Different Kinds of mineral Veins. — Ordinary metalliferous Veins or
Lodes. — Their frequent Coincidence with Faults. — Proofs that they
originated in Fissures in solid Rock. — Veins shifting other Veins. —
Polishing of their Walls or “Slicken sides”. — Shells and Pebbles in
Lodes. — Evidence of the successive Enlargement and Reopening of veins.
— Examples in Cornwall and in Auvergne. — Dimensions of Veins. — Why
some alternately swell out and contract. — Filling of Lodes by
Sublimation from below. — Supposed relative Age of the precious Metals.
— Copper and lead Veins in Ireland older than Cornish Tin. — Lead Vein
in Lias, Glamorganshire. — Gold in Russia, California, and Australia. —
Connection of hot Springs and mineral Veins.
INDEX
PREFACE.
THE LAST or sixth EDITION of my “Elements of Geology” was already out
of print before the end of 1868, in which year I brought out the tenth
edition of my “Principles of Geology.”
In writing the last-mentioned work I had been called upon to pass in
review almost all the leading points of speculation and controversy to
which the rapid advance of the science had given rise, and when I
proposed to bring out a new edition of the “Elements” I was strongly
urged by my friends not to repeat these theoretical discussions, but to
confine myself in the new treatise to those parts of the “Elements”
which were most indispensable to a beginner. This was to revert, to a
certain extent, to the original plan of the first edition; but I found,
after omitting a great number of subjects, that the necessity of
bringing up to the day those which remained, and adverting, however
briefly, to new discoveries, made it most difficult to confine the
proposed abridgment within moderate limits. Some chapters had to be
entirely recast, some additional illustrations to be introduced, and
figures of some organic remains to be replaced by new ones from
specimens more perfect than those which had been at my command on
former occasions. By these changes the work assumed a form so different
from the sixth edition of the “Elements,” that I resolved to give it a
new title and call it the “Student’s Elements of Geology.”
In executing this task I have found it very difficult to meet the
requirements of those who are entirely ignorant of the science. It is
only the adept who has already overcome the first steps as an observer,
and is familiar with many of the technical terms, who can profit by a
brief and concise manual. Beginners wish for a short and cheap book in
which they may find a full explanation of the leading facts and
principles of Geology. Their wants, I fear, somewhat resemble those of
the old woman in New England, who asked a bookseller to supply her with
“the cheapest Bible in the largest possible print.”
But notwithstanding the difficulty of reconciling brevity with the
copiousness of illustration demanded by those who have not yet mastered
the rudiments of the science, I have endeavoured to abridge the work in
the manner above hinted at, so as to place it within the reach of many
to whom it was before inaccessible.
CHARLES LYELL.
73 HARLEY STREET, LONDON,
_December_, 1870.
CHAPTER I.
ON THE DIFFERENT CLASSES OF ROCKS.
Geology defined. — Successive Formation of the Earth’s Crust. —
Classification of Rocks according to their Origin and Age. — Aqueous
Rocks. — Their Stratification and imbedded Fossils. — Volcanic Rocks,
with and without Cones and Craters. — Plutonic Rocks, and their
Relation to the Volcanic. — Metamorphic Rocks, and their probable
Origin. — The term Primitive, why erroneously applied to the
Crystalline Formations. — Leading Division of the Work.
Of what materials is the earth composed, and in what manner are these
materials arranged? These are the first inquiries with which Geology is
occupied, a science which derives its name from the Greek _ge_, the
earth, and _logos_, a discourse. Previously to experience we might have
imagined that investigations of this kind would relate exclusively to
the mineral kingdom, and to the various rocks, soils, and metals, which
occur upon the surface of the earth, or at various depths beneath it.
But, in pursuing such researches, we soon find ourselves led on to
consider the successive changes which have taken place in the former
state of the earth’s surface and interior, and the causes which have
given rise to these changes; and, what is still more singular and
unexpected, we soon become engaged in researches into the history of
the animate creation, or of the various tribes of animals and plants
which have, at different periods of the past, inhabited the globe.
All are aware that the solid parts of the earth consist of distinct
substances, such as clay, chalk, sand, limestone, coal, slate, granite,
and the like; but previously to observation it is commonly imagined
that all these had remained from the first in the state in which we now
see them—that they were created in their present form, and in their
present position. The geologist soon comes to a different conclusion,
discovering proofs that the external parts of the earth were not all
produced in the beginning of things in the state in which we now behold
them, nor in an instant of time. On the contrary, he can show that they
have acquired their actual configuration and condition gradually, under
a great variety of circumstances, and at successive periods, during
each of which distinct races of living beings have flourished on the
land and in the waters, the remains of these creatures still lying
buried in the crust of the earth.
By the “earth’s crust,” is meant that small portion of the exterior of
our planet which is accessible to human observation. It comprises not
merely all of which the structure is laid open in mountain precipices,
or in cliffs overhanging a river or the sea, or whatever the miner may
reveal in artificial excavations; but the whole of that outer covering
of the planet on which we are enabled to reason by observations made at
or near the surface. These reasonings may extend to a depth of several
miles, perhaps ten miles; and even then it may be said, that such a
thickness is no more than 1/400 part of the distance from the surface
to the centre. The remark is just: but although the dimensions of such
a crust are, in truth, insignificant when compared to the entire globe,
yet they are vast, and of magnificent extent in relation to man, and to
the organic beings which people our globe. Referring to this standard
of magnitude, the geologist may admire the ample limits of his domain,
and admit, at the same time, that not only the exterior of the planet,
but the entire earth, is but an atom in the midst of the countless
worlds surveyed by the astronomer.
The materials of this crust are not thrown together confusedly; but
distinct mineral masses, called rocks, are found to occupy definite
spaces, and to exhibit a certain order of arrangement. The term _rock_
is applied indifferently by geologists to all these substances, whether
they be soft or stony, for clay and sand are included in the term, and
some have even brought peat under this denomination. Our old writers
endeavoured to avoid offering such violence to our language, by
speaking of the component materials of the earth as consisting of rocks
and _soils._ But there is often so insensible a passage from a soft and
incoherent state to that of stone, that geologists of all countries
have found it indispensable to have one technical term to include both,
and in this sense we find _roche_ applied in French, _rocca_ in
Italian, and _felsart_ in German. The beginner, however, must
constantly bear in mind that the term rock by no means implies that a
mineral mass is in an indurated or stony condition.
The most natural and convenient mode of classifying the various rocks
which compose the earth’s crust, is to refer, in the first place, to
their origin, and in the second to their relative age. I shall
therefore begin by endeavouring briefly to explain to the student how
all rocks may be divided into four great classes by reference to their
different origin, or, in other words, by reference to the different
circumstances and causes by which they have been produced.
The first two divisions, which will at once be understood as natural,
are the aqueous and volcanic, or the products of watery and those of
igneous action at or near the surface.
Aqueous Rocks.—The aqueous rocks, sometimes called the sedimentary, or
fossiliferous, cover a larger part of the earth’s surface than any
others. They consist chiefly of mechanical deposits (pebbles, sand, and
mud), but are partly of chemical and some of them of organic origin,
especially the limestones. These rocks are _stratified,_ or divided
into distinct layers, or strata. The term _stratum_ means simply a bed,
or any thing spread out or _strewed_ over a given surface; and we infer
that these strata have been generally spread out by the action of
water, from what we daily see taking place near the mouths of rivers,
or on the land during temporary inundations. For, whenever a running
stream charged with mud or sand, has its velocity checked, as when it
enters a lake or sea, or overflows a plain, the sediment, previously
held in suspension by the motion of the water, sinks, by its own
gravity to the bottom. In this manner layers of mud and sand are thrown
down one upon another.
If we drain a lake which has been fed by a small stream, we frequently
find at the bottom a series of deposits, disposed with considerable
regularity, one above the other; the uppermost, perhaps, may be a
stratum of peat, next below a more dense and solid variety of the same
material; still lower a bed of shell-marl, alternating with peat or
sand, and then other beds of marl, divided by layers of clay. Now, if a
second pit be sunk through the same continuous lacustrine _formation_
at some distance from the first, nearly the same series of beds is
commonly met with, yet with slight variations; some, for example, of
the layers of sand, clay, or marl, may be wanting, one or more of them
having thinned out and given place to others, or sometimes one of the
masses first examined is observed to increase in thickness to the
exclusion of other beds.
The term _formation,_ which I have used in the above explanation,
expresses in geology any assemblage of rocks which have some character
in common, whether of origin, age, or composition. Thus we speak of
stratified and unstratified, fresh-water and marine, aqueous and
volcanic, ancient and modern, metalliferous and non-metalliferous
formations.
In the estuaries of large rivers, such as the Ganges and the
Mississippi, we may observe, at low water, phenomena analogous to those
of the drained lakes above mentioned, but on a grander scale, and
extending over areas several hundred miles in length and breadth. When
the periodical inundations subside, the river hollows out a channel to
the depth of many yards through horizontal beds of clay and sand, the
ends of which are seen exposed in perpendicular cliffs. These beds vary
in their mineral composition, or colour, or in the fineness or
coarseness of their particles, and some of them are occasionally
characterised by containing drift-wood. At the junction of the river
and the sea, especially in lagoons nearly separated by sand-bars from
the ocean, deposits are often formed in which brackish and salt-water
shells are included.
