
only slowly crumbles down before weathering to form a soil. While
tuff-beds provide light and fertile lands, the lava-streams remain
marked out among them, as sinuous bands of rock, given over to an
irregular growth of woodland. By repeated outflows, lavas tend to fill
up the interspaces between the earlier streams, just as these have
filled up the hollows in the country over which they spread. A uniform
surface thus arises, and _lava-plains_ eventually bury a varied land
of hill and dale. Where a number of small vents have opened, perhaps
along parallel fissures in the earth, the flooding of the country with
igneous rock may lead to an appearance of stratification in masses
extending over hundreds of square miles. Sections in the igneous
series, however, show that the individual flows dove-tail into and
overlap one another, more rapidly than is the case with the lenticular
masses that constitute an ordinary sedimentary series.
After the constituents of the lava have begun to crystallise, and when
the rock may be considered solid, cracks due to contraction are set up.
The upper part of the flow, radiating its heat and parting with its
gases into the air above, solidifies comparatively rapidly, and cracks
arise without much regularity. Now and then, columnar structure, like
that of dried starch, appears on a small scale, the columns starting
from various oblique surfaces of cooling, and lying in consequence in
various directions in the rock.
J. P. Iddings shows that curvature of the columns will result if one
portion of the surface loses heat more rapidly than another. As the
contraction-cracks bounding the columns spread inwards, the layer
reached by them at any time in the lava will be farther in from a part
of the surface where cooling is rapid than it will be from a part where
it is slow. Hence the layer in the lava where contractional stresses
are producing cracks, _i.e._ the layer reached at any time by the
inner ends of the contraction-columns, will be a curved one, and its
curvature will increase as it occupies positions more and more removed
from the surface of the lava-flow. The axes of the contraction-columns,
as they spread, are perpendicular to this layer, and the columns will
thus curve as their development proceeds.
The base of a massive lava-flow, however, cools under much more uniform
conditions, and the columns, stretching upwards from the ground and
produced by slow contraction, give rise to the regular prismatic
structures long ago known as "giants' causeways." The original Giant's
Causeway in the county of Antrim is the lower part of a basaltic flow,
exposed by denudation on the shore. Fingal's Cave in Staffa owes its
tough compact roof to the preservation of that portion of the flow
which cooled downwards from the upper surface. G. P. Scrope[63] long
ago observed this dual structure in columnar lavas.
The columns, or the more irregular joint-blocks that sometimes
represent them, are often subdivided by further contraction into
spheroids, the coats of which peel off, as the rock weathers, like
those of an onion. The curved cross-joints of massive columns, now
convex upwards, now concave, represent the same tendency towards
globular contraction.
A lava-flow is sometimes divided into large rudely spheroidal masses,
which fit into one another, and which show signs of more rapid cooling
on their surfaces. These were particularly observed on the mountains
near Mont Genèvre by Cole and Gregory[64], who compared the forms
to "pillows or soft cushions pressed upon and against one another."
It was suggested that these forms were produced by the seething of
viscid lavas, masses being heaved up and falling over, and the outer
layers having time to cool in a glassy state before they were deformed
by contact with others. This _pillow-structure_ has been widely
recognised, and J. J. H. Teall has remarked how often "pillow-lavas"
are associated with radiolarian cherts. He regarded them, therefore,
as of submarine origin. Sir A. Geikie[65], moreover, stated that the
spheroidal sack-like structure was produced by the flow of such lavas
into water or watery silt. This acute suggestion has now been verified
by Tempest Anderson[66], who has observed in Samoa the chilling of
the lobes of lava, as they are thrust off into the sea and washed
over by the waves. H. Dewey and J. S. Flett[67] have pointed out that
pillow-structure commonly occurs in lavas in which there has been a
conversion of lime soda felspars into albite, a change frequent in a
series of rocks which they call the "spilitic suite." The importation
of soda is attributed to vapours entering soon after the consolidation
of the rock, and it is urged that any excess of sodium silicate
must have escaped into the sea-water in which the pillow-lavas
were produced. Hence radiolaria will flourish in the neighbourhood
(presuming that a decomposition of the silicate can be brought about),
and their remains will in time form flint in the hollows of the lavas.
