THE LIMESTONES
INTRODUCTION
The term Limestone covers, by common consent, rocks consisting mainly
of calcium carbonate. Dolomite (properly Dolomite-rock), in which
half or nearly half the molecules consist of magnesium carbonate, is,
however, generally included. The convenience of limestones as building
materials has given them a world-wide interest. Their stratified and
jointed structure appealed to the early Egyptian architect, when he
sought blocks for his pyramids. The ease with which limestones could
be carved, combined with a reasonable resistance to decay, gave them
a pre-eminence with the designers of our rich cathedrals. The Romans
found in the stained and altered varieties colour-schemes for basilicas
and baths, and their luxurious taste in limestone has been inherited by
the modern builders of hotels.
The rock suffers, however, from its solubility in water containing even
a mild acid. In the gases dissolved by rain-water from the atmosphere,
carbon dioxide assumes a far larger proportion than that which it
possesses in the air itself. The surface of limestone slabs becomes
in consequence pitted and corroded by every rain that falls. The
sulphuric acid in the air of modern coal-consuming cities is, however,
still more deadly in its action. J. A. Howe, in his recent work on
building stones, is of opinion that limestone is unsuitable for towns.
Limestones may broadly be recognised by their solubility in cold dilute
acids, with brisk evolution of carbon dioxide. Dolomitic varieties
require hot acid.
Limestones divide themselves into types produced by chemical
precipitation and those due to the accumulation of the hard parts of
organisms; but in many of the latter types chemical precipitation
also plays a part. Organic action, moreover, frequently promotes the
deposition of the chemical types. _Detrital limestones_, that is,
limestones formed from the debris of older ones, are comparatively
unimportant. They occur in certain zones of the Chalk and of the
Carboniferous Limestone in our islands, and record the breaking up in
shallow water of beds that had already become consolidated. The Miocene
_Nagelfluh_ conglomerates of the north side of the Swiss Alps are
often formed of pebbles of the far older Mesozoic limestones. Similar
conglomerates, cemented by calcium carbonate, are now being formed in
the river-beds of the limestone karstland of Hercegovina. Limestone,
however, as a rule goes to pieces before the buffetings sustained by
mixed rocks on a shore. Even if it survives for a time in gravels,
percolating waters ultimately dissolve it, and only a porous skeleton,
formed of its impurities, remains.
LIMESTONES DEPOSITED FROM SOLUTION
Though calcium carbonate is far less soluble than calcium sulphate,
large quantities are carried invisibly, owing to the presence of carbon
dioxide, in river waters, and thus accumulate in inland seas that
have no outlet except by evaporation. Here _Calcareous Tufa_ may be
deposited as a crust upon the shores and on the growing islets, as the
water shrinks away, and before the more soluble gypsum and rock-salt
can separate out. Hot springs of volcanic origin, like the Sprudel of
Karlsbad in Bohemia, may deposit calcium carbonate as the water cools
and is relieved from pressure. At Karlsbad, little grains of granite,
or of the minerals of granite, serve as centres, and encrusting layers
are formed round them, until pea-like bodies are produced. These become
cemented together, giving rise to the well-known freshwater _pisolitic
limestone_ or _roestone_.
On the shores of the Great Salt Lake of Utah, calcareous tufa occurs
also in the form of grains resembling little eggs. These are the
oolitic grains that were first known as constituents of fossil
limestones. The calcium carbonate of oolitic grains at Karlsbad, from
the Great Salt Lake, and from the sea, is deposited in a form that
gives the reaction of aragonite when boiled in cobalt nitrate. A.
Lacroix, however, finds that the material at Karlsbad has a specific
gravity lower even than that of calcite, and that its double refraction
is also distinctly weaker. He has called this form of calcium carbonate
"ktypeite."
_Travertine_ is a tufa laid down on twigs and other vegetation, where
springs emerge laden with calcium carbonate. In a massive form, it
builds tufa-basins, as in the Mammoth Hot Springs of the Yellowstone
Park. Both here and at Karlsbad, it appears that vegetation of humble
type, multiplying under warm conditions, materially assists the deposit
by withdrawing carbon dioxide from the water. The unstable calcium
bicarbonate is thus converted into the carbonate, which is thrown down
as a quickly increasing crust.
Among the limestone regions of the Dinaric Alps, calcareous tufas or
travertines, laid down by ordinary streams, form massive beds that tend
to choke the hollows of the hills. The basin of Jajce in Bosnia is thus
partially filled up, and the town is built on materials brought in
solution from the mountains. The modern waters are still adding to this
deposit, and Fr. Katzer[4] has pointed out that the falls of the Pliva
are prevented from cutting their way down to the level of the Vrbas
ravine, into which they plunge, by the mass of tufa which they build up
in their own course.
Another type of limestone deposited from solution is of considerable
interest in arid lands, or lands with only a seasonal rainfall. Where
evaporation goes on steadily at the surface, while water is brought up
by capillary action from below, calcium carbonate may form a cement
to the soil, or to the crumbling rock near the surface, and a solid
calc-tufa may arise by continued transference of matter in solution
from lower levels. In the Cape of Good Hope such formations are
conspicuous[5].
In a careful series of experiments, G. Linck[6] showed in 1903 that
sea-water at 17° C. can only hold ·0191 per cent. of calcium carbonate
in solution. Though this quantity is not realised in the open ocean,
yet near shores rivers may bring down an excess. The Thames, though
flowing for a long distance over a limestone area, contains only ·0116
per cent. of calcium carbonate; but springs traversing limestone often
carry ·03 per cent., or ten times as much as that found in ordinary
seas. Hence a precipitation of calcium carbonate from the bicarbonate
state may take place not far from land. The mineral deposited is
calcite in temperate climates and aragonite under warm tropical
conditions. That such a precipitation actually occurs is proved by the
massive grey limestones, containing modern shells, which have been
recorded for our islands from the sea-floor off the Isle of Man and
off the coast of Mayo. In the case of the Irish Channel, the excess of
calcium carbonate may be supplied by springs rising through the glacial
gravels, which contain abundant pebbles of limestone.