In Egypt, where the Nile is always adding to its delta by filling up
part of the Mediterranean with mud, the newly deposited sediment is
_stratified,_ the thin layer thrown down in one season differing
slightly in colour from that of a previous year, and being separable
from it, as has been observed in excavations at Cairo and other
places.[1]
When beds of sand, clay, and marl, containing shells and vegetable
matter, are found arranged in a similar manner in the interior of the
earth, we ascribe to them a similar origin; and the more we examine
their characters in minute detail, the more exact do we find the
resemblance. Thus, for example, at various heights and depths in the
earth, and often far from seas, lakes, and rivers, we meet with layers
of rounded pebbles composed of flint, limestone, granite, or other
rocks, resembling the shingles of a sea-beach or the gravel in a
torrent’s bed. Such layers of pebbles frequently alternate with others
formed of sand or fine sediment, just as we may see in the channel of a
river descending from hills bordering a coast, where the current sweeps
down at one season coarse sand and gravel, while at another, when the
waters are low and less rapid, fine mud and sand alone are carried
seaward.[2]
If a stratified arrangement, and the rounded form of pebbles, are alone
sufficient to lead us to the conclusion that certain rocks originated
under water, this opinion is farther confirmed by the distinct and
independent evidence of _fossils,_ so abundantly included in the
earth’s crust. By a _fossil_ is meant any body, or the traces of the
existence of any body, whether animal or vegetable, which has been
buried in the earth by natural causes. Now the remains of animals,
especially of aquatic species, are found almost everywhere imbedded in
stratified rocks, and sometimes, in the case of limestone, they are in
such abundance as to constitute the entire mass of the rock itself.
Shells and corals are the most frequent, and with them are often
associated the bones and teeth of fishes, fragments of wood,
impressions of leaves, and other organic substances. Fossil shells, of
forms such as now abound in the sea, are met with far inland, both near
the surface, and at great depths below it. They occur at all heights
above the level of the ocean, having been observed at elevations of
more than 8000 feet in the Pyrenees, 10,000 in the Alps, 13,000 in the
Andes, and above 18,000 feet in the Himalaya.[3]
These shells belong mostly to marine testacea, but in some places
exclusively to forms characteristic of lakes and rivers. Hence it is
concluded that some ancient strata were deposited at the bottom of the
sea, and others in lakes and estuaries.
We have now pointed out one great class of rocks, which, however they
may vary in mineral composition, colour, grain, or other characters,
external and internal, may nevertheless be grouped together as having a
common origin. They have all been formed under water, in the same
manner as modern accumulations of sand, mud, shingle, banks of shells,
reefs of coral, and the like, and are all characterised by
stratification or fossils, or by both.
Volcanic Rocks.—The division of rocks which we may next consider are
the volcanic, or those which have been produced at or near the surface
whether in ancient or modern times, not by water, but by the action of
fire or subterranean heat. These rocks are for the most part
unstratified, and are devoid of fossils. They are more partially
distributed than aqueous formations, at least in respect to horizontal
extension. Among those parts of Europe where they exhibit characters
not to be mistaken, I may mention not only Sicily and the country round
Naples, but Auvergne, Velay, and Vivarais, now the departments of Puy
de Dome, Haute Loire, and Ardêche, towards the centre and south of
France, in which are several hundred conical hills having the forms of
modern volcanoes, with craters more or less perfect on many of their
summits. These cones are composed moreover of lava, sand, and ashes,
similar to those of active volcanoes. Streams of lava may sometimes be
traced from the cones into the adjoining valleys, where they have
choked up the ancient channels of rivers with solid rock, in the same
manner as some modern flows of lava in Iceland have been known to do,
the rivers either flowing beneath or cutting out a narrow passage on
one side of the lava. Although none of these French volcanoes have been
in activity within the period of history or tradition, their forms are
often very perfect. Some, however, have been compared to the mere
skeletons of volcanoes, the rains and torrents having washed their
sides, and removed all the loose sand and scoriæ, leaving only the
harder and more solid materials. By this erosion, and by earthquakes,
their internal structure has occasionally been laid open to view, in
fissures and ravines; and we then behold not only many successive beds
and masses of porous lava, sand, and scoriæ, but also perpendicular
walls, or _dikes,_ as they are called, of volcanic rock, which have
burst through the other materials. Such dikes are also observed in the
structure of Vesuvius, Etna, and other active volcanoes. They have been
formed by the pouring of melted matter, whether from above or below,
into open fissures, and they commonly traverse deposits of _volcanic
tuff,_ a substance produced by the showering down from the air, or
incumbent waters, of sand and cinders, first shot up from the interior
of the earth by the explosions of volcanic gases.
Besides the parts of France above alluded to, there are other
countries, as the north of Spain, the south of Sicily, the Tuscan
territory of Italy, the lower Rhenish provinces, and Hungary, where
spent volcanoes may be seen, still preserving in many cases a conical
form, and having craters and often lava-streams connected with them.
There are also other rocks in England, Scotland, Ireland, and almost
every country in Europe, which we infer to be of igneous origin,
although they do not form hills with cones and craters. Thus, for
example, we feel assured that the rock of Staffa, and that of the
Giant’s Causeway, called basalt, is volcanic, because it agrees in its
columnar structure and mineral composition with streams of lava which
we know to have flowed from the craters of volcanoes. We find also
similar basaltic and other igneous rocks associated with beds of _tuff_
in various parts of the British Isles, and forming _dikes,_ such as
have been spoken of; and some of the strata through which these dikes
cut are occasionally altered at the point of contact, as if they had
been exposed to the intense heat of melted matter.
The absence of cones and craters, and long narrow streams of
superficial lava, in England and many other countries, is principally
to be attributed to the eruptions having been submarine, just as a
considerable proportion of volcanoes in our own times burst out beneath
the sea. But this question must be enlarged upon more fully in the
chapters on Igneous Rocks, in which it will also be shown, that as
different sedimentary formations, containing each their characteristic
fossils, have been deposited at successive periods, so also volcanic
sand and scoriæ have been thrown out, and lavas have flowed over the
land or bed of the sea, at many different epochs, or have been injected
into fissures; so that the igneous as well as the aqueous rocks may be
classed as a chronological series of monuments, throwing light on a
succession of events in the history of the earth.
Plutonic Rocks (_Granite,_ etc).—We have now pointed out the existence
of two distinct orders of mineral masses, the aqueous and the volcanic:
but if we examine a large portion of a continent, especially if it
contain within it a lofty mountain range, we rarely fail to discover
two other classes of rocks, very distinct from either of those above
alluded to, and which we can neither assimilate to deposits such as are
now accumulated in lakes or seas, nor to those generated by ordinary
volcanic action. The members of both these divisions of rocks agree in
being highly crystalline and destitute of organic remains. The rocks of
one division have been called Plutonic, comprehending all the granites
and certain porphyries, which are nearly allied in some of their
characters to volcanic formations. The members of the other class are
stratified and often slaty, and have been called by some the
_crystalline schists,_ in which group are included gneiss,
micaceous-schist (or mica-slate), hornblende-schist, statuary marble,
the finer kinds of roofing slate, and other rocks afterwards to be
described.
As it is admitted that nothing strictly analogous to these crystalline
productions can now be seen in the progress of formation on the earth’s
surface, it will naturally be asked, on what data we can find a place
for them in a system of classification founded on the origin of rocks.
I cannot, in reply to this question, pretend to give the student, in a
few words, an intelligible account of the long chain of facts and
reasonings from which geologists have been led to infer the nature of
the rocks in question. The result, however, may be briefly stated. All
the various kinds of granites which constitute the Plutonic family are
supposed to be of igneous or aqueo-igneous origin, and to have been
formed under great pressure, at a considerable depth in the earth, or
sometimes, perhaps, under a certain weight of incumbent ocean. Like the
lava of volcanoes, they have been melted, and afterwards cooled and
crystallised, but with extreme slowness, and under conditions very
different from those of bodies cooling in the open air. Hence they
differ from the volcanic rocks, not only by their more crystalline
texture, but also by the absence of tuffs and breccias, which are the
products of eruptions at the earth’s surface, or beneath seas of
inconsiderable depth. They differ also by the absence of pores or
cellular cavities, to which the expansion of the entangled gases gives
rise in ordinary lava.
Metamorphic, or Stratified Crystalline Rocks.—The fourth and last great
division of rocks are the crystalline strata and slates, or schists,
called gneiss, mica-schist, clay-slate, chlorite-schist, marble, and
the like, the origin of which is more doubtful than that of the other
three classes. They contain no pebbles, or sand, or scoriæ, or angular
pieces of imbedded stone, and no traces of organic bodies, and they are
often as crystalline as granite, yet are divided into beds,
corresponding in form and arrangement to those of sedimentary
formations, and are therefore said to be stratified. The beds sometimes
consist of an alternation of substances varying in colour, composition,
and thickness, precisely as we see in stratified fossiliferous
deposits. According to the Huttonian theory, which I adopt as the most
probable, and which will be afterwards more fully explained, the
materials of these strata were originally deposited from water in the
usual form of sediment, but they were subsequently so altered by
subterranean heat, as to assume a new texture. It is demonstrable, in
some cases at least, that such a complete conversion has actually taken
place, fossiliferous strata having exchanged an earthy for a highly
crystalline texture for a distance of a quarter of a mile from their
contact with granite. In some cases, dark limestones, replete with
shells and corals, have been turned into white statuary marble; and
hard clays, containing vegetable or other remains, into slates called
mica-schist or hornblende-schist, every vestige of the organic bodies
having been obliterated.
Although we are in a great degree ignorant of the precise nature of the
influence exerted in these cases, yet it evidently bears some analogy
to that which volcanic heat and gases are known to produce; and the
action may be conveniently called Plutonic, because it appears to have
been developed in those regions where Plutonic rocks are generated, and
under similar circumstances of pressure and depth in the earth.
