The great eruptions of _ancient granite_ are supposed to have occurred
during the primary epoch, and chiefly in the carboniferous period. They
present themselves sometimes in considerable masses, for the earth’s
crust being still thin and permeable, it was prepared as it were for
absorbing the granite masses. In consequence of its weak cohesion, the
primitive crust of the globe would be rent and penetrated in all
directions, as represented in the following section of Cape Wrath, in
Sutherlandshire, in which the veins of granite ramify in a very
irregular manner across the gneiss and hornblende-schist, there
associated with it. (Fig. 3.)
[Illustration: Fig. 3.--Veins of granite traversing the gneiss of Cape
Wrath.]
Granite, when it is sound, furnishes a fine building-stone, but we must
not suppose that it deserves that character of extreme hardness with
which the poets have gratuitously gifted it. Its granular texture
renders it unfit for road-stone, where it gets crushed too quickly to
dust. With his hammer the geologist easily shapes his specimens; and in
the Russian War, at the bombardment of Bomarsund, the shot from our
ships demonstrated that ramparts of granite could be as easily
demolished as those constructed of limestone.
The component minerals of granite are felspar, quartz, and mica, in
varying proportions; felspar being generally the predominant ingredient,
and quartz more plentiful than mica--the whole being united into a
confusedly granular or crystalline mass. Occasionally it passes
insensibly from fine to coarse-grained granite, and the finer grained is
even sometimes found embedded in the more coarsely granular variety:
sometimes it assumes a porphyritic texture. Porphyritic granite is a
variety of granite, the components of which--quartz, felspar, and
mica--are set in a non-crystallised paste, uniting the mass in a manner
which will be familiar to many of our readers who may have seen the
granite of the Land’s End, in Cornwall. Alongside these orthoclase
crystals, quartz is implanted, usually in grains of irregular shape,
more rarely crystallised, and seldom in the form of perfect crystals. To
these ingredients are added thin scales or small hexagonal plates and
crystals of white, brown, black, or greenish-coloured mica. Finally, the
name of _quartziferous porphyry_ is reserved for those varieties which
present crystals of quartz; the other varieties are simply called
_porphyritic granite_. _True_ porphyry presents a paste essentially
composed of compact felspar, in which the crystals of orthoclase--that
is, felspar with a potash base--are abundantly disseminated, and
sometimes with great regularity.
Granite is supposed to have been “formed at considerable depths in the
earth, where it has cooled and crystallised slowly under great pressure,
where the contained gases could not expand.”[11] “The influence,” says
Lyell, “of subterranean heat may extend downwards from the crater of
every active volcano to a great depth below, perhaps several miles or
leagues, and the effects which are produced deep in the bowels of the
earth may, or rather must, be distinct; so that volcanic and plutonic
rocks, each different in texture, and sometimes even in composition, may
originate simultaneously, the one at the surface, the other far beneath
it.” Other views, however, of its origin are not unknown to science:
Professor Ramsay and some other geologists consider granite to be
metamorphic. “For my own part,” says the Professor, “I believe that in
one sense it is an igneous rock; that is to say, that it has been
completely fused. But in another sense it is a metamorphic rock, partly
because it is impossible in many cases to draw any definite line between
gneiss and granite, for they pass into each other by insensible
gradations; and granite frequently _occupies the space that ought to be
filled with gneiss_, were it not that the gneiss has been entirely
fused. I believe therefore that granite and its allies are simply the
effect of the extreme of metamorphism, brought about by great heat with
presence of water. In other words, when the metamorphism has been so
great that all traces of the semi-crystalline laminated structure have
disappeared, a more perfect crystallisation has taken place.”[12] It is
obvious that the very result on which the Professor founds his theory,
namely, the difficulty “in many cases,” of drawing a line between the
granite and the gneiss, would be produced by the sudden injection of the
fluid minerals into gneiss, composed of the same materials. Moreover, it
is only in some cases that the difficulty exists; in many others the
line of separation is definable enough.[13]
[11] Lyell’s “Elements of Geology,” p. 694.
[12] “Physical Geology and Geography of Great Britain,” by A. C.
Ramsay, p. 38, 2nd ed.
