CONSOLIDATION OF STRATA AND PETRIFACTION OF FOSSILS.
Chemical and mechanical deposits--Cementing together of
particles--Hardening by exposure to air--Concretionary
nodules--Consolidating effects of pressure--Mineralization of organic
remains--Impressions and casts how formed--Fossil wood--Goeppert's
experiments--Precipitation of stony matter most rapid where
putrefaction is going on--Source of lime in solution--Silex derived
from decomposition of felspar--Proofs of the lapidification of some
fossils soon after burial, of others when much decayed.
Having spoken in the preceding chapters of the characters of sedimentary
formations, both as dependent on the deposition of inorganic matter and the
distribution of fossils, I may next treat of the consolidation of
stratified rocks, and the petrifaction of imbedded organic remains.
_Chemical and mechanical deposits._--A distinction has been made by
geologists between deposits of a chemical, and those of a mechanical,
origin. By the latter name are designated beds of mud, sand, or pebbles
produced by the action of running water, also accumulations of stones and
scoriae thrown out by a volcano, which have fallen into their present place
by the force of gravitation. But the matter which forms a chemical deposit
has not been mechanically suspended in water, but in a state of solution
until separated by chemical action. In this manner carbonate of lime is
often precipitated upon the bottom of lakes and seas in a solid form, as
may be well seen in many parts of Italy, where mineral springs abound, and
where the calcareous stone, called travertin, is deposited. In these
springs the lime is usually held in solution by an excess of carbonic acid,
or by heat if it be a hot spring, until the water, on issuing from the
earth, cools or loses part of its acid. The calcareous matter then falls
down in a solid state, encrusting shells, fragments of wood and leaves, and
binding them together.[34-A]
In coral reefs, large masses of limestone are formed by the stony skeletons
of zoophytes; and these, together with shells, become cemented together by
carbonate of lime, part of which is probably furnished to the sea-water by
the decomposition of dead corals. Even shells of which the animals are
still living, on these reefs, are very commonly found to be encrusted over
with a hard coating of limestone.[34-B]
If sand and pebbles are carried by a river into the sea, and these are
bound together immediately by carbonate of lime, the deposit may be
described as of a mixed origin, partly chemical, and partly mechanical.
Now, the remarks already made in Chapter II. on the original horizontality
of strata are strictly applicable to mechanical deposits, and only
partially to those of a mixed nature. Such as are purely chemical may be
formed on a very steep slope, or may even encrust the vertical walls of a
fissure, and be of equal thickness throughout; but such deposits are of
small extent, and for the most part confined to veinstones.
_Cementing of particles._--It is chiefly in the case of calcareous rocks
that solidification takes place at the time of deposition. But there are
many deposits in which a cementing process comes into operation long
afterwards. We may sometimes observe, where the water of ferruginous or
calcareous springs has flowed through a bed of sand or gravel, that iron
or carbonate of lime has been deposited in the interstices between the
grains or pebbles, so that in certain places the whole has been bound
together into a stone, the same set of strata remaining in other parts
loose and incoherent.
Proofs of a similar cementing action are seen in a rock at Kelloway in
Wiltshire. A peculiar band of sandy strata, belonging to the group called
Oolite by geologists, may be traced through several counties, the sand
being for the most part loose and unconsolidated, but becoming stony near
Kelloway. In this district there are numerous fossil shells which have
decomposed, having for the most part left only their casts. The calcareous
matter hence derived has evidently served, at some former period, as a
cement to the siliceous grains of sand, and thus a solid sandstone has been
produced. If we take fragments of many other argillaceous grits, retaining
the casts of shells, and plunge them into dilute muriatic or other acid, we
see them immediately changed into common sand and mud; the cement of lime,
derived from the shells, having been dissolved by the acid.