Intensely heated water or steam permeating stratified masses under
great pressure have no doubt played their part in producing the
crystalline texture and other changes, and it is clear that the
transforming influence has often pervaded entire mountain masses of
strata.
In accordance with the hypothesis above alluded to, I proposed in the
first edition of the Principles of Geology (1833) the term
“Metamorphic” for the altered strata, a term derived from meta, _
trans,_ and morphe, _forma._
Hence there are four great classes of rocks considered in reference to
their origin—the aqueous, the volcanic, the Plutonic, and the
metamorphic. In the course of this work it will be shown that portions
of each of these four distinct classes have originated at many
successive periods. They have all been produced contemporaneously, and
may even now be in the progress of formation on a large scale. It is
not true, as was formerly supposed, that all granites, together with
the crystalline or metamorphic strata, were first formed, and therefore
entitled to be called “primitive,” and that the aqueous and volcanic
rocks were afterwards superimposed, and should, therefore, rank as
secondary in the order of time. This idea was adopted in the infancy of
the science, when all formations, whether stratified or unstratified,
earthy or crystalline, with or without fossils, were alike regarded as
of aqueous origin. At that period it was naturally argued that the
foundation must be older than the superstructure; but it was afterwards
discovered that this opinion was by no means in every instance a
legitimate deduction from facts; for the inferior parts of the earth’s
crust have often been modified, and even entirely changed, by the
influence of volcanic and other subterranean causes, while superimposed
formations have not been in the slightest degree altered. In other
words, the destroying and renovating processes have given birth to new
rocks below, while those above, whether crystalline or fossiliferous,
have remained in their ancient condition. Even in cities, such as
Venice and Amsterdam, it cannot be laid down as universally true that
the upper parts of each edifice, whether of brick or marble, are more
modern than the foundations on which they rest, for these often consist
of wooden piles, which may have rotted and been replaced one after the
other, without the least injury to the buildings above; meanwhile,
these may have required scarcely any repair, and may have been
constantly inhabited. So it is with the habitable surface of our globe,
in its relation to large masses of rock immediately below; it may
continue the same for ages, while subjacent materials, at a great
depth, are passing from a solid to a fluid state, and then
reconsolidating, so as to acquire a new texture.
As all the crystalline rocks may, in some respects, be viewed as
belonging to one great family, whether they be stratified or
unstratified, metamorphic or Plutonic, it will often be convenient to
speak of them by one common name. It being now ascertained, as above
stated, that they are of very different ages, sometimes newer than the
strata called secondary, the terms primitive and primary which were
formerly used for the whole must be abandoned, as they would imply a
manifest contradiction. It is indispensable, therefore, to find a new
name, one which must not be of chronological import, and must express,
on the one hand, some peculiarity equally attributable to granite and
gneiss (to the Plutonic as well as the _altered_ rocks), and, on the
other, must have reference to characters in which those rocks differ,
both from the volcanic and from the _unaltered_ sedimentary strata. I
proposed in the Principles of Geology (first edition, vol. iii) the
term “hypogene” for this purpose, derived from upo, _ under,_ and
ginomai, _to be,_ or _to be born_; a word implying the theory that
granite, gneiss, and the other crystalline formations are alike
_netherformed_ rocks, or rocks which have not assumed their present
form and structure at the surface. They occupy the lowest place in the
order of superposition. Even in regions such as the Alps, where some
masses of granite and gneiss can be shown to be of comparatively modern
date, belonging, for example, to the period hereafter to be described
as tertiary, they are still _underlying_ rocks. They never repose on
the volcanic or trappean formations, nor on strata containing organic
remains. They are _hypogene,_ as “being under” all the rest.
From what has now been said, the reader will understand that each of
the four great classes of rocks may be studied under two distinct
points of view; first, they may be studied simply as mineral masses
deriving their origin from particular causes, and having a certain
composition, form, and position in the earth’s crust, or other
characters both positive and negative, such as the presence or absence
of organic remains. In the second place, the rocks of each class may be
viewed as a grand chronological series of monuments, attesting a
succession of events in the former history of the globe and its living
inhabitants.
I shall accordingly proceed to treat of each family of rocks; first, in
reference to those characters which are not chronological, and then in
particular relation to the several periods when they were formed.
[1] See Principles of Geology, by the Author, Index, “Nile,” “Rivers,”
etc.
[2] See p. 44, Fig. 7.
[3] Col. R. J. Strachey found oolitic fossils 18,400 feet high in the
Himalaya.
CHAPTER II.
AQUEOUS ROCKS.—THEIR COMPOSITION AND FORMS OF STRATIFICATION.
Mineral Composition of Strata. — Siliceous Rocks. — Argillaceous. —
Calcareous. — Gypsum. — Forms of Stratification. — Original
Horizontality. — Thinning out. — Diagonal Arrangement. — Ripple-mark.
In pursuance of the arrangement explained in the last chapter, we shall
begin by examining the aqueous or sedimentary rocks, which are for the
most part distinctly stratified, and contain fossils. We may first
study them with reference to their mineral composition, external
appearance, position, mode of origin, organic contents, and other
characters which belong to them as aqueous formations, independently of
their age, and we may afterwards consider them chronologically or with
reference to the successive geological periods when they originated.
I have already given an outline of the data which led to the belief
that the stratified and fossiliferous rocks were originally deposited
under water; but, before entering into a more detailed investigation,
it will be desirable to say something of the ordinary materials of
which such strata are composed. These may be said to belong principally
to three divisions, the siliceous, the argillaceous, and the
calcareous, which are formed respectively of flint, clay, and carbonate
of lime. Of these, the siliceous are chiefly made up of sand or flinty
grains; the argillaceous, or clayey, of a mixture of siliceous matter
with a certain proportion, about a fourth in weight, of aluminous
earth; and, lastly, the calcareous rocks, or limestones, of carbonic
acid and lime.
Siliceous and Arenaceous Rocks.—To speak first of the sandy division:
beds of loose sand are frequently met with, of which the grains consist
entirely of silex, which term comprehends all purely siliceous
minerals, as quartz and common flint. Quartz is silex in its purest
form. Flint usually contains some admixture of alumina and oxide of
iron. The siliceous grains in sand are usually rounded, as if by the
action of running water. Sandstone is an aggregate of such grains,
which often cohere together without any visible cement, but more
commonly are bound together by a slight quantity of siliceous or
calcareous matter, or by oxide of iron or clay.
Pure siliceous rocks may be known by not effervescing when a drop of
nitric, sulphuric or other acid is applied to them, or by the grains
not being readily scratched or broken by ordinary pressure. In nature
there is every intermediate gradation, from perfectly loose sand to the
hardest sandstone. In _micaceous sandstones_ mica is very abundant; and
the thin silvery plates into which that mineral divides are often
arranged in layers parallel to the planes of stratification, giving a
slaty or laminated texture to the rock.
When sandstone is coarse-grained, it is usually called _ grit._ If the
grains are rounded, and large enough to be called pebbles, it becomes a
_conglomerate_ or _pudding-stone,_ which may consist of pieces of one
or of many different kinds of rock. A conglomerate, therefore, is
simply gravel bound together by cement.
Argillaceous Rocks.—Clay, strictly speaking, is a mixture of silex or
flint with a large proportion, usually about one fourth, of alumina, or
argil; but in common language, any earth which possesses sufficient
ductility, when kneaded up with water, to be fashioned like paste by
the hand, or by the potter’s lathe, is called a _clay_; and such clays
vary greatly in their composition, and are, in general, nothing more
than mud derived from the decomposition or wearing down of rocks. The
purest clay found in nature is porcelain clay, or kaolin, which results
from the decomposition of a rock composed of feldspar and quartz, and
it is almost always mixed with quartz. The kaolin of China consists of
71·15 parts of silex, 15·86 of alumine, 1·92 of lime, and 6·73 of
water;[1] but other porcelain clays differ materially, that of Cornwall
being composed, according to Boase, of nearly equal parts of silica and
alumine, with 1 per cent of magnesia.[2] _ Shale_ has also the
property, like clay, of becoming plastic in water: it is a more solid
form of clay, or argillaceous matter, condensed by pressure. It always
divides into laminæ more or less regular.
One general character of all argillaceous rocks is to give out a
peculiar, earthy odour when breathed upon, which is a test of the
presence of alumine, although it does not belong to pure alumine, but,
apparently, to the combination of that substance with oxide of iron.[3]
Calcareous Rocks.—This division comprehends those rocks which, like
chalk, are composed chiefly of lime and carbonic acid. Shells and
corals are also formed of the same elements, with the addition of
animal matter. To obtain pure lime it is necessary to calcine these
calcareous substances, that is to say, to expose them to heat of
sufficient intensity to drive off the carbonic acid, and other volatile
matter. White chalk is sometimes pure carbonate of lime; and this rock,
although usually in a soft and earthy state, is occasionally
sufficiently solid to be used for building, and even passes into a
_compact_ stone, or a stone of which the separate parts are so minute
as not to be distinguishable from each other by the naked eye.
Many limestones are made up entirely of minute fragments of shells and
coral, or of calcareous sand cemented together. These last might be
called “calcareous sandstones;” but that term is more properly applied
to a rock in which the grains are partly calcareous and partly
siliceous, or to quartzose sandstones, having a cement of carbonate of
lime.
The variety of limestone called _oolite_ is composed of numerous small
egg-like grains, resembling the roe of a fish, each of which has
usually a small fragment of sand as a nucleus, around which concentric
layers of calcareous matter have accumulated.