The paper quoted contains numerous references to previous work, and is
a suggestive example of how petrographic study may go hand in hand with
the appreciation of rocks from a natural history point of view. It is
only characteristic of the subject of petrology that G. Steinmann[68]
has with equal ingenuity explained the relations between radiolaria
and spilitic lavas by reminding us that gravity-determinations show
an excess of basic material under the oceans and of lighter material,
rich in silica, under continental land. Hence, when deep-sea deposits
are crumpled by earth-movements, basic types of rock, graduating even
into serpentine, become associated with radiolarian chert, partly as
extruded lavas, but usually as intrusive sheets injected at the epoch
of mountain-building.
The characters of igneous rocks in _dykes_, that is, of those types
that have consolidated in fissures, resemble in many respects the
characters of lava-flows. Chilling being usually equal on both
surfaces, glassy or compact types of rock occur on both sides, and
the dyke is, as previously observed, more crystalline in the centre.
Columnar structures arise from both surfaces, the dyke also shrinking
parallel to its margins. In the outer layers so formed, the columns
are small, and they increase in diameter nearer the centre. In small
dykes and veins, the columns may run continuously from side to side;
in larger ones, they meet along a central surface, which forms, on
weathering, a plane of weakness in the rock. Dykes may thus become worn
away, decay spreading from the central region, and leaving the more
resisting and more glassy portions clinging to the bounding walls.
Where, however, the surrounding rocks are more easily worn away than
the igneous invader, as very often happens, the dykes stand out on the
surface as great ribs and walls.
The rocks cooled in the deep-seated cauldrons, under what are styled
_plutonic_ conditions, have parted with their gases so slowly that
they do not show scoriaceous structure. They may become very coarsely
crystalline, like many of the Scandinavian granites; minerals,
moreover, may be produced which are unstable or difficult to form
nearer the surface. Crystals developed in plutonic surroundings become
carried forward when the partially consolidated mass is pressed up
to a volcanic orifice, and may undergo resorption on the way. Many,
however, escape, and impart a _porphyritic structure_ to lavas. The
deep-seated rock, from causes that promote the growth of one mineral
and the retention of another in solution, may also become "porphyritic"
_in situ_, smaller crystals, or even a eutectic intergrowth, finally
filling in the ground.
The viscidity of igneous rocks may cause any of the types to show a
_fluidal structure_. Constituents already formed become dragged along
in parallel series as the mass moves forward. Sometimes a group of
spherulites, or a knot of "felsitic" matter caused by the dense growth
of embryo-crystals, is stretched out into a sheet, and on fractured
surfaces a _banded structure_ characterises the mass. These banded
rocks record, in their crumpled and obviously fluidal layers, the
formerly molten condition of the mass. Even completely crystalline
rocks may show parallel arrangement of their minerals, owing to flow
during the last stages of consolidation, or to pressure from the walls
of the cauldron, influencing the positions taken up by crystals that
possess a rod-like or platy form.
[Illustration: Fig. 16. Granite invading Mica-Schist. Clifton, near
Cape Town. Adjacent sections were studied by Charles Darwin (see p.
156).]
The conspicuously banded structures in some crystalline rocks that
are often grouped with the metamorphic gneisses may, however, be best
explained by their composite origin, and the history of the structure
is easily determinable in the field. A common case arises where a
granite magma, perhaps already bearing crystals, is intruded, under
pressure operating from a distance, into a well-bedded series of
sedimentary rocks. The sediments open up like the leaves of a book and
admit the invader along their planes of stratification. Even limestone
may thus become interlaminated with an igneous rock, just as basalt
has been known to separate the annual rings of trees involved in it.