Ammonium carbonate, again, derived from the decay of organisms, or
sodium carbonate, will precipitate calcium carbonate as aragonite
from the calcium sulphate and chloride, but not from the calcium
bicarbonate, of salt water. Films of aragonite are at present
accumulating by this process on the floor of the Black Sea, and marine
oolitic grains, also consisting of aragonite, are produced by the same
reaction.
In the case of oolitic grains, deposition is no doubt helped by
evaporation, since they seem to arise in shallow waters. The _Oolitic
Limestones_ that have proved so admirable as building stones, whether
from the quarries of Caen or Portland, are cemented representatives of
the loose deposits formed in modern tropical seas. De la Beche long
ago compared their grains with those from West Indian coral-reefs.
These small egg-like bodies develop round fragments of foraminiferal
and other shells, round the ossicles of echinoderms, and round broken
bits of coral. At first they have the general form of the nucleus; but,
as they are rolled by the waves during their growth, they become more
and more spheroidal as they enlarge. Boring algæ make tubular passages
in them, and these have led to the view that algæ of thread-like form
actually originate oolitic structure. Doelter, Linck, and others
conclude, with much reason, that the mode of deposition is inorganic.
When the grains are unusually large, they are often flattened and
irregular, as in the marine _Pisolites_ or _Pea-grits_.
For building purposes, the fine-grained oolites without large fossils
are much sought after, since they can be trimmed equally in any desired
direction.
Before leaving the question of the inorganic deposition of limestone,
we may note that R. A. Daly[7] has suggested that the pre-Cambrian
and early Cambrian limestones were entirely products of chemical
precipitation. He believes that the continental areas were at first
relatively small, and that the abundance of decaying soft-bodied
organisms on the sea-floor led to a continuous precipitation of such
calcium carbonate as was available. Hence the ocean was limeless,
and it was only when continental land became more extended that
a sufficient quantity of lime salts was brought in by rivers to
counterbalance that thrown down by ammonium carbonate and sodium
carbonate on the sea-floor. Daly urges that, on this account, the
earlier organisms could not form calcareous shells or skeletons, and
he also believes that pre-Cambrian and Cambrian limestones, even
when unaltered, show no signs of having originated from fragmental
organic remains. Linck's researches (p. 17) show that limestones thus
precipitated must have originally consisted of aragonite.
LIMESTONES FORMED OF ORGANIC REMAINS
These limestones present an immense variety, according to the nature
of the originating organisms, and the amount of foreign material
brought down into the water where they accumulated. The calcareous
remains of Chara may form a white deposit on the floors of freshwater
lakes. The part played by calcareous algæ in the formation of marine
limestones has long been recognised; but the detailed exploration in
1904 of the atoll of Funafuti in the Pacific showed that Halimeda may
be responsible for a considerable portion of an ordinary "coral-reef."
Lithothamnium occurs in immense quantities, associated with molluscan
remains, near many shores, and forms a large part of the material of
the raised beaches in Spitsbergen.
Animal, not vegetable, activity, however, is responsible for the
majority of our limestones, and the humbler organisms, by reason of
their abundance, play a prominent part in rock-formation. Analogies
between the Globigerina-ooze of deep waters and the groundwork of the
soft white limestone known as _Chalk_ have been freely pointed out.
Early in the nineteenth century, Ehrenberg, in a series of researches
with the microscope, proved the organic origin of the compact ground
of marine limestones. The occurrence of foraminifera from the shore
outwards to truly oceanic waters provides a fine-grained calcareous
material which forms deposits at very various depths. The milioline
types, often with a surface like that of glazed porcelain, are common
in the sandy beds formed near a coast. Few rocks are more fascinating
under the microscope than those in which such types are seen in
section, associated with detrital grains of quartz, washed down from
the land, and perhaps with bright green grains of the marine mineral,
glauconite. In Ireland white chalks occur, speckled throughout with
glauconite, which looks dark in the rock-mass, but which reveals its
green tint when streaked out by the hammer. When formed still farther
from land, pure chalk arises from the consolidation of foraminiferal
ooze, and the probable depth in which it accumulated must be judged
from the nature of the associated organisms. A white limestone may,
however, arise in a comparatively shallow sea, where the rivers
bring down little solid matter from the land. A coast formed of pure
limestone, with clear streams flowing from a land of similar rock
behind, may allow of the development of pure limestone on its shores.
It is generally agreed that the Upper Chalk of the British Isles and
of northern France was laid down in water one thousand fathoms or more
in depth; yet the corresponding white limestone of northern Ireland in
places follows rapidly on conglomeratic and glauconitic deposits, and
seems to owe its purity to the comparative absence of rain and rivers
on the highland of crystalline rocks which stretched westward from its
shore.
There are two epochs of the earth's history in which foraminifera were
remarkable for their size as well as their abundance. The first gave
us the grey Fusulina limestone of Upper Carboniferous times, when this
spindle-shaped shell spread freely from the United States through the
arctic regions to the east of Asia. The second gave us, in the Eocene
period, the great beds formed of Nummulites and Orbitoides, which
we meet with in Europe on the Lake of Thun, but which are far more
important in Lower Egypt. The disc-like forms of the nummulites in the
white limestone of the Pyramids are familiar to hundreds of travellers,
and forms are recorded up to four and a half inches across.
The foraminiferal origin of many compact limestones can often be
appreciated on smooth surfaces with a pocket-lens. The older examples
have commonly become stained and darkened, and crystallisation of
calcite throughout the ground has in part destroyed the original
organic structures. This tendency to crystallise affects even the
larger fossils, and brachiopods and molluscs have sometimes disappeared
from our Carboniferous limestones, without the intervention of
"metamorphic" heat or pressure. In most limestones older than the
Eocene period, the shells and other fossils, such as corals, that
were originally formed of aragonite have passed into the calcite
state, without the destruction of their characteristic shapes. Shells,
however, have been found still preserved as aragonite in beds as old as
the Jurassic period[8].