Any limestone which is sufficiently hard to take a fine polish is
called _marble._ Many of these are fossiliferous; but statuary marble,
which is also called saccharoid limestone, as having a texture
resembling that of loaf-sugar, is devoid of fossils, and is in many
cases a member of the metamorphic series.
_Siliceous limestone_ is an intimate mixture of carbonate of lime and
flint, and is harder in proportion as the flinty matter predominates.
The presence of carbonate of lime in a rock may be ascertained by
applying to the surface a small drop of diluted sulphuric, nitric, or
muriatic acid, or strong vinegar; for the lime, having a greater
chemical affinity for any one of these acids than for the carbonic,
unites immediately with them to form new compounds, thereby becoming a
sulphate, nitrate or muriate of lime. The carbonic acid, when thus
liberated from its union with the lime, escapes in a gaseous form, and
froths up or effervesces as it makes its way in small bubbles through
the drop of liquid. This effervescence is brisk or feeble in proportion
as the limestone is pure or impure, or, in other words, according to
the quantity of foreign matter mixed with the carbonate of lime.
Without the aid of this test, the most experienced eye cannot always
detect the presence of carbonate of lime in rocks.
The above-mentioned three classes of rocks, the siliceous,
argillaceous, and calcareous, pass continually into each other, and
rarely occur in a perfectly separate and pure form. Thus it is an
exception to the general rule to meet with a limestone as pure as
ordinary white chalk, or with clay as aluminous as that used in
Cornwall for porcelain, or with sand so entirely composed of siliceous
grains as the white sand of Alum Bay, in the Isle of Wight, employed in
the manufacture of glass, or sandstone so pure as the grit of
Fontainebleau, used for pavement in France. More commonly we find sand
and clay, or clay and marl, intermixed in the same mass. When the sand
and clay are each in considerable quantity, the mixture is called
_loam._ If there is much calcareous matter in clay it is called _marl_;
but this term has unfortunately been used so vaguely, as often to be
very ambiguous. It has been applied to substances in which there is no
lime; as, to that red loam usually called red marl in certain parts of
England. Agriculturists were in the habit of calling any soil a marl
which, like true marl, fell to pieces readily on exposure to the air.
Hence arose the confusion of using this name for soils which,
consisting of loam, were easily worked by the plough, though devoid of
lime.
_Marl slate_ bears the same relation to marl which shale bears to clay,
being a calcareous shale. It is very abundant in some countries, as in
the Swiss Alps. Argillaceous or marly limestone is also of common
occurrence.
There are few other kinds of rock which enter so largely into the
composition of sedimentary strata as to make it necessary to dwell here
on their characters. I may, however, mention two others—magnesian
limestone or dolomite, and gypsum. _Magnesian limestone_ is composed of
carbonate of lime and carbonate of magnesia; the proportion of the
latter amounting in some cases to nearly one half. It effervesces much
more slowly and feebly with acids than common limestone. In England
this rock is generally of a yellowish colour; but it varies greatly in
mineralogical character, passing from an earthy state to a white
compact stone of great hardness. _Dolomite,_ so common in many parts of
Germany and France, is also a variety of magnesian limestone, usually
of a granular texture.
_Gypsum_ is a rock composed of sulphuric acid, lime, and water. It is
usually a soft whitish-yellow rock, with a texture resembling that of
loaf-sugar, but sometimes it is entirely composed of lenticular
crystals. It is insoluble in acids, and does not effervesce like chalk
and dolomite, because it does not contain carbonic acid gas, or fixed
air, the lime being already combined with sulphuric acid, for which it
has a stronger affinity than for any other. Anhydrous gypsum is a rare
variety, into which water does not enter as a component part. _
Gypseous marl_ is a mixture of gypsum and marl. _Alabaster_ is a
granular and compact variety of gypsum found in masses large enough to
be used in sculpture and architecture. It is sometimes a pure
snow-white substance, as that of Volterra in Tuscany, well known as
being carved for works of art in Florence and Leghorn. It is a softer
stone than marble, and more easily wrought.
Forms of Stratification.—A series of strata sometimes consists of one
of the above rocks, sometimes of two or more in alternating beds.
Thus, in the coal districts of England, for example, we often pass
through several beds of sandstone, some of finer, others of coarser
grain, some white, others of a dark colour, and below these, layers of
shale and sandstone or beds of shale, divisible into leaf-like laminæ,
and containing beautiful impressions of plants. Then again we meet with
beds of pure and impure coal, alternating with shales and sandstones,
and underneath the whole, perhaps, are calcareous strata, or beds of
limestone, filled with corals and marine shells, each bed
distinguishable from another by certain fossils, or by the abundance of
particular species of shells or zoophytes.
This alternation of different kinds of rock produces the most distinct
stratification; and we often find beds of limestone and marl,
conglomerate and sandstone, sand and clay, recurring again and again,
in nearly regular order, throughout a series of many hundred strata.
The causes which may produce these phenomena are various, and have been
fully discussed in my treatise on the modern changes of the earth’s
surface.[4] It is there seen that rivers flowing into lakes and seas
are charged with sediment, varying in quantity, composition, colour,
and grain according to the seasons; the waters are sometimes flooded
and rapid, at other periods low and feeble; different tributaries,
also, draining peculiar countries and soils, and therefore charged with
peculiar sediment, are swollen at distinct periods. It was also shown
that the waves of the sea and currents undermine the cliffs during
wintry storms, and sweep away the materials into the deep, after which
a season of tranquillity succeeds, when nothing but the finest mud is
spread by the movements of the ocean over the same submarine area.
It is not the object of the present work to give a description of these
operations, repeated as they are, year after year, and century after
century; but I may suggest an explanation of the manner in which some
micaceous sandstones have originated, namely, those in which we see
innumerable thin layers of mica dividing layers of fine quartzose sand.
I observed the same arrangement of materials in recent mud deposited in
the estuary of Laroche St. Bernard in Brittany, at the mouth of the
Loire. The surrounding rocks are of gneiss, which, by its waste,
supplies the mud: when this dries at low water, it is found to consist
of brown laminated clay, divided by thin seams of mica. The separation
of the mica in this case, or in that of micaceous sandstones, may be
thus understood. If we take a handful of quartzose sand, mixed with
mica, and throw it into a clear running stream, we see the materials
immediately sorted by the water, the grains of quartz falling almost
directly to the bottom, while the plates of mica take a much longer
time to reach the bottom, and are carried farther down the stream. At
the first instant the water is turbid, but immediately after the flat
surfaces of the plates of mica are seen all alone, reflecting a silvery
light, as they descend slowly, to form a distinct micaceous lamina. The
mica is the heavier mineral of the two; but it remains a longer time
suspended in the fluid, owing to its greater extent of surface. It is
easy, therefore, to perceive that where such mud is acted upon by a
river or tidal current, the thin plates of mica will be carried
farther, and not deposited in the same places as the grains of quartz;
and since the force and velocity of the stream varies from time to
time, layers of mica or of sand will be thrown down successively on the
same area.
Original Horizontality.—It is said generally that the upper and under
surfaces of strata, or the “planes of stratification,” are parallel.
Although this is not strictly true, they make an approach to
parallelism, for the same reason that sediment is usually deposited at
first in nearly horizontal layers. Such an arrangement can by no means
be attributed to an original evenness or horizontality in the bed of
the sea: for it is ascertained that in those places where no matter has
been recently deposited, the bottom of the ocean is often as uneven as
that of the dry land, having in like manner its hills, valleys, and
ravines. Yet if the sea should go down, or be removed from near the
mouth of a large river where a delta has been forming, we should see
extensive plains of mud and sand laid dry, which, to the eye, would
appear perfectly level, although, in reality, they would slope gently
from the land towards the sea.
This tendency in newly-formed strata to assume a horizontal position
arises principally from the motion of the water, which forces along
particles of sand or mud at the bottom, and causes them to settle in
hollows or depressions where they are less exposed to the force of a
current than when they are resting on elevated points. The velocity of
the current and the motion of the superficial waves diminish from the
surface downward, and are least in those depressions where the water is
deepest.
A good illustration of the principle here alluded to may be sometimes
seen in the neighbourhood of a volcano, when a section, whether natural
or artificial, has laid open to view a succession of various-coloured
layers of sand and ashes, which have fallen in showers upon uneven
ground. Thus let A B (Fig. 1) be two ridges, with an intervening
valley. These original inequalities of the surface have been gradually
effaced by beds of sand and ashes _c, d, e,_ the surface at _e_ being
quite level. It will be seen that, although the materials of the first
layers have accommodated themselves in a great degree to the shape of
the ground A B, yet each bed is thickest at the bottom. At first a
great many particles would be carried by their own gravity down the
steep sides of A and B, and others would afterwards be blown by the
wind as they fell off the ridges, and would settle in the hollow, which
would thus become more and more effaced as the strata accumulated from
_c_ to _e._ Now, water in motion can exert this levelling power on
similar materials more easily than air, for almost all stones lose in
water more than a third of the weight which they have in air, the
specific gravity of rocks being in general as 2½ when compared to that
of water, which is estimated at 1. But the buoyancy of sand or mud
would be still greater in the sea, as the density of salt-water exceeds
that of fresh.
Fig. 2. Section of strata of sandtone, grit, and congolmerate.
Yet, however uniform and horizontal may be the surface of new deposits
in general, there are still many disturbing causes, such as eddies in
the water, and currents moving first in one and then in another
direction, which frequently cause irregularities. We may sometimes
follow a bed of limestone, shale, or sandstone, for a distance of many
hundred yards continuously; but we generally find at length that each
individual stratum thins out, and allows the beds which were previously
above and below it to meet. If the materials are coarse, as in grits
and conglomerates, the same beds can rarely be traced many yards
without varying in size, and often coming to an end abruptly. (See Fig.