This intimate admixture permits of extensive mineral changes, and the
two types of rock, probably very different in geological age, become
welded together into a _composite gneiss_, both members of which have
influenced one another by contact-metamorphism, often across a wide
stretch of country (Fig. 16).
Intrusive igneous rocks in the field will, however, ordinarily prove
their character by cutting somewhere across the prevalent structure of
the district. When the materials that elsewhere form dykes penetrate
between strata for considerable distances as _intrusive sheets_,
they may yet be traced to some point where they have made use of a
crack across the bedding. The necks or plugs of old volcanic centres
sometimes seem to occupy orifices drilled, or rather shattered, by
explosion right through the overlying obstacles. The approximately
circular necks in South Africa, filled by brecciated masses of
serpentinous rock, are notable examples. The underground cauldrons
themselves, when brought to light by denudation, are represented by
regions of crystalline rock, which may have various relations to their
surroundings. We may trace, in every case, upon their margins the
ramifying veins that first proved to James Hutton that granite was
younger than the rocks among which it lay. But the portion exposed may
be merely the top of a huge body or _batholite_ of igneous matter,
stretching far down into the crust; or it may be part of a localised
knot, which filled up some cavity provided for it by earth-movement,
oozing in step by step as room was made for its advance. In the latter
case, it was originally bounded above by some series of strata which
was arched up as a dome or as an anticline. Or possibly strata have
been moved apart from one another, the upper ones sliding over the
lower ones and at the same time bulging upwards, so as to leave a
cavity of roughly hemispherical form. Such a space, allowing relief
from pressure, will be occupied by igneous rock, which may or may not
have a direct root through the stratum underneath it. The igneous mass
may in such cases be merely an expansion of a large intrusive sheet. It
sends off veins into the roof above, and can only be distinguished from
a batholite by the presence of stratified rock beneath it. Occurrences
of this kind were first described in the Henry Mountains of Utah by G.
K. Gilbert, who gave them the name of "stone-cisterns" or _laccoliths_,
a word now commonly written _laccolites_. It may be questioned if the
expansion of the gases in the intruding igneous rock is sufficient in
itself to form the laccolitic dome. The igneous rock has probably been
pressed into position by the forces that produced the earth-movements.
In many cases, batholites seem to have worked their way upwards
without any relation to earth-movements in the district. The processes
by which they come into place among other rocks are worthy of separate
consideration.
THE INTRUSION OF LARGE BODIES OF IGNEOUS ROCK
Attention has been already called to the composite gneisses formed by
the intrusion of an igneous magma between the leaves, as it were, of
sediments. Such occurrences are often seen on the margins of batholites
or of any kind of igneous dome, and they no doubt represent the picking
off of layer after layer from the walls surrounding the intrusive mass.
If these layers can become absorbed into the igneous rock, the crest
of the dome can advance, and the dome itself can widen, so long as
sufficient heat is supplied to it from below. Space is found for the
intrusive mass at the expense of the marginal rocks; but it is obvious
that the portions absorbed merely add to the bulk of the igneous
material. The composition of the latter must also undergo modification.
Its great size, reaching as it does far down into the crust, in
comparison with the quantity of matter absorbed in the upper regions,
may render such modification very difficult to trace beyond the latest
zone of contact.
Petrologists differ very widely as to the extent to which igneous
masses assume their place in the upper regions of the crust by
processes of "stoping," absorption, and assimilation. The statement,
however, in a recent work that "the assimilation hypothesis" is "still
supported by some French geologists" is calculated to surprise those
who recognise the trend of modern opinion both in America and on the
continent of Europe. Far from the views of A. Michel Lévy, C. Barrois,
and A. Lacroix, surviving as an expression of national perversity,
they have been supported to a remarkable degree by the observations of
Sederholm in Finland, of Lepsius and H. Credner in Saxony, of A. Lawson
and F. D. Adams in North America, and by the careful reasoning of C.
Doelter[69] based largely on his own experimental work. A. Harker[70]
and J. P. Iddings[71] have argued that assimilation is merely a local
phenomenon, of little importance in the theory of igneous intrusion. W.