Traces of impressions and casts are often extremely faint. In some loose
sands of recent date we meet with shells in so advanced a stage of
decomposition as to crumble into powder when touched. It is clear that
water percolating such strata may soon remove the calcareous matter of the
shell; and, unless circumstances cause the carbonate of lime to be again
deposited, the grains of sand will not be cemented together; in which case
no memorial of the fossil will remain. The absence of organic remains from
many aqueous rocks may be thus explained; but we may presume that in many
of them no fossils were ever imbedded, as there are extensive tracts on the
bottoms of existing seas even of moderate depth on which no fragment of
shell, coral, or other living creature can be detected by dredging. On the
other hand, there are depths where the zero of animal life has been
approached; as, for example, in the Mediterranean, at the depth of about
230 fathoms, according to the researches of Prof. E. Forbes. In the AEgean
Sea a deposit of yellowish mud of a very uniform character, and closely
resembling chalk, is going on in regions below 230 fathoms, and this
formation must be wholly devoid of organic remains.[35-A]
In what manner silex and carbonate of lime may become widely diffused in
small quantities through the waters which permeate the earth's crust will
be spoken of presently, when the petrifaction of fossil bodies is
considered; but I may remark here that such waters are always passing in
the case of thermal springs from hotter to colder parts of the interior of
the earth; and as often as the temperature of the solvent is lowered,
mineral matter has a tendency to separate from it and solidify. Thus a
stony cement is often supplied to any sand, pebbles, or fragmentary
mixture. In some conglomerates, like the pudding-stone of Hertfordshire,
pebbles of flint and grains of sand are united by a siliceous cement so
firmly, that if a block be fractured the rent passes as readily through the
pebbles as through the cement.
It is probable that many strata became solid at the time when they emerged
from the waters in which they were deposited, and when they first formed a
part of the dry land. A well-known fact seems to confirm this idea: by far
the greater number of the stones used for building and road-making are much
softer when first taken from the quarry than after they have been long
exposed to the air; and these, when once dried, may afterwards be immersed
for any length of time in water without becoming soft again. Hence it is
found desirable to shape the stones which are to be used in architecture
while they are yet soft and wet, and while they contain their
"quarry-water," as it is called; also to break up stone intended for roads
when soft, and then leave it to dry in the air for months that it may
harden. Such induration may perhaps be accounted for by supposing the
water, which penetrates the minutest pores of rocks, to deposit, on
evaporation, carbonate of lime, iron, silex, and other minerals previously
held in solution, and thereby to fill up the pores partially. These
particles, on crystallizing, would not only be themselves deprived of
freedom of motion, but would also bind together other portions of the rock
which before were loosely aggregated. On the same principle wet sand and
mud become as hard as stone when frozen; because one ingredient of the
mass, namely, the water, has crystallized, so as to hold firmly together
all the separate particles of which the loose mud and sand were composed.
Dr. MacCulloch mentions a sandstone in Skye, which may be moulded like
dough when first found; and some simple minerals, which are rigid and as
hard as glass in our cabinets, are often flexible and soft in their native
beds; this is the case with asbestos, sahlite, tremolite, and chalcedony,
and it is reported also to happen in the case of the beryl.[36-A]
The marl recently deposited at the bottom of Lake Superior, in North
America, is soft, and often filled with freshwater shells; but if a
piece be taken up and dried, it becomes so hard that it can only be
broken by a smart blow of the hammer. If the lake therefore was drained,
such a deposit would be found to consist of strata of marlstone, like
that observed in many ancient European formations, and like them
containing freshwater shells.[36-B]
It is probable that some of the heterogeneous materials which rivers
transport to the sea may at once set under water, like the artificial
mixture called pozzolana, which consists of fine volcanic sand charged
with about 20 per cent. of oxide of iron, and the addition of a small
quantity of lime. This substance hardens, and becomes a solid stone in
water, and was used by the Romans in constructing the foundations of
buildings in the sea.
Consolidation in these cases is brought about by the action of chemical
affinity on finely comminuted matter previously suspended in water. After
deposition similar particles seem to exert a mutual attraction on each
other, and congregate together in particular spots, forming lumps, nodules,
and concretions. Thus in many argillaceous deposits there are calcareous
balls, or spherical concretions, ranged in layers parallel to the general
stratification; an arrangement which took place after the shale or marl had
been thrown down in successive laminae; for these laminae are often traced
in the concretions, remaining parallel to those of the surrounding
unconsolidated rock. (See fig. 55.) Such nodules of limestone have often a
shell or other foreign body in the centre.[37-A]
[Illustration: Fig. 55. Calcareous nodules in Lias.]