2.)
Fig. 3: Section of sand at Sandy Hill, near Biggleswade, Bedfordshire.
Diagonal or Cross Stratification.—There is also another phenomenon of
frequent occurrence. We find a series of larger strata, each of which
is composed of a number of minor layers placed obliquely to the general
planes of stratification. To this diagonal arrangement the name of
“false or cross bedding” has been given. Thus in the section (Fig. 3)
we see seven or eight large beds of loose sand, yellow and brown, and
the lines _a, b, c_ mark some of the principal planes of
stratification, which are nearly horizontal. But the greater part of
the subordinate laminæ do not conform to these planes, but have often a
steep slope, the inclination being sometimes towards opposite points of
the compass. When the sand is loose and incoherent, as in the case here
represented, the deviation from parallelism of the slanting laminæ
cannot possibly be accounted for by any rearrangement of the particles
acquired during the consolidation of the rock. In what manner, then,
can such irregularities be due to original deposition? We must suppose
that at the bottom of the sea, as well as in the beds of rivers, the
motions of waves, currents, and eddies often cause mud, sand, and
gravel to be thrown down in heaps on particular spots, instead of being
spread out uniformly over a wide area. Sometimes, when banks are thus
formed, currents may cut passages through them, just as a river forms
its bed.
Fig. 4Fig. 5
Suppose the bank A (Fig. 4) to be thus formed with a steep sloping
side, and, the water being in a tranquil state, the layer of sediment
No. 1 is thrown down upon it, conforming nearly to its surface.
Afterwards the other layers, 2, 3, 4, may be deposited in succession,
so that the bank B C D is formed. If the current then increases in
velocity, it may cut away the upper portion of this mass down to the
dotted line e, and deposit the materials thus removed farther on, so as
to form the layers 5, 6, 7, 8. We have now the bank B, C, D, E (Fig.
5), of which the surface is almost level, and on which the nearly
horizontal layers, 9, 10, 11, may then accumulate. It was shown in Fig.
3 that the diagonal layers of successive strata may sometimes have an
opposite slope. This is well seen in some cliffs of loose sand on the
Suffolk coast. A portion of one of these is represented in Fig. 6,
where the layers, of which there are about six in the thickness of an
inch, are composed of quartzose grains. This arrangement may have been
due to the altered direction of the tides and currents in the same
place.
Fig. 6: Cliff between Mismer and Dunwich.
Fig. 7: Section from Monte Calvo to the sea by the valley of the
Magnan, near Nice.
The description above given of the slanting position of the minor
layers constituting a single stratum is in certain cases applicable on
a much grander scale to masses several hundred feet thick, and many
miles in extent. A fine example may be seen at the base of the Maritime
Alps near Nice. The mountains here terminate abruptly in the sea, so
that a depth of one hundred fathoms is often found within a stone’s
throw of the beach, and sometimes a depth of 3000 feet within half a
mile. But at certain points, strata of sand, marl, or conglomerate
intervene between the shore and the mountains, as in the section (Fig.
7), where a vast succession of slanting beds of gravel and sand may be
traced from the sea to Monte Calvo, a distance of no less than nine
miles in a straight line. The dip of these beds is remarkably uniform,
being always southward or towards the Mediterranean, at an angle of
about 25°. They are exposed to view in nearly vertical precipices,
varying from 200 to 600 feet in height, which bound the valley through
which the river Magnan flows. Although, in a general view, the strata
appear to be parallel and uniform, they are nevertheless found, when
examined closely, to be wedge-shaped, and to thin out when followed for
a few hundred feet or yards, so that we may suppose them to have been
thrown down originally upon the side of a steep bank where a river or
Alpine torrent discharged itself into a deep and tranquil sea, and
formed a delta, which advanced gradually from the base of Monte Calvo
to a distance of nine miles from the original shore. If subsequently
this part of the Alps and bed of the sea were raised 700 feet, the
delta may have emerged, a deep channel may then have been cut through
it by the river, and the coast may at the same time have acquired its
present configuration.
It is well known that the torrents and streams which now descend from
the Alpine declivities to the shore, bring down annually, when the snow
melts, vast quantities of shingle and sand, and then, as they subside,
fine mud, while in summer they are nearly or entirely dry; so that it
may be safely assumed that deposits like those of the valley of the
Magnan, consisting of coarse gravel alternating with fine sediment, are
still in progress at many points, as, for instance, at the mouth of the
Var. They must advance upon the Mediterranean in the form of great
shoals terminating in a steep talus; such being the original mode of
accumulation of all coarse materials conveyed into deep water,
especially where they are composed in great part of pebbles, which
cannot be transported to indefinite distances by currents of moderate
velocity. By inattention to facts and inferences of this kind, a very
exaggerated estimate has sometimes been made of the supposed depth of
the ancient ocean. There can be no doubt, for example, that the strata
_a_, Fig. 7, or those nearest to Monte Calvo, are older than those
indicated by _b_, and these again were formed before _c_; but the
vertical depth of gravel and sand in any one place cannot be proved to
amount even to 1000 feet, although it may perhaps be much greater, yet
probably never exceeding at any point 3000 or 4000 feet. But were we to
assume that all the strata were once horizontal, and that their present
dip or inclination was due to subsequent movements, we should then be
forced to conclude that a sea several miles deep had been filled up
with alternate layers of mud and pebbles thrown down one upon another.
In the locality now under consideration, situated a few miles to the
west of Nice, there are many geological data, the details of which
cannot be given in this place, all leading to the opinion that, when
the deposit of the Magnan was formed, the shape and outline of the
Alpine declivities and the shore greatly resembled what we now behold
at many points in the neighbourhood. That the beds _a, b, c, d_ are of
comparatively modern date is proved by this fact, that in seams of
loamy marl intervening between the pebbly beds are fossil shells, half
of which belong to species now living in the Mediterranean.
Fig. 8: Slab of ripple-marked (New Red) sandstone from Cheshire.
Ripple-mark.—The ripple-mark, so common on the surface of sandstones of
all ages (see Fig. 8), and which is so often seen on the sea-shore at
low tide, seems to originate in the drifting of materials along the
bottom of the water, in a manner very similar to that which may explain
the inclined layers above described. This ripple is not entirely
confined to the beach between high and low water mark, but is also
produced on sands which are constantly covered by water. Similar
undulating ridges and furrows may also be sometimes seen on the surface
of drift snow and blown sand.
The ripple-mark is usually an indication of a sea-beach, or of water
from six to ten feet deep, for the agitation caused by waves even
during storms extends to a very slight depth. To this rule, however,
there are some exceptions, and recent ripple-marks have been observed
at the depth of 60 or 70 feet. It has also been ascertained that
currents or large bodies of water in motion may disturb mud and sand at
the depth of 300 or even 450 feet.[5] Beach ripple, however, may
usually be distinguished from current ripple by frequent changes in its
direction. In a slab of sandstone, not more than an inch thick, the
furrows or ridges of an ancient ripple may often be seen in several
successive laminæ to run towards different points of the compass.
[1] W. Phillips, Mineralogy, p.33.
[2] Phil. Mag., vol. x, 1837.
[3] See W. Phillips’s Mineralogy, “Alumine.”
[4] Consult Index to Principles of Geology, “Stratification,”
“Currents,” “Deltas,” “Water,” etc.
[5] Darwin, Volcanic Islands, p. 134.
CHAPTER III.
ARRANGEMENT OF FOSSILS IN STRATA.—FRESH-WATER AND MARINE FOSSILS.
Successive Deposition indicated by Fossils. — Limestones formed of
Corals and Shells. — Proofs of gradual Increase of Strata derived from
Fossils. — Serpula attached to Spatangus. — Wood bored by Teredina. —
Tripoli formed of Infusoria. — Chalk derived principally from Organic
Bodies. — Distinction of Fresh-water from Marine Formations. — Genera
of Fresh-water and Land Shells. — Rules for recognising Marine
Testacea. — Gyrogonite and Chara. — Fresh-water Fishes. — Alternation
of Marine and Fresh-water Deposits. — Lym-Fiord.
Having in the last chapter considered the forms of stratification so
far as they are determined by the arrangement of inorganic matter, we
may now turn our attention to the manner in which organic remains are
distributed through stratified deposits. We should often be unable to
detect any signs of stratification or of successive deposition, if
particular kinds of fossils did not occur here and there at certain
depths in the mass. At one level, for example, univalve shells of some
one or more species predominate; at another, bivalve shells; and at a
third, corals; while in some formations we find layers of vegetable
matter, commonly derived from land plants, separating strata.
It may appear inconceivable to a beginner how mountains, several
thousand feet thick, can have become full of fossils from top to
bottom; but the difficulty is removed, when he reflects on the origin
of stratification, as explained in the last chapter, and allows
sufficient time for the accumulation of sediment. He must never lose
sight of the fact that, during the process of deposition, each separate
layer was once the uppermost, and immediately in contact with the water
in which aquatic animals lived. Each stratum, in fact, however far it
may now lie beneath the surface, was once in the state of shingle, or
loose sand or soft mud at the bottom of the sea, in which shells and
other bodies easily became enveloped.
Rate of Deposition indicated by Fossils.—By attending to the nature of
these remains, we are often enabled to determine whether the deposition
was slow or rapid, whether it took place in a deep or shallow sea, near
the shore or far from land, and whether the water was salt, brackish,
or fresh. Some limestones consist almost exclusively of corals, and in
many cases it is evident that the present position of each fossil
zoophyte has been determined by the manner in which it grew originally.