[13] At the same time it may be safely assumed (as Professor Ramsay
believes to be the case) that granite in most cases is a
metamorphic rock; yet are there many instances in which it may
with greater truth be considered as a true plutonic rock.
The granitic rock called _Syenite_, in which a part of the mica is
replaced by hornblende or amphibole, has to all appearance been erupted
to the surface subsequently to the granite, and very often alongside of
it. Thus the two extremities of the Vosges, towards Belfort and
Strasburg, are eminently syenitic, while the intermediate part, towards
Colmar, is as markedly granitic. In the Lyonnais, the southern region is
granitic; the northern region, from Arbresle, is in great part syenitic.
Syenite also makes its appearance in the Limousin.
Syenite, into which rose-coloured felspar often enters, forms a
beautiful rock, because the green or nearly black hornblende heightens,
by contrast, the effect of its colour. This rock is a valuable adjunct
for architectural ornament; it is that out of which the ancient
Egyptians shaped many of their monumental columns, sphinxes, and
sarcophagi; the most perfect type of it is found in Egypt, not far from
the city of Syene, from which it derives its name. The obelisk of Luxor
now in Paris, several of the Egyptian obelisks in Rome, and the
celebrated sphinxes, of which copies may be seen in front of the
Egyptian Court at the Crystal Palace, the pedestal of the statue of
Peter the Great at St. Petersburg, and the facing of the sub-basement of
the column in the Place Vendôme in Paris, are of this stone, of which
there are quarries in the neighbourhood of Plancher-les-Mines in the
Vosges.
Syenite disintegrates more readily than granite, and it contains
indurated nodular concretions, which often remain in the form of large
spherical balls, in the midst of the débris resulting from
disintegration of the mass. It remains to be added that syenitic masses
are often very variable as regards their composition; the hornblende is
sometimes wanting, in which case we can only recognise an ancient
granite. In other instances the hornblende predominates to such a
degree, that a large or small-grained _diorite_, or greenstone, results.
The geologist should be prepared to observe these transitions, which are
apt to lead him into error if passed over without being noticed.
_Protogine_ is a talcose granite, composed of felspar, quartz, and talc
or _chlorite_, or decomposed mica, which take the place of the usual
mica. Excessively variable in its texture, protogine passes from the
most perfect granitic aspect to that of a porphyry, in such a manner as
to present continual subjects of uncertainty, rendering it very
difficult to determine its geological age. Nevertheless, it is supposed
to have come to the surface before and during the coal-period; in short,
at Creusot, protogine covers the coal-fields so completely, that it is
necessary to sink the pits through the protogine, in order to penetrate
to the coal, and the rock has so manifestly acted on the coal-measure
strata, as to have contorted and metamorphosed them. Something analogous
to this manifests itself near Mont Blanc, where the colossal mass which
predominates in that chain, and the peaks which belong to it, consist of
protogine. But as no such action can be perceived in the overlying rocks
of the Triassic period, it may be assumed that at the time of the
deposition of the New Red Sandstone the protoginous eruptions had
ceased.
It is necessary to add, however, that if the protogine rises in such
bold pinnacles round Mont Blanc, the circumstance only applies to the
more elevated parts of the mountain, and is influenced by the excessive
rigour of the seasons, which demolishes and continually wears away all
the parts of the rock which have been decomposed by atmospheric agency.
Where protogine occurs in milder climates--around Creusot, and at
Pierre-sur-Autre, in the Forez chain, for instance--the mountains show
none of the scarped and bristling peaks exhibited in the chain of Mont
Blanc. Only single isolated masses occasionally form _rocking-stones_,
so called because, resting with a convex base upon a pedestal also
convex, but in a contrary way, it is easy to move these naturally
balanced blocks, although from their vast size it would require very
considerable force to displace them. This tendency to fashion themselves
into rounded or ellipsoidal forms belongs, also, to other granitic
rocks, and even to some of the variegated sandstones. The rocking-stones
have often given rise to legends and popular myths.