C. Brögger[72], however, who strongly supports the laccolitic view for
the Christiania district, expresses himself with far more caution, and
leaves the way clear for conclusions as to absorption and mingling of
molten products in the _lower regions_ of the crust.
Doelter lays stress on the influence of high temperature, and
especially of the highly heated gases in the igneous rock, in promoting
corrosion of the cauldron-walls. He attributes greater power of
corrosion to the magmas rich in silica, and agrees with R. A. Daly
that the rapidly moving basic magmas reach the upper layers of the
crust in a condition of comparative purity. Daly[73] may be looked
on as an extremist in this matter; but it is hard for those who have
studied regions where the deep-seated cauldrons have been cut across
by denudation to avoid very large views of igneous absorption. The
contact-zones between the igneous mass and the surrounding rocks are
often seen merely in cross-section on the flanks of a batholite or
laccolite. In the areas of Archæan rocks, on the other hand, where
prolonged denudation has exposed the zones of repeated interaction over
hundreds of square miles on an approximately horizontal surface, one
may form some idea of the processes that are still effective in the
depths.
G. V. Hawes[74], in 1881, recognised the importance of the process
known by the mining term of "stoping," as a means whereby igneous rocks
work their way upward in the crust. Cracks in the overlying roof are
entered by the magma, blocks are wedged off, and these are ultimately
absorbed in the molten mass. In this matter Hawes stands as a pioneer.
As the viscosity of the magma increases during cooling, the blocks
last detached may remain embedded in the marginal zone. The remarkable
purity of this zone, however, in many cases has raised an obvious
difficulty; but it has been pointed out[75] that the modified marginal
and composite rock may continuously sink down into the depths, aided
by any of the causes that promote magmatic differentiation, while a
fairly pure magma, almost of the original composition, is left on the
crest of the advancing dome. R. A. Daly[76] has developed the stoping
theory with considerable boldness. The areas most likely to carry
conviction to those who doubt that igneous masses can be intruded at
the expense of their surroundings are those where banded gneisses have
arisen on a regional scale (see p. 160).
THE RANGE OF COMPOSITION IN IGNEOUS ROCKS
The broad division of igneous rocks into those of light colour and
of low specific gravity on the one hand and those that are dark and
heavy on the other is a very natural one, and Bunsen and Durocher
insisted that two magmas were fundamental in the crust. In one of
these, the "acid" magma, which gives rise to granites and rhyolites,
silica formed about 70 per cent. by weight of the ultimate rocks; in
the other, it formed about 50 per cent., and the products are basic
diorites, gabbros, and basalts[77]. The former group of rocks is rich
in alkalies, the latter, the "basic" group, in calcium, magnesium, and
iron. The mixture of these more extreme types of magma was held to give
rise to what are now called "intermediate" rocks.
Two other views are of course possible. If the composition of the
globe was originally uniform, the two magmas must have arisen by
separation from one of intermediate nature. Hence, in any cauldron in
the crust, in place of one of two magmas, an intermediate magma may
be presumed to exist, and to split up, from various causes, into a
number of parts which are separately erupted at the surface. Charles
Darwin's[78] remarks as to the sinking of crystals in a cooling magma,
and the consequent production of a trachytic and basaltic type in
the same cauldron, led the way to a general acceptance of the theory
of _magmatic differentiation_ in laccolites and batholites. W. C.
Brögger's[79] brilliant explanation of the variation and succession of
types of igneous rock in the Christiania district has had a profound
influence on workers in other fields, and has perhaps directed
attention away from the parallel possibilities of differentiation by
assimilation.