Among the most remarkable examples of concretionary structure are those
described by Professor Sedgwick as abounding in the magnesian limestone
of the north of England. The spherical balls are of various sizes, from
that of a pea to a diameter of several feet, and they have both a
concentric and radiated structure, while at the same time the laminae of
original deposition pass uninterruptedly through them. In some cliffs
this limestone resembles a great irregular pile of cannon balls. Some of
the globular masses have their centre in one stratum, while a portion of
their exterior passes through to the stratum above or below. Thus the
larger spheroid in the annexed section (fig. 56.) passes from the
stratum _b_ upwards into _a_. In this instance we must suppose the
deposition of a series of minor layers, first forming the stratum _b_,
and afterwards the incumbent stratum _a_; then a movement of the
particles took place, and the carbonates of lime and magnesia separated
from the more impure and mixed matter forming the still unconsolidated
parts of the stratum. Crystallization, beginning at the centre, must
have gone on forming concentric coats, around the original nucleus
without interfering with the laminated structure of the rock.
[Illustration: Fig. 56. Spheroidal concretions in magnesian limestone.]
When the particles of rocks have been thus re-arranged by chemical forces,
it is sometimes difficult or impossible to ascertain whether certain lines
of division are due to original deposition or to the subsequent aggregation
of similar particles. Thus suppose three strata of grit, A, B, C, are
charged unequally with calcareous matter, and that B is the most
calcareous. If consolidation takes place in B, the concretionary action may
spread upwards into a part of A, where the carbonate of lime is more
abundant than in the rest; so that a mass, _d_, _e_, _f_, forming a portion
of the superior stratum, becomes united with B into one solid mass of
stone. The original line of division _d_, _e_, being thus effaced, the line
_d_, _f_, would generally be considered as the surface of the bed B, though
not strictly a true plane of stratification.
[Illustration: Fig. 57. Block section.]
_Pressure and heat._--When sand and mud sink to the bottom of a deep
sea, the particles are not pressed down by the enormous weight of the
incumbent ocean; for the water, which becomes mingled with the sand and
mud, resists pressure with a force equal to that of the column of fluid
above. The same happens in regard to organic remains which are filled
with water under great pressure as they sink, otherwise they would be
immediately crushed to pieces and flattened. Nevertheless, if the
materials of a stratum remain in a yielding state, and do not set or
solidify, they will be gradually squeezed down by the weight of other
materials successively heaped upon them, just as soft clay or loose sand
on which a house is built may give way. By such downward pressure
particles of clay, sand, and marl, may become packed into a smaller
space, and be made to cohere together permanently.
Analogous effects of condensation may arise when the solid parts of the
earth's crust are forced in various directions by those mechanical
movements afterwards to be described, by which strata have been bent,
broken, and raised above the level of the sea. Rocks of more yielding
materials must often have been forced against others previously
consolidated, and, thus compressed, may have acquired a new structure. A
recent discovery may help us to comprehend how fine sediment derived
from the detritus of rocks may be solidified by mere pressure. The
graphite or "black lead" of commerce having become very scarce, Mr.
Brockedon contrived a method by which the dust of the purer portions of
the mineral found in Borrowdale might be recomposed into a mass as dense
and compact as native graphite. The powder of graphite is first
carefully prepared and freed from air, and placed under a powerful press
on a strong steel die, with air-tight fittings. It is then struck
several blows, each of a power of 1000 tons; after which operation the
powder is so perfectly solidified that it can be cut for pencils, and
exhibits when broken the same texture as native graphite.
But the action of heat at various depths in the earth is probably the most
powerful of all causes in hardening sedimentary strata. To this subject I
shall refer again when treating of the metamorphic rocks, and of the slaty
and jointed structure.
_Mineralization of organic remains._--The changes which fossil organic
bodies have undergone since they were first imbedded in rocks, throw
much light on the consolidation of strata. Fossil shells in some modern
deposits have been scarcely altered in the course of centuries, having
simply lost a part of their animal matter. But in other cases the shell
has disappeared, and left an impression only of its exterior, or a cast
of its interior form, or thirdly, a cast of the shell itself, the
original matter of which has been removed. These different forms of
fossilization may easily be understood if we examine the mud recently
thrown out from a pond or canal in which there are shells. If the mud be
argillaceous, it acquires consistency on drying, and on breaking open a
portion of it we find that each shell has left impressions of its
external form. If we then remove the shell itself, we find within a
solid nucleus of clay, having the form of the interior of the shell.