The lamellibranchs, the ordinary bivalves, came into prominence as
limestone-builders with the Carboniferous period, and are now rivalled
by the univalve gastropods, which displayed no widespread activity
until Eocene times. The most massive existing shell, however, is a
lamellibranch, the giant Tridacna of Australian seas, a single valve
of which may weigh 250 lbs. The cephalopods, though lying far nearer to
the crown of molluscan development, became important from the Silurian
Orthoceras onwards, and nautiloids of various forms are common fossils
in the Carboniferous limestone. Their large size attracts attention
from our present point of view. The cephalopods, however, swell the
bulk of many limestones, not by the thickness of their shells, but
through their chambered character, which has prevented complete
infilling of the shell, and which thus allows of cavities in the mass.
This is notably the case with the ammonites, which contribute so
largely to Jurassic limestones. Crystalline calcite has often been
deposited by infiltration on the septa and on the inner layer of the
shell, thus reducing the hollow spaces. The massive calcite guards of
the belemnites form a considerable part of many limestones.
Even freshwater lakes possess molluscan deposits, producing a white
limestone of their own. Where streams flow over pure pre-existing
limestone, there is no alluvial mud to choke the basins. In the hard
lake-waters, gastropods such as Limnæa and Planorbis, and a few
bivalves, can then flourish freely, and a "shell-marl" accumulates
at the bottom, unmixed with sediment. Limestone of this type is
conspicuous in hollows in the Dinaric Alps, which were once occupied
by lakes, and is often found beneath peat in the limestone lowland of
central Ireland.
In older days, two groups of organisms, now relatively unimportant,
had a powerful place. The brachiopods, including in early Palæozoic
times an interesting series of thin shells largely composed of calcium
phosphate, were for long the predominant shell-bearing organisms. The
stout Spiriferidæ and the well-known _Productus giganteus_ of the
Carboniferous period illustrate their dominance. The group became much
restricted in variety in Jurassic times; but even then Terebratula and
Rhynchonella occurred so abundantly that they now fall out of many
rock-faces like pebbles from a loose conglomerate.
The sea-lilies have similarly lost their place as limestone-builders,
though their "ossicles," notably from their stems, furnish crinoidal
or "encrinital" masses from Silurian to Carboniferous times. The
broken portions of their stems, resembling tubes of tobacco-pipes, are
conspicuous when they are weathered out on rock-surfaces or revealed in
polished slabs of marble. The fact that each joint or ossicle, as is
the universal case in the echinodermata, consists of a single crystal
of calcite causes the fragments to break with the characteristic
cleavage of that mineral. The smooth glancing surfaces thus seen on
fractured specimens readily call attention to them in a rock.
Those humble colonial organisms, the compound corals, have so special
a place as limestone-formers that they have been reserved for more
detailed treatment. The accumulation of their skeletons, and the fact
that they may form large continuous masses by their very mode of
growth, promotes the formation of solid rock at an unusual rate. Von
Richthofen long ago pointed out how foraminifera and other drifted
material became caught in the interstices of coral, producing even a
stratified structure in the hollows of a reef; and subsequent research
has shown the composite character of reefs in various portions of the
tropic seas. Calcareous algæ as already remarked, and the massive and
often encrusting skeletons of hydrozoa, such as Millepora, are freely
associated with the products of true corals.
Charles Darwin, in his famous theory of the formation of atolls and
barrier-reefs, showed how, in a subsiding area, corals might keep pace
with the downward movement. Hence reefs might arise of great vertical
thickness, although the polypes themselves could flourish only in the
upper twenty fathoms or so of water. This conclusion, which appears
strictly logical, has met with much opposition from Karl Semper,
Alexander Agassiz, and Sir John Murray. Murray in particular urges the
importance of banks of calcareous organisms in building up platforms
on which corals may ultimately dwell. The extension of reefs outward
into deep water has been attributed to the rolling down of wave-worn
coral debris over submarine mountain-slopes. From this point of view,
an apparently thick atoll may be formed as a comparatively thin mass
of limestone at the summit of a volcanic cone that fails to reach the
sea-level.
The opponents of the view that thick coral-limestones are formed at the
present day in the Pacific have been unwilling to accept the results
even of the deep boring in the atoll of Funafuti[9], which penetrated
materials like those of the superficial layers of the reef to a depth
of 1114 feet. They have also refused to see in the huge dolomitic
rocks of Tyrol the remains of Triassic reefs four thousand feet in
thickness. None the less, most geologists regard the Funafuti boring
as a strong support for Darwin's contention. Whatever may be proved as
to the origin of this or that atoll at the present day, it is clear
that the possibility of subsidence leads us to expect considerable
coral-limestones among our ancient rocks. The same problem arises
wherever we have a rich molluscan fauna continuously represented in two
or three thousand feet of limestone, or where we find shore-deposits
of any kind accumulated to an unusual thickness. Darwin, at the end of
the fifth chapter of his work on "The structure and distribution of
Coral-Reefs," gives a vivid account of the features that would appear
in a section of an atoll that has grown large through subsidence of
its inorganic floor, and he emphasises the occurrence of conglomerates
of broken coral-rock on the outer zone. The stratification of material
by wave-action in this zone, and the horizontal deposition of finer
material in the lagoon, would give to the dissected mass a general
sedimentary aspect. Darwin concluded that the ring of solid coral,
the true reef, might be denuded away during an epoch of elevation,
and that only stratified portions might remain. He does not seem to
have discussed the contemporaneous deposition of pelagic material from
foraminiferal and other sources against the outer surface of the reef
whereby an interlocking of two facies of limestone might arise.