The _assimilation theory_ provides the second possible view above
referred to. A magma may be modified by the rocks into which it
intrudes, so that a "basic" fluid may become charged with silica from
a sandstone, the product crystallising as a granite; while an "acid"
fluid may become so charged with limestone that diorite ultimately
results. A. Harker[80] has discussed both theories clearly, with a
strong leaning to the acceptance of magmatic differentiation in the
cauldron as the only important cause of variation. R. A. Daly, on the
other hand, goes at least as far as Lacroix in France in supporting
the theory of assimilation. For him, the primitive igneous magma is
already basic, and basalts are therefore the prevalent type of igneous
rock. They reach us, moreover, from considerable depths. The acid
rocks are formed by amalgamation of this magma with siliceous material
lying nearer the earth's surface. Igneous rocks exceptionally rich in
alkalies, the so-called "alkaline" series, result from the absorption
of limestone in the magma; denser lime-bearing silicates are thus
formed, which sink by gravitation, leaving a lighter magma above in
which soda has become concentrated. Carbon dioxide liberated from the
limestone also plays a part in carrying up the alkalies that might
otherwise remain in a lower portion[81].
E. H. L. Schwarz[82] extends Daly's views with an almost romantic
fulness. He holds, with Chamberlin, that the primitive globe resulted
from the aggregation of basic meteoritic material. The more siliceous
crust arose from the withdrawal of magnesium and iron into the depths
by long-continued processes of leaching and gravitation. The melting of
this crust produces the acid igneous rocks. Igneous cauldrons originate
in the heat due to faulting, or to crumpling, or even to the impact
of gigantic meteorites. When a molten magma is locally established,
variation occurs in it by assimilation of different types of material
round it.
The balance of judgment as to differentiation and assimilation, which
should be regarded as parallel probabilities rather than as rival
propositions, is admirably held by C. Doelter[83], whose chapters on
this matter can be appreciated by all geologists.
It is of course possible that differentiation of type, from various
causes, has already proceeded so far in the earth's crust as to produce
noteworthy contrasts in the rocks erupted in different areas. The
interior of our globe, on Chamberlin's planetesimal hypothesis, need
not have been uniform in constitution, either at the outset or at any
subsequent time. J. W. Judd[84] has called attention to the existence
of _petrographical provinces_, a conception that has been very fruitful
in results. These provinces have been grouped by Harker[85] in two
branches, characterised respectively by rocks rich in alkalies and by
rocks rich in lime. The former branch appears to be associated with the
movements of faulting and block-structure, rather than of crumpling,
that have produced E. Suess's "Atlantic" type of coast. The rocks rich
in lime, on the other hand, are said to be characteristic of areas
that have been folded like the countries bordering the Pacific. The
names "Atlantic" and "Pacific" have consequently been given to the two
branches, but these terms seem too geographical in their suggestion.
Dewey and Flett[86] have put forward a third type of magma, giving
rise especially to albite as a primary or secondary constituent, and
characterised by the production of pillow-lavas. This type is held to
be associated with areas that have steadily subsided, without much
folding. G. Steinmann[87], however, has connected the spilites and
"ophiolitic" rocks with regions of intense over-folding.
So far, there are many cases where it is difficult to assign a
petrographic province to one or other of these branches, and the system
seems to demand more simplicity within the provinces than nature is
prepared to yield.
Whatever the causes of variation, it is necessary to mark out by names
certain kinds of igneous material, and it is generally accepted that
the types thus set up are best based on chemical composition. At the
same time, the minerals present in the rock, and also its structure,
record certain phases of its history, and deserve an important place in
any system of classification. The natural history of an igneous rock
is concerned with its mode of occurrence, and no isolated specimen can
satisfy the geological investigator. In the field, the porphyritic
crystals, which have an important influence on the total chemical
composition, may be found to be strangers to the magma, and to have
been derived from some mass imperfectly absorbed. The dark flecks and
patches in a granitoid rock, so often ascribed, somewhat mysteriously,
to local "segregation" in the magma, again and again prove to be
metamorphosed and minutely injected fragments of foreign rocks[88].
None the less, a broad classification is possible on chemical grounds,
and the _acid_, _intermediate_, _basic_, and _ultrabasic_ grouping
adopted by Judd has been found of great convenience. Among acid rocks
we have _granite_ as the coarsely crystalline type, with potassium
felspars prevalent and the excess of silica manifest as quartz.