The great eruptions of granite, protogine, and porphyry took place,
according to M. Fournet, during the carboniferous period, for
porphyritic pebbles are found in the conglomerates of the Coal-measure
period. “The granite of Dartmoor, in Devonshire,” says Lyell,[14] “was
formerly supposed to be one of the most ancient of the plutonic rocks,
but it is now ascertained to be posterior in date to the culm-measures
of that county, which from their position, and as containing true
coal-plants, are regarded by Professor Sedgwick and Sir R. Murchison as
members of the true Carboniferous series. This granite, like the
syenitic granite of Christiana, has broken through the stratified
formations without much changing their strike. Hence, on the north-west
side of Dartmoor, the successive members of the Culm-measures abut
against the granite, and become metamorphic as they approach. The
granite of Cornwall is probably of the same date, and therefore as
modern as the Carboniferous strata, if not newer.”
[14] “Elements of Geology,” p. 716, 6th edition.
The _ancient granites_ show themselves in France in the Vosges, in
Auvergne, at Espinouse in Languedoc, at Plan-de-la-Tour in Provence, in
the chain of the Cévennes, at Mont Pilat near Lyons, and in the southern
part of the Lyonnaise chain. They rarely impart boldness or grandeur to
the landscape, as might be expected from their crystallised texture and
hardness; for having been exposed to the effects of atmospheric changes
from the earliest times of the earth’s consolidation, the rocks have
become greatly worn away and rounded in the outlines of their masses. It
is only when recent dislocations have broken them up that they assume a
picturesque character.
The Christiania granite alluded to above was at one time thought to have
belonged to the Silurian period. But, in 1813, Von Buch announced that
the strata in question consisted of limestones containing orthoceratites
and trilobites; the shales and limestone being only penetrated by
granite-veins, and altered for a considerable distance from the point of
contact.[15] The same granite is found to penetrate the ancient gneiss
of the country on which the fossiliferous beds rest--unconformably, as
the geologists say; that is, they rest on the edges of the gneiss, from
which other stratified deposits had been washed away, leaving the gneiss
denuded before the sedimentary beds were deposited. “Between the origin,
therefore, of the gneiss and the granite,”[16] says Lyell, “there
intervened, first, the period when the strata of gneiss were denuded;
secondly, the period of the deposition of the Silurian deposits. Yet
the granite produced after this long interval is often so intimately
blended with the ancient gneiss at the point of the junction, that it is
impossible to draw any other than an arbitrary line of separation
between them; and where this is not the case, tortuous veins of granite
pass freely through gneiss, ending sometimes in threads, as if the older
rock had offered no resistance to their passage.” From this example Sir
Charles concludes that it is impossible to conjecture whether certain
granites, which send veins into gneiss and other metamorphic rocks, have
been so injected while the gneiss was scarcely solidified, or at some
time during the Secondary or Tertiary period. As it is, no single mass
of granite can be pointed out more ancient than the oldest known
fossiliferous deposits; no Lower Cambrian stratum is known to rest
immediately on granite; no pebbles of granite are found in the
conglomerates of the Lower Cambrian. On the contrary, granite is usually
found, as in the case of Dartmoor, in immediate contact with primary
formations, with every sign of elevation subsequent to their deposition.
Porphyritic pebbles are found in the Coal-measures; porpyhries continue
during the Triassic period; since, in some parts of Germany, veins of
porphyry are found traversing the New Red Sandstone, or _grès bigarré_
of French geologists. Syenites have especially reacted upon the Silurian
deposits and other old sedimentary rocks, up to those of the Lower
Carboniferous period.
[15] “Elements of Geology,” p. 717.
[16] Ibid, p. 718.
The term porphyry is usually applied to a rock with a paste or base of
compact felspar, in which felspathic crystals of various sizes assume
their natural form. The variety of their mineralogical characters, the
admirable polish which can be given to them, and which renders them
eminently useful for ornamentation, give to the porphyries an artistic
and industrial importance, which would be greatly enhanced if the
difficulty of working such a hard material did not render the price so
high.
The porphyries possess various degrees of hardness and compactness. When
a fine dark-red colour--which contrasts well with the white of the
felspar--is combined with hardness, a magnificent stone is the result,
susceptible of taking a polish, and fit for any kind of ornamental work;
for the decoration of buildings, for the construction of vases, columns,
&c. The red Egyptian porphyry, called _Rosso antico_, was particularly
sought after by the ancients, who made sepulchres, baths, and obelisks
of it. The grandest known mass of this kind of porphyry is the Obelisk
of Sextus V. at Rome. In the Museum of the Louvre, in Paris, some
magnificent basins and statues, made of the same stone, may also be
seen.