These features, together with those predicted by Darwin, have been
recognised by von Richthofen and Mojsisovics in the Tyrol dolomites,
and have afforded Austrian geologists good evidence that large parts
of these limestones originated as coral-reefs. Faulting, however, has
undoubtedly taken place in this region, producing here and there a
subsidence of the limestone blocks among the surrounding more normal
sediments. Rothpletz, Ogilvie Gordon[10], and other critics of von
Richthofen's view have seen in this faulting the cause of the abrupt
change from a facies of massive dolomite to one of normal sedimentation
on the same horizontal level. They have also urged that shell-banks
may accumulate locally so as to simulate reefs by their contrast with
their surroundings, while the change to dolomite has obliterated their
original features (see p. 30). It cannot be denied, however, that
coral-reefs and their associated detrital deposits must exercise a very
important influence in the formation of solid limestone.
Even small knots and local groups of compound corals are seen in
ordinary limestones to serve as a mesh in which other organic remains
have become entrapped. The ease with which the aragonite of their
skeletons becomes silicified causes them often to stand out on
weathered surfaces with all the delicacy of structure displayed upon a
modern reef.
Where limestones and shales are associated together, a "knoll
structure" may be found, the limestone occurring in masses of a
somewhat hemispherical form, with the shales fitted against and round
them. In some cases this may be due to the local distribution of
patches of growing coral on the old sea-floor; but in other cases
the structure has arisen from compression and brecciation of the
strata, the original beds of limestone becoming broken up and the
more yielding beds flowing round them. This structure is well seen
on a small scale in many "crush-conglomerates," where the limestone
appears as knots and eyes, resembling pebbles. Yet near at hand the
true bedding may be traced, bands of limestone alternating with shale,
and a few cross-joints indicating the possibility of a separation
of the limestone into blocks. These blocks become rounded in the
general rock-flow; but Gardiner and Reynolds[11] suggest solution by
infiltering water as an explanation of certain remarkable examples
studied by them.
ALTERED FORMS OF MASSIVE LIMESTONE
A certain amount of magnesium carbonate is present in the skeletons
of some marine organisms. This has been shown both by Forchammer and
Walther[911]. A foraminifer, _Nubecularia novorossica_, has been found
with 26 per cent. of magnesium carbonate, and a serpula with 7·64
per cent.; alcyonarian corals contain up to 9·32 per cent., while
calcareous algæ, such as Lithothamnium and Halimeda, contain about 12
per cent.[12]. The magnesium salt is not, however, here combined with
calcium carbonate to form the mineral dolomite; none the less it is
clear that such organisms introduce magnesium in appreciable quantities
into the constitution of marine limestones.
Marine limestones are very commonly "dolomitised." Dolomite, the joint
carbonate, CaMg(CO_{3})_{2}, contains 54·35 per cent. of calcium
carbonate and 45·65 per cent. of magnesium carbonate, or carbon dioxide
47·8, lime 30·4, and magnesia 21·8. Its specific gravity is 2·85.
The occurrence of dolomite in intimate association with calcite has
been proved by E. W. Skeats[13] in the case of modern coral-reefs,
and the secondary deposition of the mineral has been made clear. The
skeletons of the corals themselves may now consist of dolomite, while
calcite has crystallised in their interstices, or remains as part of
the original infilling of mud. The presence of dolomite in reefs has,
of course, long been known, having been observed by J. D. Dana in 1849,
and it has been realised that, by prolonged alteration, masses of
_Dolomite Rock_ become built up[14].
Commonly, the process produces a _Dolomitic Limestone_, in which
calcium carbonate is still in excess of the 54 per cent. which is
present in the mineral dolomite.
The alteration of the original limestone is, however, sufficiently
profound. The ready crystallisation of dolomite as rhombohedra destroys
the organic structure, and traces of corals or molluscan shells
disappear from great thicknesses of rock. It is uncertain whether the
process of dolomitisation proceeds most rapidly in the evaporating
waters of the lagoons, or, as Pfaff believes, at considerable depths,
where the pressure may reach 100 atmospheres. Magnesium carbonate, as
we shall note later, may be removed from dolomite in solution under
pressure at a greater rate than calcium carbonate. If this occurs in
sea-water, it would seem to militate against the production of dolomite
in the lower levels of a reef.
The magnesium required for dolomitisation is derived from the magnesium
sulphate and chloride of sea-water, calcium being removed during the
change. C. Klement in particular urges that a concentrated solution of
sodium chloride at 60° C. assists the process in the case of magnesium
sulphate. Aragonite, the material of coral skeletons and of most
molluscan shells, is more susceptible than calcite. The temperature of
Klement's experiments may be realised in lagoons or between tide-marks,
and Doelter suggests that the element of time in nature may allow the
reaction to take place at lower temperatures.
The intimate structure of modern dolomitic limestone, as exhibited
in coral-reefs, satisfies us that many older or fossil dolomites
were formed from marine calcareous deposits while these were still
accumulating. In other cases we must admit that the dolomite has
developed in the neighbourhood of joints after the consolidation of the
rock. The view that dolomitisation results from the mere removal of
calcium, the magnesium originally present in organic skeletons becoming
thus more concentrated, is not borne out by recent observations.
Skeats[15] has carefully compared the dolomite-rocks of Tyrol with the
materials of recent coral-reefs. In both there is a striking absence
of detritus of inorganic origin, and his work goes far to show that
the much-discussed Alpine dolomites were formed under conditions which
occur in the neighbourhood of existing reefs. This, however, does not
solve the question as to whether we are dealing in Tyrol with fossil
coral-reefs, or with the calcareous type of ordinary marine sediments,
which might undergo the same kind of alteration. While Skeats finds in
two dolomites from recent reefs 43 per cent. of magnesium carbonate,
the substitution seems usually to terminate when 40 per cent. has been
introduced. In Tyrol, however, the process has gone so far as to give
rise to true dolomites, with 45·65 of magnesium carbonate.