The axis of the coral, for example, if its natural growth is erect,
still remains at right angles to the plane of stratification. If the
stratum be now horizontal, the round spherical heads of certain species
continue uppermost, and their points of attachment are directed
downward. This arrangement is sometimes repeated throughout a great
succession of strata. From what we know of the growth of similar
zoophytes in modern reefs, we infer that the rate of increase was
extremely slow, and some of the fossils must have flourished for ages
like forest-trees, before they attained so large a size. During these
ages, the water must have been clear and transparent, for such corals
cannot live in turbid water.
Fossil Gryphæ, covered both on the outside and inside with fossil
serpulæ.
In like manner, when we see thousands of full-grown shells dispersed
everywhere throughout a long series of strata, we cannot doubt that
time was required for the multiplication of successive generations; and
the evidence of slow accumulation is rendered more striking from the
proofs, so often discovered, of fossil bodies having lain for a time on
the floor of the ocean after death before they were imbedded in
sediment. Nothing, for example, is more common than to see fossil
oysters in clay, with Serpulæ, or barnacles (acorn-shells), or corals,
and other creatures, attached to the inside of the valves, so that the
mollusk was certainly not buried in argillaceous mud the moment it
died. There must have been an interval during which it was still
surrounded with clear water, when the creatures whose remains now
adhere to it grew from an embryonic to a mature state. Attached shells
which are merely external, like some of the Serpulæ (_a_) in Fig. 9,
may often have grown upon an oyster or other shell while the animal
within was still living; but if they are found on the inside, it could
only happen after the death of the inhabitant of the shell which
affords the support. Thus, in Fig. 9, it will be seen that two Serpulæ
have grown on the interior, one of them exactly on the place where the
adductor muscle of the _Gryphæa_ (a kind of oyster) was fixed.
Fig. 10: Serpula attached to a fossil. Fig. 11: Recent Spatangus with
spines removed from one side.
Some fossil shells, even if simply attached to the _ outside_ of
others, bear full testimony to the conclusion above alluded to, namely,
that an interval elapsed between the death of the creature to whose
shell they adhere, and the burial of the same in mud or sand. The
sea-urchins, or _Echini_, so abundant in white chalk, afford a good
illustration. It is well known that these animals, when living, are
invariably covered with spines supported by rows of tubercles. These
last are only seen after the death of the sea-urchin, when the spines
have dropped off. In Fig. 11 a living species of _Spatangus_, common on
our coast, is represented with one half of its shell stripped of the
spines. In Fig. 10 a fossil of a similar and allied genus from the
white chalk of England shows the naked surface which the individuals of
this family exhibit when denuded of their bristles. The full-grown _
Serpula_, therefore, which now adheres externally, could not have begun
to grow till the _Micraster_ had died, and the spines became detached.
Fig. 12: Ananchytes from the chalk.
Now the series of events here attested by a single fossil may be
carried a step farther. Thus, for example, we often meet with a
sea-urchin (_Ananchytes_) in the chalk (see Fig. 12) which has fixed to
it the lower valve of a _Crania_, a genus of bivalve mollusca. The
upper valve (_b_, Fig. 12) is almost invariably wanting, though
occasionally found in a perfect state of preservation in white chalk at
some distance. In this case, we see clearly that the sea-urchin first
lived from youth to age, then died and lost its spines, which were
carried away. Then the young _Crania_ adhered to the bared shell, grew
and perished in its turn; after which the upper valve was separated
from the lower before the _Ananchytes_ became enveloped in chalky mud.
Fig. 13: Fossil wood bored by Teredina.
Fig. 14: Recent wood bored by Teredo.
It may be well to mention one more illustration of the manner in which
single fossils may sometimes throw light on a former state of things,
both in the bed of the ocean and on some adjoining land. We meet with
many fragments of wood bored by ship-worms at various depths in the
clay on which London is built. Entire branches and stems of trees,
several feet in length, are sometimes found drilled all over by the
holes of these borers, the tubes and shells of the mollusk still
remaining in the cylindrical hollows. In Fig. 14, _ e_, a
representation is given of a piece of recent wood pierced by the
_Teredo navalis_, or common ship-worm, which destroys wooden piles and
ships. When the cylindrical tube _d_ has been extracted from the wood,
the valves are seen at the larger or anterior extremity, as shown at
_c._ In like manner, a piece of fossil wood (_a_, Fig. 13) has been
perforated by a kindred but extinct genus, the _Teredina_ of Lamarck.
The calcareous tube of this mollusk was united and, as it were,
soldered on to the valves of the shell (_b_), which therefore cannot be
detached from the tube, like the valves of the recent _Teredo._ The
wood in this fossil specimen is now converted into a stony mass, a
mixture of clay and lime; but it must once have been buoyant and
floating in the sea, when the _ Teredinæ_ lived upon, and perforated
it. Again, before the infant colony settled upon the drift wood, part
of a tree must have been floated down to the sea by a river, uprooted,
perhaps, by a flood, or torn off and cast into the waves by the wind:
and thus our thoughts are carried back to a prior period, when the tree
grew for years on dry land, enjoying a fit soil and climate.
Strata of Organic Origin.—It has been already remarked that there are
rocks in the interior of continents, at various depths in the earth,
and at great heights above the sea, almost entirely made up of the
remains of zoophytes and testacea. Such masses may be compared to
modern oyster-beds and coral-reefs; and, like them, the rate of
increase must have been extremely gradual. But there are a variety of
stone deposits in the earth’s crust, now proved to have been derived
from plants and animals of which the organic origin was not suspected
until of late years, even by naturalists. Great surprise was therefore
created some years since by the discovery of Professor Ehrenberg, of
Berlin, that a certain kind of siliceous stone, called tripoli, was
entirely composed of millions of the remains of organic beings, which
were formerly referred to microscopic Infusoria, but which are now
admitted to be plants. They abound in rivulets, lakes, and ponds in
England and other countries, and are termed Diatomaceæ by those
naturalists who believe in their vegetable origin. The subject alluded
to has long been well-known in the arts, under the name of infusorial
earth or mountain meal, and is used in the form of powder for polishing
stones and metals. It has been procured, among other places, from the
mud of a lake at Dolgelly, in North Wales, and from Bilin, in Bohemia,
in which latter place a single stratum, extending over a wide area, is
no less than fourteen feet thick. This stone, when examined with a
powerful microscope, is found to consist of the siliceous plates or
frustules of the above-figured Diatomaceæ, united together without any
visible cement. It is difficult to convey an idea of their extreme
minuteness; but Ehrenberg estimates that in the Bilin tripoli there are
41,000 millions of individuals of the _Gaillonella distans_ (see Fig.
16) in every cubic inch (which weighs about 220 grains), or about 187
millions in a single grain. At every stroke, therefore, that we make
with this polishing powder, several millions, perhaps tens of millions,
of perfect fossils are crushed to atoms.
Figs 15 and 16: Gaillonella; Fig. 17: Bacillaria parodoxa
A well-known substance, called bog-iron ore, often met with in
peat-mosses, has often been shown by Ehrenberg to consist of
innumerable articulated threads, of a yellow ochre colour, composed of
silica, argillaceous matter, and peroxide of iron. These threads are
the cases of a minute microscopic body, called _Gaillonella ferruginea_
(Fig. 15), associated with the siliceous frustules of other fresh-water
algæ. Layers of this iron ore occurring in Scotch peat bogs are often
called “the pan,” and are sometimes of economical value.
It is clear much time must have been required for the accumulation of
strata to which countless generations of Diatomaceæ have contributed
their remains; and these discoveries lead us naturally to suspect that
other deposits, of which the materials have been supposed to be
inorganic, may in reality be composed chiefly of microscopic organic
bodies. That this is the case with the white chalk, has often been
imagined, and is now proved to be the fact. It has, moreover, been
lately discovered that the chambers into which these Foraminifera are
divided are actually often filled with thousands of well-preserved
organic bodies, which abound in every minute grain of chalk, and are
especially apparent in the white coating of flints, often accompanied
by innumerable needle-shaped spiculæ of sponges (see Chapter XVII).
“The dust we tread upon was once alive!”—BYRON.
How faint an idea does this exclamation of the poet convey of the real
wonders of nature! for here we discover proofs that the calcareous and
siliceous dust of which hills are composed has not only been once
alive, but almost every particle, albeit invisible to the naked eye,
still retains the organic structure which, at periods of time
incalculably remote, was impressed upon it by the powers of life.
Fresh-water and Marine Fossils.—Strata, whether deposited in salt or
fresh water, have the same forms; but the imbedded fossils are very
different in the two cases, because the aquatic animals which frequent
lakes and rivers are distinct from those inhabiting the sea. In the
northern part of the Isle of Wight formations of marl and limestone,
more than 50 feet thick occur, in which the shells are of extinct
species. Yet we recognise their fresh-water origin, because they are of
the same genera as those now abounding in ponds, lakes, and rivers,
either in our own country or in warmer latitudes.
In many parts of France—in Auvergne, for example—strata occur of
limestone, marl, and sandstone hundreds of feet thick, which contain
exclusively fresh-water and land shells, together with the remains of
terrestrial quadrupeds. The number of land-shells scattered through
some of these fresh-water deposits is exceedingly great; and there are
districts in Germany where the rocks scarcely contain any other fossils
except snail-shells (_helices_); as, for instance, the limestone on the
left bank of the Rhine, between Mayence and Worms, at Oppenheim,
Findheim, Budenheim, and other places. In order to account for this
phenomenon, the geologist has only to examine the small deltas of
torrents which enter the Swiss lakes when the waters are low, such as
the newly-formed plain where the Kander enters the Lake of Thun. He
there sees sand and mud strewn over with innumerable dead land-shells,
which have been brought down from the valleys in the Alps in the
preceding spring, during the melting of the snows. Again, if we search
the sands on the borders of the Rhine, in the lower part of its course,
we find countless land-shells mixed with others of species belonging to
lakes, stagnant pools, and marshes. These individuals have been washed
away from the alluvial plains of the great river and its tributaries,
some from mountainous regions, others from the low country.