This form is often very different from that of the outer shell. Thus a
cast such as _a_, fig. 58., commonly called a fossil screw, would never
be suspected by an inexperienced conchologist to be the internal shape
of the fossil univalve, _b_, fig. 58. Nor should we have imagined at
first sight that the shell _a_ and the cast _b_, fig. 59., were
different parts of the same fossil. The reader will observe, in the
last-mentioned figure (_b_, fig. 59.), that an empty space shaded dark,
which the _shell itself_ once occupied, now intervenes between the
enveloping stone and the cast of the smooth interior of the whorls. In
such cases the shell has been dissolved and the component particles
removed by water percolating the rock. If the nucleus were taken out a
hollow mould would remain, on which the external form of the shell with
its tubercles and striae, as seen in _a_, fig. 59., would be seen
embossed. Now if the space alluded to between the nucleus and the
impression, instead of being left empty, has been filled up with
calcareous spar, flint, pyrites, or other mineral, we then obtain from
the mould an exact cast both of the external and internal form of the
original shell. In this manner silicified casts of shells have been
formed; and if the mud or sand of the nucleus happen to be incoherent,
or soluble in acid, we can then procure in flint an empty shell, which
in shape is the exact counterpart of the original. This cast may be
compared to a bronze statue, representing merely the superficial form,
and not the internal organization; but there is another description of
petrifaction by no means uncommon, and of a much more wonderful kind,
which may be compared to certain anatomical models in wax, where not
only the outward forms and features, but the nerves, blood-vessels, and
other internal organs are also shown. Thus we find corals, originally
calcareous, in which not only the general shape, but also the minute and
complicated internal organization are retained in flint.
[Illustration: Fig. 58. _Phasianella Heddingtonensis_, and cast of the
same. Coral Rag.]
[Illustration: Fig. 59. _Trochus Anglicus_ and cast. Lias.]
Such a process of petrifaction is still more remarkably exhibited in fossil
wood, in which we often perceive not only the rings of annual growth, but
all the minute vessels and medullary rays. Many of the minute pores and
fibres of plants, and even those spiral vessels which in the living
vegetable can only be discovered by the microscope, are preserved. Among
many instances, I may mention a fossil tree, 72 feet in length, found at
Gosforth near Newcastle, in sandstone strata associated with coal. By
cutting a transverse slice so thin as to transmit light, and magnifying it
about fifty-five times, the texture seen in fig. 60. is exhibited. A
texture equally minute and complicated has been observed in the wood of
large trunks of fossil trees found in the Craigleith quarry near Edinburgh,
where the stone was not in the slightest degree siliceous, but consisted
chiefly of carbonate of lime, with oxide of iron, alumina, and carbon. The
parallel rows of vessels here seen are the rings of annual growth, but in
one part they are imperfectly preserved, the wood having probably decayed
before the mineralizing matter had penetrated to that portion of the tree.
[Illustration: Fig. 60. Texture of a tree from the coal strata, magnified.
(Witham.) Transverse section.]
In attempting to explain the process of petrifaction in such cases, we
may first assume that strata are very generally permeated by water
charged with minute portions of calcareous, siliceous, and other earths
in solution. In what manner they become so impregnated will be
afterwards considered. If an organic substance is exposed in the open
air to the action of the sun and rain, it will in time putrefy, or be
dissolved into its component elements, which consist chiefly of oxygen,
hydrogen, and carbon. These will readily be absorbed by the atmosphere
or be washed away by rain, so that all vestiges of the dead animal or
plant disappear. But if the same substances be submerged in water, they
decompose more gradually; and if buried in earth, still more slowly, as
in the familiar example of wooden piles or other buried timber. Now, if
as fast as each particle is set free by putrefaction in a fluid or
gaseous state, a particle equally minute of carbonate of lime, flint, or
other mineral, is at hand and ready to be precipitated, we may imagine
this inorganic matter to take the place just before left unoccupied by
the organic molecule. In this manner a cast of the interior of certain
vessels may first be taken, and afterwards the more solid walls of the
same may decay and suffer a like transmutation. Yet when the whole is
lapidified, it may not form one homogeneous mass of stone or metal. Some
of the original ligneous, osseous, or other organic elements may remain
mingled in certain parts, or the lapidifying substance itself may be
differently coloured at different times, or so crystallized as to
reflect light differently, and thus the texture of the original body may
be faithfully exhibited.