The dolomites of the Jurassic series in north Bavaria are massive rocks
almost devoid of fossils, traversed by shrinkage cracks, and associated
with richly fossiliferous stratified limestones. The relations of these
two types of rock are those of coral-reefs to the bedded deposits on
their flanks, and the dolomite seems to merge horizontally into the
stratified series. As in Tyrol, fossils and corals are rare in the
bosses of dolomite, but the structural evidence is strongly in favour
of their having originated as steeply sided reefs.
The dolomitic facies of the Carboniferous limestone in our islands
is an example of the second type of origin. The dolomite here
frequently occurs in irregular veins and patches. The introduction
of iron carbonate with the magnesium salt stains the dolomite brown
on exposure to oxidation, and its limits are thus clearly seen in the
general blue-grey mass. The dolomitisation has evidently proceeded
from joint-surfaces inwards. It is often sufficiently thorough to
obliterate all traces of fossils, and the shrinkage accompanying the
chemical change has produced numerous cavities, in which calcite has
subsequently crystallised. An expansion takes place when aragonite is
altered into dolomite, unless more of the calcium carbonate is removed
than is necessary to give place to the magnesium carbonate introduced.
In the change from calcite, with a density of 2·72, to dolomite, with
a density of 2·85, there is, on the other hand, a shrinkage of 4·56
per cent. Where the alteration, then, takes place while the aragonite
organisms still remain as aragonite, and not as calcite, an expansion
rather than a contraction should occur in the substance of a reef; but
when an old limestone, in which all the calcium carbonate is present as
calcite, becomes dolomitised, a considerable shrinkage will occur, and
rifts and hollows may remain obvious.
Very few dolomites, except those found in association with rock-salt
and other products of the evaporation of lagoons, can now be attributed
to direct chemical deposition from the sea.
Daly[7] has argued that the first Palæozoic and the pre-Cambrian
dolomites were formed by precipitation, since the calcium salts in
those early days were completely removed from the sea-water. Ammonium
carbonate, though effective in precipitating the calcium salts, does
not act on those of magnesium until the calcium salts have been brought
down. But, under the conditions postulated for the river-waters that
reached the sea from the earliest continental lands, conditions
involving the presence of only small quantities of salts of calcium,
the decay of organisms on the sea-floor might lead to a deposition of
all the magnesium salts, following on those of calcium, both coming
down in the form of carbonates.
The experimental work of Pfaff[16] should be considered in connexion
with Daly's suggestions, since means are there indicated whereby basic
magnesium carbonate, precipitated from sea-water, may associate itself
with calcium carbonate to form dolomite; shallow-water conditions, with
concentration by evaporation, are required.
Daly compares analyses of river-waters now running over pre-Cambrian
rocks with analyses of pre-Cambrian limestones, and the ratio of the
carbonates of magnesium and calcium is shown to be the same in both
series.
From what we have said, it now seems probable that the great majority
of dolomitic limestones owe their magnesium to substitution from
without. Direct precipitation of dolomite has, however, been invoked
to account for several cases of Permian age, such as the Magnesian
Limestone of the county of Durham. Near Sunderland, this rock is
greatly modified, containing ball-like and other concretions,
associated with frequent cavities. Traces of the original bedding
remain, running through the concretions, and marine fossils are
abundant. Conybeare and Phillips, so far back as 1822, stated that the
nodules were devoid of magnesia, though formed in a magnesian rock. In
spite of this, these objects long appeared as dolomite in collections.
E. J. Garwood[17] showed conclusively that they resulted from the
concentration of calcium carbonate in a concretionary form. The process
whereby a dolomite may thus revert towards the ordinary limestone
condition, with removal of magnesium in most cases, has been styled
"dedolomitisation." Water containing calcium sulphate after passing
through a dolomite is found to carry magnesium sulphate by a chemical
exchange. Skeats[18], moreover, points out that, under a pressure of
five atmospheres the magnesium carbonate of dolomite becomes more
soluble than the calcium carbonate in fresh water containing carbon
dioxide. The ordinary relations are thus reversed under pressure, and a
cause of dedolomitisation may be indicated.
Under the influence of contact-action from igneous rocks, dolomite may
separate into calcium carbonate, magnesium oxide, and carbon dioxide.
The magnesium oxide takes up water and yields the flaky colourless
mineral brucite. Where silica is present, either as an impurity in the
dolomite, or introduced from an invading siliceous magma, magnesium
and calcium silicates may be built up[19]. Olivine thus arises, and,
on becoming hydrated and passing into serpentine, stains the rock
in various shades of green. The calcium carbonate crystallises as a
ground of granular calcite, and the whole mass becomes a handsome
_Ophicalcite_, or serpentinous marble. The famous rock of Connemara,
used in polished slabs, has arisen through contact with intrusive
diorite.
Dolomitic limestones are liable to decay rapidly in towns, owing to
the formation of magnesium sulphate, which, as shown above, is even
more soluble in water than is the accompanying calcium sulphate. In the
country, the crystals of dolomite resist ordinary weathering by the
carbon dioxide of the rain-water better than those of calcite; and the
rock thus becomes loosened through the loss of one constituent, and
crumbles into a dolomite sand[20]. Compact dolomites, however, have
furnished some excellent building-stones for country use, since here
the more resisting mineral forms the bulk of the rock.
The _Phosphatic Limestones_ are commercially even more important.
Tricalcium orthophosphate, derived, perhaps, in the first instance
from the decay of bones of fishes and the excreta known as coprolites,
tends to become aggregated in certain limestones, as in the chalk
of Mons in Belgium and of Taplow in Buckinghamshire. The phosphate
replaces foraminiferal and other shells, and frequently forms
internal casts of fossils. In the latter case, it has replaced the
calcareous mud that first occupied the shells. The observations of the
"Challenger" expedition show that concretionary calcium phosphate is
forming among the calcareous and glauconitic oozes of existing oceans,
nodular masses collecting, in which foraminiferal shells are united and
even replaced by calcium phosphate. Where deposits of guano are formed
by sea-birds on surfaces of coral limestone, as at Christmas Island
to the south of Java and at Sombrero in the Windward Islands, calcium
phosphate becomes washed downwards and replaces part of the calcium
carbonate of the rock. The resulting phosphatic limestone is quarried
on a commercial scale, and the very existence of Christmas Island is
said to be threatened by the energy of excavators. The "phosphorites
du Quercy," well known to agriculturists in France, are accumulations
in hollows and fissures of Jurassic limestone, and are associated with
the bones of fossil mammals. But in this and in other cases there is
much doubt as to whether the phosphate is derived from the bones, or
is locally concentrated, with other impurities, such as sand and clay,
through solution of the adjacent limestone.