Although fresh-water formations are often of great thickness, yet they
are usually very limited in area when compared to marine deposits, just
as lakes and estuaries are of small dimensions in comparison with seas.
The absence of many fossil forms usually met with in marine strata,
affords a useful negative indication of the fresh-water origin of a
formation. For example, there are no sea-urchins, no corals, no
chambered shells, such as the nautilus, nor microscopic Foraminifera in
lacustrine or fluviatile deposits. In distinguishing the latter from
formations accumulated in the sea, we are chiefly guided by the forms
of the mollusca. In a fresh-water deposit, the number of individual
shells is often as great as in a marine stratum, if not greater; but
there is a smaller variety of species and genera. This might be
anticipated from the fact that the genera and species of recent
fresh-water and land shells are few when contrasted with the marine.
Thus, the genera of true mollusca according to Woodward’s system,
excluding those altogether extinct and those without shells, amount to
446 in number, of which the terrestrial and fresh-water genera scarcely
form more than a fifth.[1]
Fig. 18: Cyrena obovata. Fig. 19: Cyrena fluminatis.
Fig. 20: Anodonta Cordierii. Fig. 21: Anodonta latimarginata. Fig. 22:
Unio littoralis.
Almost all bivalve shells, or those of acephalous mollusca, are marine,
about sixteen only out of 140 genera being fresh-water. Among these
last, the four most common forms, both recent and fossil, are _Cyclas,
Cyrena, Unio,_ and _Anodonta_ (see Figures); the two first and two last
of which are so nearly allied as to pass into each other.
Fig. 23: Gryphæa incurva.
Lamarck divided the bivalve mollusca into the Dimyary, or those having
two large muscular impressions in each valve, as _a b_ in the Cyclas,
Fig. 18, and Unio, Fig. 22, and the _ Monomyary,_ such as the oyster
and scallop, in which there is only one of these impressions, as is
seen in Fig. 23. Now, as none of these last, or the unimuscular
bivalves, are fresh-water,[2] we may at once presume a deposit
containing any of them to be marine.
Fig. 24: Planorbis enomphalus. Fig. 25: Limnæa longiscala. Fig. 26:
Pauldina lenta. Fig. 27: Succinea amphibia. Fig. 28: Ancylus velletia.
Fig. 29: Valvata piscinalis. Fig. 30: Physa hypnorum. Fig. 31:
Auricula. Fig. 32: Melania inquinata. Fig. 33: Physa columnaris. Fig.
34: Melanopsis buccinoidea.
Fig. 35: Neritina globulud. Fig. 36: Nerita granulosa.
The univalve shells most characteristic of fresh-water deposits are,
_Planorbis, Limnæa,_ and _Paludina._ (See Figures.) But to these are
occasionally added _Physa, Succinea, Ancylus, Valvata, Melanopsis,
Melania, Potamides,_ and _ Neritina_ (see Figures), the four last being
usually found in estuaries.
Fig. 37: Potamides cinctus.
Some naturalists include _Neritina_ (Fig. 35) and the marine _Nerita_
(Fig. 36) in the same genus, it being scarcely possible to distinguish
the two by good generic characters. But, as a general rule, the
fluviatile species are smaller, smoother, and more globular than the
marine; and they have never, like the _Neritæ,_ the inner margin of the
outer lip toothed or crenulated. (See Fig. 36.)
The Potamides inhabit the mouths of rivers in warm latitudes, and are
distinguishable from the marine Cerithia by their orbicular and
multispiral opercula. The genus Auricula (Fig. 31) is amphibious,
frequenting swamps and marshes within the influence of the tide.
The terrestrial shells are all univalves. The most important genera
among these, both in a recent and fossil state, are _ Helix_ (Fig. 38),
_Cyclostoma_ (Fig. 39), _Pupa_ (Fig. 40), _Clausilia_ (Fig. 41),
_Bulimus_ (Fig. 42), _ Glandina_ and _Achatina._
Fig. 38: Helix Turomensis. Fig. 39: Cyclostoma elegans. Fig. 40: Pupa
tridens. Fig. 41: Clausilia bidens. Fig. 42: Bulimus lubricus.
_Ampullaria_ (Fig. 43) is another genus of shells inhabiting rivers and
ponds in hot countries. Many fossil species formerly referred to this
genus, and which have been met with chiefly in marine formations, are
now considered by conchologists to belong to _Natica_ and other marine
genera.
Fig. 43: Ampullaria glauca.
All univalve shells of land and fresh-water species, with the exception
of _Melanopsis_ (Fig. 34), and _Achatina,_ which has a slight
indentation, have entire mouths; and this circumstance may often serve
as a convenient rule for distinguishing fresh-water from marine strata;
since, if any univalves occur of which the mouths are not entire, we
may presume that the formation is marine. The aperture is said to be
entire in such shells as the fresh-water _Ampullaria_ and the
land-shells (Figs 38-42), when its outline is not interrupted by an
indentation or notch, such as that seen at _b_ in _ Ancillaria_ (Fig.
45); or is not prolonged into a canal, as that seen at _a_ in
_Pleurotoma_ (Fig. 44).
Fig. 44: Pleurotoma exorta. Fig. 45: Ancillaria subulata.
The mouths of a large proportion of the marine univalves have these
notches or canals, and almost all species are carnivorous; whereas
nearly all testacea having entire mouths are plant-eaters, whether the
species be marine, fresh-water, or terrestrial.
There is, however, one genus which affords an occasional exception to
one of the above rules. The _Potamides_ (Fig. 37), a subgenus of
Cerithium, although provided with a short canal, comprises some species
which inhabit salt, others brackish, and others fresh-water, and they
are said to be all plant-eaters.
Among the fossils very common in fresh-water deposits are the shells of
_Cypris,_ a minute bivalve crustaceous animal.[3] Many minute living
species of this genus swarm in lakes and stagnant pools in Great
Britain; but their shells are not, if considered separately, conclusive
as to the fresh-water origin of a deposit, because the majority of
species in another kindred genus of the same order, the _Cytherina_ of
Lamarck, inhabit salt-water; and, although the animal differs slightly,
the shell is scarcely distinguishable from that of the Cypris.
Fresh-water Fossil Plants.—The seed-vessels and stems of _ Chara,_ a
genus of aquatic plants, are very frequent in fresh-water strata. These
seed-vessels were called, before their true nature was known,
gyrogonites, and were supposed to be foraminiferous shells. (See Fig.
46, _a_.)
The _Charæ_ inhabit the bottom of lakes and ponds, and flourish mostly
where the water is charged with carbonate of lime. Their seed-vessels
are covered with a very tough integument, capable of resisting
decomposition; to which circumstance we may attribute their abundance
in a fossil state. The annexed figure (Fig. 47) represents a branch of
one of many new species found by Professor Amici in the lakes of
Northern Italy. The seed-vessel in this plant is more globular than in
the British _Charæ,_) and therefore more nearly resembles in form the
extinct fossil species found in England, France, and other countries.
The stems, as well as the seed-vessels, of these plants occur both in
modern shell-marl and in ancient fresh-water formations. They are
generally composed of a large central tube surrounded by smaller ones;
the whole stem being divided at certain intervals by transverse
partitions or joints. (See _b,_ Fig. 46.)
Fig. 46: Chara medicaginula. Fig. 47: Chara elastica.
It is not uncommon to meet with layers of vegetable matter, impressions
of leaves, and branches of trees, in strata containing fresh-water
shells; and we also find occasionally the teeth and bones of land
quadrupeds, of species now unknown. The manner in which such remains
are occasionally carried by rivers into lakes, especially during
floods, has been fully treated of in the “Principles of Geology.”
Fresh-water and Marine Fish.—The remains of fish are occasionally
useful in determining the fresh-water origin of strata. Certain genera,
such as carp, perch, pike, and loach (_Cyprinus, Perca, Esox,_ and
_Cobitis_), as also _Lebias,_ being peculiar to fresh-water. Other
genera contain some fresh-water and some marine species, as _Cottus,
Mugil,_ and _Anguilla,_ or eel. The rest are either common to rivers
and the sea, as the salmon; or are exclusively characteristic of
salt-water. The above observations respecting fossil fishes are
applicable only to the more modern or tertiary deposits; for in the
more ancient rocks the forms depart so widely from those of existing
fishes, that it is very difficult, at least in the present state of
science, to derive any positive information from ichthyolites
respecting the element in which strata were deposited.
The alternation of marine and fresh-water formations, both on a small
and large scale, are facts well ascertained in geology. When it occurs
on a small scale, it may have arisen from the alternate occupation of
certain spaces by river-water and the sea; for in the flood season the
river forces back the ocean and freshens it over a large area,
depositing at the same time its sediment; after which the salt-water
again returns, and, on resuming its former place, brings with it sand,
mud, and marine shells.
There are also lagoons at the mouth of many rivers, as the Nile and
Mississippi, which are divided off by bars of sand from the sea, and
which are filled with salt and fresh water by turns. They often
communicate exclusively with the river for months, years, or even
centuries; and then a breach being made in the bar of sand, they are
for long periods filled with salt-water.