In spite of its compact texture porphyry disintegrates, like other
rocks, when exposed to air and water. One of the sphinxes transported
from Egypt to Paris, being accidentally placed under a gutter of the
Louvre, soon began to exhibit signs of exfoliation, notwithstanding it
had remained sound for ages under the climate of Egypt. In this country,
and even in France, where the climate is much drier, the porphyries
frequently decompose so as to become scarcely recognisable. They crop
out in various parts of France, but are only abundant in the
north-eastern part of the central plateau, and in some parts of the
south. They form mountains of a conical form, presenting, nearly always,
considerable depressions on their flanks. In the Vosges they attain a
height of from three to four thousand feet.
The _Serpentine_ rocks are a sort of compact _talc_, which owe their
soapy texture and greasy feel to silicate of magnesia. Their softness
permits of their being turned in a lathe and fashioned into vessels of
various forms. Even stoves are constructed of this substance, which
bears heat well. The serpentine quarried on the banks of Lake Como,
which bears the name of pierre ollaire, or pot-stone, is excellently
adapted for this purpose. Serpentine shows itself in the Vosges, in the
Limousin, in the Lyonnais, and in the Var; it occupies an immense tract
in the Alps, as well as in the Apennines. Mona marble is an example of
serpentine; and the Lizard Point, Cornwall, is a mass of it. A portion
of the stratified rocks of Tuscany, and also those of the Island of
Elba, have been upheaved and overturned by eruptions of it.
As for the British Islands, plutonic rocks are extensively developed in
Scotland, where the Cambrian and Silurian rocks, often of gneissic
character--associated here and there with great bosses of granite and
syenite--form by far the greater part of the region known as the
Highlands. In the Isle of Arran a circular mass of coarse-grained
granite protrudes through the schists of the northern part of the
island; while, in the southern part, a finer-grained granite and veins
of porphyry and coarse-grained granite have broken through the
stratified rocks.[17] In Devonshire and Cornwall there are four great
bosses of granite; in the southern parts of Cornwall the mineral axis is
defined by a line drawn through the centre of the several bosses from
south-west to north-east; but in the north of Cornwall, and extending
into Devonshire, it strikes nearly east and west. The great granite
mass in Cornwall lies on the moors north of St. Austell, and indicates
the existence of more than one disturbing force. “There was an elevating
force,” says Professor Sedgwick,[18] “protruding from the St. Austell
granite; and, if I interpret the phenomena correctly, there was a
contemporaneous elevating force acting from the south; and between these
two forces, the beds, now spread over the surface from the St. Austell
granite to the Dodman and Narehead, were broken, contorted, and placed
in their present disturbed position. Some great disturbing forces,” he
observes, “have modified the symmetry of this part of Cornwall,
affecting,” he believes, “the whole transverse section of the country
from the headlands near Fowey to those south of Padstow.” This great
granite-axis was upheaved in a line commencing at the west end of
Cornwall, rising through the slate-rocks of the older Devonian group,
continuing in association with them as far as the boss north of St.
Austell, producing much confusion in the stratified masses; the
granite-mass between St. Clear and Camelford rose between the deposition
of the Petherwin and that of the Plymouth group; lastly, the Dartmoor
granite rose, partially moving the adjacent slates in such a manner that
its north end abuts against and tilts up the base of the Culm-trough,
mineralising the great Culm-limestone, while on the south it does the
same to the base of the Plymouth slates. These facts prove that the
granite of Dartmoor, which was formerly thought to be the most ancient
of the Plutonic rocks, is of a date subsequent to the Culm-measures of
Devonshire, which are now regarded as forming part of the true
carboniferous series.
[17] “Geology of the Island of Arran,” by Andrew C. Ramsay. “Geology
of Arran and Clydesdale,” by James Bryce.
[18] See _Quarterly Journal of Geological Society_, vol. viii., pp. 9
and 10.