The most common substance that replaces calcium carbonate in limestones
is silica, in the form of _Flint_. The nodules of this material, white
on the outside and richly black within, mark bands of stratification
in the Cretaceous chalk, and are among the best known materials in
south-east England. Their fantastic forms have given rise to many
speculations. Sometimes, however, when fractured, they are clearly
seen to include the remains of fossil sponges. The sponges may be
represented merely as hollow casts; but there is abundant evidence in
other cases that they belong to genera which secreted skeletons of
amorphous (non-crystalline) silica during life.
The nodular flint has collected round the sponge, while the sponge
itself has often disappeared. G. J. Hinde[21] has shown how readily
the spicules of siliceous sponges go into solution. Even at the bottom
of existing seas they become rounded at the ends, while their canals
become enlarged. In some fossil instances, they are replaced by
calcite. W. J. Sollas[22], emphasising this point, remarks that "it may
be taken as an almost invariable rule that the replacement of organic
silica by calcite is always accompanied by a subsequent deposition
of the silica in some form or other." This subsequent deposition is
frequently at the expense of calcite in some other part of the rock.
The solid flint is a replacement of the limestone in which it occurs.
The pocket-lens will often show traces of sponge-spicules, as dull
little rods, in the translucent substance of a flint. But the
microscope shows that the mass of the flint has the structure of the
limestone in which it lies. The foraminifera and other small structural
features of the original rock are perfectly preserved in chalcedonic
(that is, minutely crystalline) silica. Larger fossils, such as thick
molluscan shells and the tests of sea-urchins, may escape alteration,
while the chalk mud, the original ooze, with which they are infilled
has become completely silicified. This explains the internal moulds
of fossils in brown oxidised flint that are found in gravel-pits on
the surface of the Chalk, and also the tubular hollows, representing
stems of crinoids, that often occur in flint from the Carboniferous
Limestone. In the latter case, the fossil remained calcareous while
the ground became silicified, and the fossil was removed by subsequent
solution.
Where great thicknesses of strata, as may happen in the Carboniferous
Limestone, have become thus silicified, it may be presumed that
siliceous skeletons were unusually abundant in the mass. But, as
L. Cayeux[23] observes, such skeletons may be in one case entirely
removed, and in another represented by massive flints; in yet
another case, the silica may remain disseminated through the rock.
The irregularity of its segregation is shown by the growth of flints
in branching or hook-like forms, running from one bed to another in a
limestone.
Oolitic limestones and the skeletons of corals, both having been
originally made of aragonite, are often replaced by flint, forming
conclusive instances, appreciable by the naked eye, of the secondary
origin of this form of silica. Traces of diatoms are comparatively
rare, though they probably contributed to the silicification of the
freshwater Calcaire de la Brie of the Paris basin. Radiolaria, however,
have now been well recognised as flint-formers, even in dark "cherts"
of Silurian age. Radiolarian cherts have been taken as an indication
that the beds in which they occur were formed in oceanic depths.
It is difficult to determine the stage in the history of a rock at
which silicification has set in. As A. Jukes-Browne[24] remarks,
solution of the silica skeletons may be accelerated by pressure,
_i.e._ by the depth of water in which the bed accumulated. Yet, in
comparison with the calcareous shells of foraminifera, radiolarian and
diatomaceous remains are only slowly soluble, and are found in the
deepest spots reached by soundings. H. B. Guppy[25], on the other hand,
has observed silicification of modern corals in reefs in the Fijis, and
believes that the process went on during the elevation of the area,
when waters containing silica became concentrated, and parts of the
mass were exposed to evaporation.
The instability of the non-crystalline siliceous skeletons in
geological time makes it probable that a rock cannot long retain them
when buried among other strata in the earth.
It is clear that there is no support for the view, current from the
time of James Hutton onwards, that nodular flints are formed by matter
in hot solutions entering pre-existing cavities in limestone rocks. But
there must be cases where the silicification of limestone has arisen
through its penetration by hot springs. The presence of tabular flint
in joints of the Chalk shows that water has imported silica along easy
lines of passage from some other portion of the rock. Just as stems of
trees become replaced by chalcedonic silica, so may beds of limestone
be converted into flint, especially in volcanic areas. A. W. Rogers[26]
records that recent limestones formed in the Cape province by the
evaporation of ascending waters have already become silicified. These
flinty rocks have been found in the Kalahari Desert and elsewhere,
though not south of the Orange River; the chemical change is probably
due to the character of local water rather than to temperature. Yet
it is remarkable how, in the vast majority of instances, the partial
or complete silicification of a limestone may be traced to an
intermediate resting stage of the silica in the form of skeletons of
the vegetable diatoms or the animal sponges or radiolarians.
The decay of flint itself, by the removal of part of its substance
in solution, is the cause of the white surface on specimens from the
Chalk, and of the crumbling white residues found in certain gravels.
This process has been fully discussed by J. W. Judd, who believes that
the material removed is silica in the opaline condition[27].
LIMESTONE AND SCENERY
Limestones in the field are characterised by joints which traverse
considerable thicknesses of strata, until some shaly bed is met with,
in which earth-stresses cannot set up such continuous planes of
fracture. Since the conditions of deposition may remain constant for
a long time in open seas, and since stratification cannot be obvious
until these conditions change, limestones may have a massive character
that is exceptional among sedimentary rocks. In some cases, however,
where muddy rivers in times of flood have brought in detritus from the
land, rapid and no doubt seasonal alternations of shale and limestone
may be observed.