The student may perhaps ask whether, on chemical principles, we have any
ground to expect that mineral matter will be thrown down precisely in
those spots where organic decomposition is in progress? The following
curious experiments may serve to illustrate this point. Professor
Goeppert of Breslau attempted recently to imitate the natural process of
petrifaction. For this purpose he steeped a variety of animal and
vegetable substances in waters, some holding siliceous, others
calcareous, others metallic matter in solution. He found that in the
period of a few weeks, or even days, the organic bodies thus immersed
were mineralized to a certain extent. Thus, for example, thin vertical
slices of deal, taken from the Scotch fir (_Pinus sylvestris_), were
immersed in a moderately strong solution of sulphate of iron. When they
had been thoroughly soaked in the liquid for several days they were
dried and exposed to a red-heat until the vegetable matter was burnt up
and nothing remained but an oxide of iron, which was found to have
taken the form of the deal so exactly that casts even of the dotted
vessels peculiar to this family of plants were distinctly visible
under the microscope.
Another accidental experiment has been recorded by Mr. Pepys in the
Geological Transactions.[41-A] An earthen pitcher containing several quarts
of sulphate of iron had remained undisturbed and unnoticed for about a
twelvemonth in the laboratory. At the end of this time when the liquor was
examined an oily appearance was observed on the surface, and a yellowish
powder, which proved to be sulphur, together with a quantity of small
hairs. At the bottom were discovered the bones of several mice in a
sediment consisting of small grains of pyrites, others of sulphur, others
of crystallized green sulphate of iron, and a black muddy oxide of iron. It
was evident that some mice had accidentally been drowned in the fluid, and
by the mutual action of the animal matter and the sulphate of iron on each
other, the metallic sulphate had been deprived of its oxygen; hence the
pyrites and the other compounds were thrown down. Although the mice were
not mineralized, or turned into pyrites, the phenomenon shows how mineral
waters, charged with sulphate of iron, may be deoxydated on coming in
contact with animal matter undergoing putrefaction, so that atom after atom
of pyrites may be precipitated, and ready, under favourable circumstances,
to replace the oxygen, hydrogen, and carbon into which the original body
would be resolved.
The late Dr. Turner observes, that when mineral matter is in a "nascent
state," that is to say, just liberated from a previous state of chemical
combination, it is most ready to unite with other matter, and form a new
chemical compound. Probably the particles or atoms just set free are of
extreme minuteness, and therefore move more freely, and are more ready
to obey any impulse of chemical affinity. Whatever be the cause, it
clearly follows, as before stated, that where organic matter newly
imbedded in sediment is decomposing, there will chemical changes take
place most actively.
An analysis was lately made of the water which was flowing off from the
rich mud deposited by the Hooghly river in the Delta of the Ganges after
the annual inundation. This water was found to be highly charged with
carbonic acid gas holding lime in solution.[41-B] Now if newly-deposited
mud is thus proved to be permeated by mineral matter in a state of
solution, it is not difficult to perceive that decomposing organic
bodies, naturally imbedded in sediment, may as readily become petrified
as the substances artificially immersed by Professor Goeppert in various
fluid mixtures.
It is well known that the water of springs, or that which is continually
percolating the earth's crust, is rarely free from a slight admixture
either of iron, carbonate of lime, sulphur, silica, potash, or some other
earthy, alkaline, or metallic ingredient. Hot springs in particular are
copiously charged with one or more of these elements; and it is only in
their waters that silex is found in abundance. In certain cases, therefore,
especially in volcanic regions, we may imagine the flint of silicified wood
and corals to have been supplied by the waters of thermal springs. In other
instances, as in tripoli and chalk-flint, it may have been derived in great
part, if not wholly, from the decomposition of infusoria or diatomaceae,
sponges, and other bodies. But even if this be granted, we have still to
inquire whence a lake or the ocean can be constantly replenished with the
calcareous and siliceous matter so abundantly withdrawn from it by the
secretions of these zoophytes.