The finer grained and sometimes compact types are the _eurites_,
_quartz-felsites_, or _quartz-porphyries_. When the rock contains more
or less residual glass, we have what are now known as _rhyolites_, of
which ordinary _obsidian_ is the most glassy representative.
The opposite types, those of the basic group, include, at the coarsely
crystalline end, _gabbro_ and _basic diorite_; the finely crystalline
forms are styled _dolerites_, and those with a trace of glass, or at
any rate very fine-grained and compact, are _basalts_. Glassy types
are naturally rare in this group, owing to the unsuitable chemical
composition.
Between granite and gabbro lie various rocks of intermediate
composition, some of them rich in soda rather than in potash.
_Syenite_, _granodiorite_, and the diorites with a prevalence of soda
over lime, are coarsely crystalline types. Compact types of these of
course occur. It will be sufficient, however, here to name the forms
with traces of residual glass, which range from _trachyte_, the type
rich in potash, to _andesite_, which connects them with basalt, in a
series where lime ultimately predominates over soda.
In the ultrabasic group are a number of exceptional types. Olivine
often becomes an important constituent, and the rocks then decompose
into the soft green or reddish masses known as _serpentine_--or, more
properly, _serpentine-rock_.
Igneous rocks, owing to their range of mineral composition and of
structure, combined with their general hardness, lend themselves to
various economic purposes. While the granites, resisting atmospheric
attack admirably in a polished state, provide our handsomest
building-stones, dolerites and fine-grained diorites, which owe their
toughness largely to the interlocked relations of their constituent
minerals, serve as our most satisfactory road-metals.
THE SCENERY OF IGNEOUS ROCKS
Volcanic landscapes, where activity is very recent or still in
progress, present a number of characteristic surface-forms. The cones
that have accumulated round the vents surpass all other hills in
regularity of outline, and the crater in the summit is often relatively
large. Lava-cones may be steep-sided bosses when formed of protrusions
of viscid rocks rich in silica, like the remarkable domes in the north
of Bohemia, or they may present very gentle slopes where fluid basic
lavas have been extruded.
Tuff-cones are liable to be breached on one side, owing to the outflow
of lava which the crater-wall could not sustain, and they then assume
the form of a mountain in which glacial influences have hollowed out a
cirque.
Rain washes down the loose materials from great volcanic cones, and
emphasises the concave curve of the mountain sides, the form that is
so beautiful in Fuji-yama in Japan, and which Hokusai, with pardonable
and affectionate exaggeration, reproduced in a hundred illustrations.
Ultimately, however, grooves appear on the flanks of the cone, in which
permanent streams gather, and the slopes are dissected and worn away.
During this process, the tuffs yield steep and fantastic forms, and
wall-like dykes weather out among them. The dykes are usually the last
features to decay.
Where the vent has been plugged with lava at the close of its activity,
the _neck_ of rock often remains standing above the surrounding
country. The site of cone after cone can be picked out in this way
in the Cainozoic volcanic areas of central Germany. The jutting crag
of trachyte or of basalt has often been seized on as the site of a
feudal castle, under which the dependent agriculturists still gather at
nightfall in their red-roofed town. The group of sheer-sided necks in
the Hegau in southern Württemberg, the Hohentwiel, Hohenkrähen, and the
rest, form a very striking landscape amid undulating Cainozoic lands.
The lava-beds that cover wide areas are naturally of basic composition.
Basalts thus form enormous plains with rugged surfaces, on which at
last a red-brown soil collects. When exposed to denudation from the
edge of the region inwards, they develop a marked terrace-structure,
through which the rivers cut steep and grim ravines. Grass may grow
on the ledges and the tables; but the scarps, controlled by the
well-marked vertical jointing of the lavas, remain sharp and prominent,
and the rock falls away from these walls in whole columns at a time.