The Chalk of north-western Europe remains typically soft, lending
itself to cliff-formation along the coast, where landslides are
frequent through undercutting from below. Were it not for the
development of flints along stratification-planes, it would be
impossible at a distance to detect any bedded structure in the rock.
Its representatives in eastern France, in the north zone of the Alps,
or in the central Apennines, are compressed into far more resisting
masses, and rear themselves as terraced crags and sheer rock-walls, in
which the structure due to vertical joints is paramount. The English
Chalk weathers into round-backed downs, clothed with thin grass, and
hollowed into combes by streams that have long ago run dry. The soil
owes hardly anything but its abundant flints to the white limestone
rock on which it lies. Residual clays and sands derived from the
breaking up of later beds allow of cultivation here and there, and
beechwoods flourish even on the crests of the high downs. But water
sinks freely into the ground, and may so far saturate the mass as to
appear again in wet seasons in hollows of the surface as temporary
springs or "bournes." When deep wells are sunk and pumping is begun,
it is found that the supply varies greatly in different spots under
seemingly uniform conditions. Even in so permeable a mass, there are
waterways where maximum flow occurs. Channels where water soaks in
from above, or weak places in the roofs of underground watercourses,
become marked at the surface by sinkings known as swallow-holes. These
increase in size with time, and are abandoned to the growth of scrub
and trees.
Among more consolidated limestones, as we have hinted, the joints
are effective in promoting bold rock-scenery. The absorptive power
of the rock, rather than its hardness, prevents it from being washed
away. Water that might round the edges of escarpments and send down
taluses to modify the slopes sinks into the ground and works out
passages by solution. On level surfaces, the solubility of limestone
in water charged with carbon dioxide from the atmosphere is apparent
by the formation of pitted hollows, with edges between them that grow
sharper until they are worn through. Where a rain-drop first secures a
resting-place, its successors deepen the little hollow. Water lies in
this after every shower, working its way gently downwards. In time the
rock may seem bored into as if prepared for blasting; the holes unite
to form vertical grooves, and the surface is cut deeply into fantastic
forms.
The face of the rock, formed by weathering on a valley-side or towards
the sea, or occurring on any mass that is being cut back and reduced
by denudation, is likely to be vertical, or at any rate perpendicular
to the bedding. The form of the surfaces of the beds is perpetuated by
their fairly uniform lowering through solution. The result is that
stratification surfaces and planes perpendicular to them control in a
very marked degree the scenery of limestone lands (Fig. 1).
[Illustration: Fig. 1. Surface of Limestone Plateau. Causse du Larzac,
Aveyron, France.]
Where the beds are level, with occasional partings of a slightly
different composition, the country will develop terraces, like those of
the Burren in northern Clare. Where they are folded, as in the Juras,
scarps and dip-slopes follow one another picturesquely, the weathered
edge of the bed, the true escarpment, being sometimes at an angle
as steep as that of the dip. Hence a false effect of sharp peaks is
produced, when these "edges" are seen end on at a distance.
The terrace-structure may be seen in miniature forms upon a rocky
shore, where the blocks loosened from the escarpments of the successive
beds are carried away by the waves. Frost-action is powerful in larger
instances, and sends down huge blocks upon the lower terraces. A
combination of shale bands and massive limestones, especially with a
dip outward from the highland, leads to destructive landslips, since
the sloping surface of shale is lubricated by water that passes through
the limestone (see Fig. 9). Outward slips of the coast are thus common
in Antrim, and have been extensive near Axmouth, two regions where
chalk rests upon Liassic clays.
Broken ground, then, occurs freely under limestone scarps, and the
falling blocks often prevent the growth of trees. The freshness of the
rock-face above and of the talus below calls attention to spots where
denudation is most active. Differences in the constitution of the beds
are indicated by differences of the slope formed by denudation on the
rocky walls. The huge cañons of Arizona afford effective illustrations.
[Illustration: Fig. 2. Ravine in Limestone. Cañon of the Dourbie,
Aveyron, France.]
These cañons owe much of their character to the presence of vertically
jointed limestone. The small rainfall of the region has allowed the
rivers to deepen their channels ahead of the wearing back of the walls.
Yet even where valleys are widened by rain and other atmospheric
agents, those formed in limestone will maintain the character of
ravines. In the valley-sides of Derbyshire, or of the Franconian
plateau, or of the Arve near Sallanches, where the crags rise a mile
or more above the stream, we see how cañon-cutting is assisted by the
joints in limestone. The ravine of the Dourbie, east of Millau in
Aveyron, in the romantic region of the Causses, is a winding gorge two
thousand feet in depth (Fig. 2). That of the Tarn, a little to the
north, has only recently been penetrated by a road, cut out for the
most part in a vertical rock-wall.
When we observe, especially from the stream itself, the details of
these sheer valley-sides excavated in limestone, we again and again
detect evidences of solution. High above the present water-level,
the rocks are rounded, and are often undercut, so that they overhang
(Fig. 3). In Millersdale in Derbyshire, above grass-grown taluses,
the surface is still smooth to the hand, and we can picture the water
swirling against it, and washing it away, as it does now in the bottom
of the grim ravines of Carniola. It has been suggested, indeed, that
some limestone cañons represent underground waterways, the roofs of
which have fallen in. This may be true of the fine gorge of Cheddar,
and in some cases is proved by the existence of rock-arches bridging
across the hollow of the stream.
[Illustration: Fig. 3. Waterworn Cliff of Limestone. Ravine of
Millersdale, Derbyshire.]