In regard to carbonate of lime there is no difficulty, because not only are
calcareous springs very numerous, but even rain-water has the power of
dissolving a minute portion of the calcareous rocks over which it flows.
Hence marine corals and mollusca may be provided by rivers with the
materials of their shells and solid supports. But pure silex, even when
reduced to the finest powder and boiled, is insoluble in water, except at
very high temperatures. Nevertheless Dr. Turner has well explained, in an
essay on the chemistry of geology[42-A], how the decomposition of felspar
may be a source of silex in solution. He has remarked that the siliceous
earth, which constitutes more than half the bulk of felspar, is intimately
combined with alumine, potash, and some other elements. The alkaline matter
of the felspar has a chemical affinity for water, as also for the carbonic
acid which is more or less contained in the waters of most springs. The
water therefore carries away alkaline matter, and leaves behind a clay
consisting of alumine and silica. But this residue of the decomposed
mineral, which in its purest state is called porcelain clay, is found to
contain a part only of the silica which existed in the original felspar.
The other part, therefore, must have been dissolved and removed; and this
can be accounted for in two ways; first, because silica when combined with
an alkali is soluble in water; secondly, because silica in what is
technically called its nascent state is also soluble in water. Hence an
endless supply of silica is afforded to rivers and the waters of the sea.
For the felspathic rocks are universally distributed, constituting, as they
do, so large a proportion of the volcanic, plutonic, and metamorphic
formations. Even where they chance to be absent in mass, they rarely fail
to occur in the superficial gravel or alluvial deposits of the basin of
every large river.
The disintegration of mica also, another mineral which enters largely
into the composition of granite and various sandstones, may yield
silica which may be dissolved in water, for nearly half of this mineral
consists of silica, combined with alumine, potash, and about a tenth
part of iron. The oxidation of this iron in the air is the principal
cause of the waste of mica.
We have still, however, much to learn before the conversion of fossil
bodies into stone is fully understood. Some phenomena seem to imply that
the mineralization must proceed with considerable rapidity, for stems of a
soft and succulent character, and of a most perishable nature, are
preserved in flint; and there are instances of the complete silicification
of the young leaves of a palm-tree when just about to shoot forth, and in
that state which in the West Indies is called the cabbage of the
palm.[43-A] It may, however, be questioned whether in such cases there may
not have been some antiseptic quality in the water which retarded
putrefaction, so that the soft parts of the buried substance may have
remained for a long time without disintegration, like the flesh of bodies
imbedded in peat.
Mr. Stokes has pointed out examples of petrifactions in which the more
perishable, and others where the more durable portions of wood are
preserved. These variations, he suggests, must doubtless have depended on
the time when the lapidifying mineral was introduced. Thus, in certain
silicified stems of palm-trees, the cellular tissue, that most destructible
part, is in good condition, while all signs of the hard woody fibre have
disappeared, the spaces once occupied by it being hollow or filled with
agate. Here, petrifaction must have commenced soon after the wood was
exposed to the action of moisture, and the supply of mineral matter must
then have failed, or the water must have become too much diluted before the
woody fibre decayed. But when this fibre is alone discoverable, we must
suppose that an interval of time elapsed before the commencement of
lapidification, during which the cellular tissue was obliterated. When both
structures, namely, the cellular and the woody fibre, are preserved, the
process must have commenced at an early period, and continued without
interruption till it was completed throughout.[43-B]
FOOTNOTES:
[34-A] See Principles, Index, "Calcareous Springs," &c.
[34-B] Ibid. "Travertin," "Coral Reefs," &c.
[35-A] Report Brit. Ass. 1843, p. 178.
[36-A] Dr. MacCulloch, Syst. of Geol. vol. i. p. 123.
[36-B] Princ. of Geol., Index, "Superior Lake."
[37-A] De la Beche, Geol. Researches, p. 95., and Geol. Observer
(1851), p. 686.
[41-A] Vol. i. p. 399. first series.
[41-B] Piddington, Asiat. Research. vol. xviii. p. 226.
[42-A] Jam. Ed. New Phil. Journ. No. 30. p. 246.
[43-A] Stokes, Geol. Trans., vol. v. p. 212. second series.
[43-B] Ibid.