This structure is characteristically seen in northern Mull and the
adjacent smaller isles, and is still more impressive from the centre
to the north of Skye, where the rain swept terraces covered by grass
and bog and scanty oatfields, and the black steps of rock between them,
present a scene of strange monotony and desolation.
In regions less exposed to stormy weather, the lava-plateaus may
provide good soils. For instance, after the great seaward scarp of the
basalts has been crossed in the counties of Antrim and of Londonderry,
the lava-fields, dropped by faults towards Lough Neagh, are seen to
be occupied by prosperous farms. In arid countries, however, the
savage surface of the flows merely becomes modified by red dust and
scoriaceous gravel, worn by wind and changes of temperature from the
upstanding portions of the land.
Where a stratified country has been freely invaded by sheets of lava
along its planes of bedding, the stratification is emphasised in any
part exposed to weathering. The resisting igneous rock stands out in
scarps along the hills, and marks out any folds that have been formed
since the epoch of its intrusion.
When the beds remain fairly level, and are also uplifted, flat-topped
hills are formed by the intrusive sheets, like those that may be carved
out of a country flooded over by lava-streams. The crystalline rock,
very probably a dolerite, protects what lies below it. The kopjes north
of the Great Karroo in the centre of the Cape of Good Hope are thus
level on the crest and bounded by a steep wall or _krans_ of rock.
The edges of similar "sills" of igneous rock have controlled much of
the scenery between the Highland border of Scotland and the Tyne. A
fine example is the indented scarp of the Great Whin Sill, a sheet of
dolerite intruded among the Carboniferous strata of Northumberland.
This mass forms a platform for Bamburgh Castle against the wild North
Sea, and is traceable south-westward across the country towards
Carlisle. North of Hexham, its escarpment is occupied by Hadrian's
wall, and the town of Borcovicus was planted on the edge, overlooking
all Northumbria.
The farmers of North Britain and Ireland have long known upstanding
igneous dykes as unprofitable "whinstones." The regularity of direction
among dykes over very wide areas points to their intrusion along cracks
produced by stretching of the crust. Radial grouping of dykes, such as
one finds near volcanic necks, or, on a gigantic scale, round Tycho on
the moon, may be due to explosive action; but the majority of dykes
seem to have followed upon earth-movement. In the north of Ireland,
from the coast of Down to that of Donegal, a series of compact rocks of
Devonian age occurs in dykes lying almost invariably north and south.
The post-Cretaceous dykes of the same region have a still more uniform
trend, from north-west to south-east. Such series of dykes modify the
scenery of coasts by forming promontories and serviceable piers for
boats.
The offshoots near the surface of a great intrusive mass are far less
regular. We are here close to the zone of attack, the "shatter-zone,"
and the structures or regular fracture-planes of the overlying rock
only partially control the position taken up by the intrusive magma.
Irregular knots and bosses appear in place of far-spreading sheets, and
a network of crossing veins occurs, instead of a system of coordinated
dykes. The resulting country is hummocky and broken, and, where the
cauldron itself has become exposed, striking contrasts of surface are
seen as we pass from the igneous core to the older and frequently
stratified rocks upon its flanks.
Some large bodies of intrusive rock have, however, been formed
sheet by sheet, and a bedded sill-like structure is then revealed
in them on weathering. Sir A. Geikie[89] calls attention to this
in his description of the heart of the black gabbro mass in Skye.
But, as a rule, the continuity of structure in batholites, and their
characteristic joint-planes set at angles to one another, cause them to
appear as massive blocks in the landscape, untraversed by any regular
lines.
Granite, with its broad tabular jointing, which is often developed
parallel to a surface of cooling, forms rounded slopes and domes
after long-continued weathering. When reared high into the zone
of frost-action, it develops spires and pinnacles, as in the huge
"aiguilles" of Mont Blanc. But, as decay goes on, the uniform descent
of boulders and sand forms spreading taluses, banked against the
lower slopes, while the curving joints, not too closely set, promote
a smoothness on the higher lands. These joints, moreover, divide the
rock into boulders almost ready-made. Tabular structure sometimes
predominates; but even in this case the exposed ends of the layers soon
become rounded, as the felspar crystals pass into a powdery state.