The characters of an unmitigated limestone region are best seen
when we travel east of the Adriatic. Here what have been styled the
_karst_ landscapes become prominent, and may be followed through the
Greek isles to the Levant. Something of the kind is realised in the
terraced lands between the Rhône and the upper reaches of the Durance;
lavender bushes form dull-green spots on almost barren hills, and
the grey walls of old stone-built towns are barely distinguishable
against equally grey hillsides. But towards Trieste the limestone
lands are barer still. The small amount of insoluble matter yielded
by the rock may accumulate in swallow-holes, which are here called
"dolinas," a Slavonic word really meaning valleys. This residue
appears in the dolinas as a red clayey earth, the "terra rossa" of the
Italian-speaking Dalmatian coast. But on the surface of the plateaus it
is washed or blown away as soon as it is extracted from the limestone.
A. Grund[28] has suggested that the frequency of frost-action in
more northern areas allows surfaces of limestone to be cumbered with
loose blocks among which soil-patches may gather; hence we do not
find karst-features on the plateaus of central Bavaria, Champagne,
or the Cotteswold Hills. Something approaching to a karst appears in
the wind-swept levels of southern Galway and of Clare, and exposure
to strong winds has probably a good deal to do with the origin of the
Causses and the Illyrian karstlands. At the same time, the amount
of impurity in the limestone must strongly influence the resulting
landscape. The noble woods in the limestone hollows of southern Ireland
are rendered possible by the clay soils derived from the limestone, as
much as by the sheltered nature of the ground.
[Illustration: Fig. 4. Limestone Country Dissected by Ravines.
Karstland of Hercegovina, from the Maklen Pass.]
In typical karstlands, water sinks in, and emerges again on low ground,
where the surface-forms cut the level of the subterranean water-table.
Streams that manage to hold their own for a time on the uplands often
disappear into the clefts. Marshes may occur in hollows, but may have
no outlet, except in vertical directions, upwards by evaporation
and downwards through the dolinas. The dolinas correspond, as the
Slavonic shepherds so aptly perceived, to the river-valleys of more
normal areas. The landscape of flowing streams has to be sought for
in a mysterious underworld, of which we can gain only a few glimpses.
What we know is largely due to explorers of singular enterprise and
resource, notably E. A. Martel and the "spelæologists" whom he has
inspired.
A view over the plateau of Hercegovina shows us how deep gorges, rather
than ordinary river-valleys, are prevalent where important streams run
across a karstland (Fig. 4). The roads are carried, where possible,
along the ravines, and the country possesses a double life, that of
the broad uplands, where tanks have to be made to preserve the water,
and that along the commercial highways, four or five thousand feet
below. Even beside the rivers there is a sense of desolation in the
barren whiteness of the rocks. The sunlight strikes on the wall of
some theatre of the limestone, carved out in old times by a side-swirl
of the stream, and the hollow glares like a white furnace in the
hills. The river in summer shrinks among broad stony reaches, to which
thin-flanked sheep are driven for a scanty pasture. Its clear green
water gives no promise of alluvium for its banks. Limestone, even in
temperate Europe, may create the features of a desert land.
The most extraordinary rock-scenery in Europe is due to limestone in
the dolomitic state. It is not clear if the crags and pinnacles of
Tyrol are caused by the change from calcium carbonate into dolomite,
whereby a granular mass has arisen, weathering freely along its
vertical joints. It may well be that these compact limestones have
developed an exceptionally jointed structure under earth-stresses, and
that faulting has intensified their tendency to break up into fort-like
blocks. Stratified masses of more normal Rhætic limestones often
provide a terraced structure near the mountain-crests; but in thousands
of feet of underlying dolomite vertical clefts prevail entirely over
planes of bedding. If, as is extremely probable, these dolomite-rocks
arose from the composite masses that we style coral-reefs,
stratification was none the less a marked feature as their limestone
grew in thickness. This structure is still plainly visible; but the
joints have been widened, and the mass is cut up into stupendous
pinnacles and dominating towers. The Drei Zinnen near Landro, the
deeply notched wall of the Langkofel and the Plattkofel, rising four
thousand feet above a grassy upland of normal Lower Triassic strata,
and the overhanging crests of the Sett Sass above Buchenstein, are
types of a country where dolomite is pre-eminent, and where the zone of
steep rock-weathering is marked by the most fantastic forms.
ON MARBLES
Any limestone the markings or colour of which render it suitable for
ornamental purposes passes as a _Marble_. "Fossil marbles" are often
mere grey limestones, in which the stems of crinoids, or the curved
sections of shells, or the radiating patterns due to corals, please
the eye with their variety on a polished surface. The Purbeck Marble
that was so much used as a grey foil to the massive white columns of
cathedrals throughout England is simply a freshwater limestone, of no
great merit as a building stone, crowded with the shells of Paludina.
The black marbles are limestones coloured by one or two per cent. of
carbon, derived from the decay of organisms, and white shells may
stand out in them conspicuously, in contrast with the ground. The
red marbles of Plymouth and of Cork have become iron-stained, and at
the same time secondary crystallisation has destroyed many of their
original features. In Little Island, near Cork city, earth-movements
have crushed the mass, which in consequence shows signs of solid flow.
The breaking of a crystalline limestone under such stresses furnishes
us with many handsome marble _Breccias_. The abrupt juxtaposition of
angular masses of various colours, torn from beds originally distinct,
renders some of these rocks almost too startling for the decoration of
rooms of moderate size.
There seems no such thing in nature as amorphous carbonate of lime, and
all limestones are therefore formed of crystalline particles; but the
further crystallisation of this material produces a true marble, in
which all traces of fossils may be lost. Heat and pressure underground
probably facilitate this change, since even soft chalk is converted
by igneous dykes into granular marble. But where the pressure is
accompanied by the possibility of movement, the shearing action breaks
down the grains, and a more delicate structure results.
We have already seen (p. 35) how dolomite may undergo striking
mineral changes through advanced metamorphic action. Lime-garnets,
wollastonite, diopside, and other silicates similarly develop in
ordinary limestones exposed to the intrusion of an igneous magma. The
extreme changes in such rocks will be described when amphibolites are
dealt with.