Commonly, a rough spheroidal structure prevails, as may be traced in
many of the Dartmoor "tors," and the blocks that slip away through
widening of the joints become more and more rounded as their surfaces
crumble on the talus (Fig. 17).
[Illustration: Fig. 17. Weathering Granite. Lundy Island.]
In tropical lands, granite exfoliates under the alternations of clear
hot days and clear cold nights, and the joint-structure allows of the
formation of great round-backed surfaces, on which spheroidal boulders
appear poised. These boulders are the relics of an overlying layer
of granite, most of which has slipped away to the hill-foot. Their
surfaces crumble, owing to the unequal expansion of the constituent
minerals. When the rainy season sets in, the decomposed crust is washed
away; during the dry season it falls off in flakes and powder. In
this way the magnificent series of monoliths that surround the grave
of Cecil Rhodes in the Matopo Hills have become separated out from a
continuous sheet of granite. They stand now like glacial boulders on
a surface almost as smooth as that of a _roche moutonnée_ (Fig. 18).
The landscape for miles around is fantastic with huge fallen masses,
and with high-perched blocks that seem about to fall. Similar scenery
is well known in central India, and exfoliation controls the form
of mountain-domes in California and Brazil. J. C. Branner[90] lays
most stress on temperature-changes in the surface-zone, and little
on original spheroidal jointing, in promoting the exfoliation of the
rounded boulders.
[Illustration: Fig. 18. Granite weathering under tropical conditions.
Rhodes's Grave, Matopo Hills, S. Rhodesia. The blocks like boulders are
residues of a sheet of granite that once overlay the hill.]
The basic rocks present far more rugged outlines. When a cauldron
occupied by basic diorite or by gabbro comes under denuding action, the
numerous crossing joints oppose the formation of domes or tables. The
weather widens one groove here, another there; the rock breaks away
in angular fragments rather than as a powder over a broad surface,
and serrated edges and jagged pinnacles arise along the crests. The
diorites among our old metamorphic rocks in Scotland or in Ireland can
be recognised on the sky-line at considerable distances. Sir A. Geikie,
in his "Scenery of Scotland," has made the contrast between granite and
gabbro in the centre of the Isle of Skye familiar to all geologists.
Here the two types of rock were erupted at no long interval, and
they have been exposed to denudation under the same conditions. J.
Macculloch dwelt in 1819[91] on the relative resistance of the gabbro
and the rapid disintegration of the granite hills, quaintly remarking
of the latter that "the loose stones, by their constant descent from
the summits, obscure the rocky surface, covering the sides with long
torrents of red rubbish even more unpleasing to the sight than their
conoidal forms." Macculloch noted that the loose blocks in the gabbro
region lay much as they had fallen, without the production of a sand.
In most mountain-chains produced by folding, igneous matter has been
forced up as an accompaniment of the earth-movements. The local knots
and laccolites, or the great cores admitted along certain anticlines,
stand out on weathering among schistose or stratified hills. Their
surfaces are marked by accidents, and each peak as it comes into
view offers something of a new surprise. The wall of Mont Blanc from
the angle near Entrèves, and the huge crag of the Matterhorn above
the valley of the Visp, have illustrated to every traveller the
dominance of igneous masses in the landscape. In our own islands, the
granites of Ben Cruachan and Cairn Gorm have resisted long ages of
denudation; an intrusive sheet of finer grain forms the long sheer
wall of Cader Idris; while obsidian lava-flows, now grey and dull and
crystalline, have furnished on Snowdon the finest scenery of Wales. The
fortress-town of Edinburgh has arisen on the relics of a dead volcano;
and the high moor of Leinster, so long the peril of the English,
records an igneous cauldron that has been exposed to denudation from
the opening of Devonian times.