Lym-Fiord.—The Lym-Fiord in Jutland offers an excellent illustration of
analogous changes; for, in the course of the last thousand years, the
western extremity of this long frith, which is 120 miles in length,
including its windings, has been four times fresh and four times salt,
a bar of sand between it and the ocean having been often formed and
removed. The last irruption of salt water happened in 1824, when the
North Sea entered, killing all the fresh-water shells, fish, and
plants; and from that time to the present, the sea-weed _Fucus
vesiculosus,_ together with oysters and other marine mollusca, have
succeeded the _Cyclas, Lymnæa, Paludina,_ and _Charæ._[4]
But changes like these in the Lym-Fiord, and those before mentioned as
occurring at the mouths of great rivers, will only account for some
cases of marine deposits of partial extent resting on fresh-water
strata. When we find, as in the south-east of England (Chapter XVIII),
a great series of fresh-water beds, 1000 feet in thickness, resting
upon marine formations and again covered by other rocks, such as the
Cretaceous, more than 1000 feet thick, and of deep-sea origin, we shall
find it necessary to seek for a different explanation of the phenomena.
[1] See Woodward’s Manual of Mollusca, 1856.
[2] The fresh-water Mulleria, when young, forms a single exception to
the rule, as it then has two muscular impressions, but it has only one
in the adult state.
[3] For figures of fossil species of Purbeck, see Chapter XIX
[4] See Principles, Index, “Lym-Fiord.”
CHAPTER IV.
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. — Goppert’s Experiments. —
Precipitation of Stony Matter most rapid where Putrefaction is going
on. — Sources of Lime and Silex in Solution.
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 mechanical, and those of a chemical,
origin. By the name mechanical are designated beds of mud, sand, or
pebbles produced by the action of running water, also accumulations of
stones and scoriæ 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 occasionally precipitated upon the bottom of lakes
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,
incrusting shells, fragments of wood and leaves, and binding them
together.
That similar travertin is formed at some points in the bed of the sea
where calcareous springs issue cannot be doubted, but as a general rule
the quantity of lime, according to Bischoff, spread through the waters
of the ocean is very small, the free carbonic acid gas in the same
waters being five times as much as is necessary to keep the lime in a
fluid state. Carbonate of lime, therefore, can rarely be precipitated
at the bottom of the sea by chemical action alone, but must be produced
by vital agency as in the case of coral reefs.
In such 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 incrusted over with a hard coating of limestone.
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 incrust 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
vein-stones.
Consolidation of Strata.—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.
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 sand, pebbles, or
any fragmentary mixture. In some conglomerates, like the pudding-stone
of Hertfordshire (a Lower Eocene deposit), 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
crystallising, 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 crystallised, 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.[1]
The marl recently deposited at the bottom of Lake Superior, in North
America, is soft, and often filled with fresh-water 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 fresh-water shells.
Fig. 48: Calcareous nodules in Lias.
Concretionary Structure.—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 twenty 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
such cases is brought about by the action of chemical affinity on
finely comminuted matter previously suspended in water. After
deposition similar particles seem often 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 laminæ; for these
laminæ are often traceable through the concretions, remaining parallel
to those of the surrounding unconsolidated rock. (See Fig. 48.) Such
nodules of limestone have often a shell or other foreign body in the
centre.
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 laminæ 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 section (Fig. 49) passes from the
stratum _b_ upward 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. Crystallisation, beginning at the centre, must
have gone on forming concentric coats around the original nucleus
without interfering with the laminated structure of the rock.
Fig. 49: Spheroidal concretions in magnesian limestone. Fig. 50:
Section through strata of grit.
When the particles of rocks have been thus rearranged 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 several 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 upward 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.
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 hereafter 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 may thus by compression 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.
Mineralisation 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,
secondly, 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 fossilisation 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. 51, commonly called a fossil
screw, would never be suspected by an inexperienced conchologist to be
the internal shape of the fossil univalve, _ b,_ Fig. 51. Nor should we
have imagined at first sight that the shell a and the cast _b,_ Fig.
52, belong to one and the same fossil. The reader will observe, in the
last-mentioned figure (_b,_ Fig. 52), 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 striæ, as seen in _a,_ Fig. 52, 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 organisation; 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 organisation is retained in flint.
Fig. 51: Phasianella Heddingtonensis. Fig. 52: Pleurotomaria Anglica.
Fig. 53: Section of a tree from the coal-measures.
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 cells 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,
seventy-two 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, as seen in Fig. 53, 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 mineralising matter had penetrated to that portion
of the tree.
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, consisting usually of oxygen,
hydrogen, nitrogen, 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 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 crystallised 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 Goppert of Breslau, with a view of imitating the natural
process of petrifaction, 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 sometimes even days, the organic bodies thus immersed were
mineralised 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.
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 holding lime in solution.[2] 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 Goppert in various
fluid mixtures.
It is well known that the waters of all springs are more or less
charged with earthy, alkaline, or metallic ingredients derived from the
rocks and mineral veins through which they percolate. Silex is
especially abundant in hot springs, and carbonate of lime is almost
always present in greater or less quantity. The materials for the
petrifaction of organic remains are, therefore, usually at hand in a
state of chemical solution wherever organic remains are imbedded in new
strata.
[1] Dr. MacCulloch, Syst. of Geol., vol. i, p. 123.
[2] Piddington, Asiat. Research., vol. xviii, p. 226.
CHAPTER V.
ELEVATION OF STRATA ABOVE THE SEA.—HORIZONTAL AND INCLINED
STRATIFICATION.
Why the Position of Marine Strata, above the Level of the Sea, should
be referred to the rising up of the Land, not to the going down of the
Sea. — Strata of Deep-sea and Shallow-water Origin alternate. — Also
Marine and Fresh-water Beds and old Land Surfaces. — Vertical,
inclined, and folded Strata. — Anticlinal and Synclinal Curves. —
Theories to explain Lateral Movements. — Creeps in Coal-mines. — Dip
and Strike. — Structure of the Jura. — Various Forms of Outcrop. —
Synclinal Strata forming Ridges. — Connection of Fracture and Flexure
of Rocks. — Inverted Strata. — Faults described. — Superficial Signs of
the same obliterated by Denudation. — Great Faults the Result of
repeated Movements. — Arrangement and Direction of parallel Folds of
Strata. — Unconformability. — Overlapping Strata.
Land has been raised, not the Sea lowered.—It has been already stated
that the aqueous rocks containing marine fossils extend over wide
continental tracts, and are seen in mountain chains rising to great
heights above the level of the sea (p. 29). Hence it follows, that what
is now dry land was once under water. But if we admit this conclusion,
we must imagine, either that there has been a general lowering of the
waters of the ocean, or that the solid rocks, once covered by water,
have been raised up bodily out of the sea, and have thus become dry
land. The earlier geologists, finding themselves reduced to this
alternative, embraced the former opinion, assuming that the ocean was
originally universal, and had gradually sunk down to its actual level,
so that the present islands and continents were left dry. It seemed to
them far easier to conceive that the water had gone down, than that
solid land had risen upward into its present position. It was, however,
impossible to invent any satisfactory hypothesis to explain the
disappearance of so enormous a body of water throughout the globe, it
being necessary to infer that the ocean had once stood at whatever
height marine shells might be detected. It moreover appeared clear, as
the science of geology advanced, that certain spaces on the globe had
been alternately sea, then land, then estuary, then sea again, and,
lastly, once more habitable land, having remained in each of these
states for considerable periods. In order to account for such phenomena
without admitting any movement of the land itself, we are required to
imagine several retreats and returns of the ocean; and even then our
theory applies merely to cases where the marine strata composing the
dry land are horizontal, leaving unexplained those more common
instances where strata are inclined, curved, or placed on their edges,
and evidently not in the position in which they were first deposited.
Geologists, therefore, were at last compelled to have recourse to the
doctrine that the solid land has been repeatedly moved upward or
downward, so as permanently to change its position relatively to the
sea. There are several distinct grounds for preferring this conclusion.
First, it will account equally for the position of those elevated
masses of marine origin in which the stratification remains horizontal,
and for those in which the strata are disturbed, broken, inclined, or
vertical. Secondly, it is consistent with human experience that land
should rise gradually in some places and be depressed in others. Such
changes have actually occurred in our own days, and are now in
progress, having been accompanied in some cases by violent convulsions,
while in others they have proceeded so insensibly as to have been
ascertainable only by the most careful scientific observations, made at
considerable intervals of time. On the other hand, there is no evidence
from human experience of a rising or lowering of the sea’s level in any
region, and the ocean cannot be raised or depressed in one place
without its level being changed all over the globe.
These preliminary remarks will prepare the reader to understand the
great theoretical interest attached to all facts connected with the
position of strata, whether horizontal or inclined, curved or vertical.
Now the first and most simple appearance is where strata of marine
origin occur above the level of the sea in horizontal position. Such
are the strata which we meet with in the south of Sicily, filled with
shells for the most part of the same species as those now living in the
Mediterranean. Some of these rocks rise to the height of more than 2000
feet above the sea. Other mountain masses might be mentioned, composed
of horizontal strata of high antiquity, which contain fossil remains of
animals wholly dissimilar from any now known to exist. In the south of
Sweden, for example, near Lake Wener, the beds of some of the oldest
fossiliferous deposits, called Silurian and Cambrian by geologists,
occur in as level a position as if they had recently formed part of the
delta of a great river, and been left dry on the retiring of the annual
floods. Aqueous rocks of equal antiquity extend for hundreds of miles
over the lake-district of North America, and exhibit in like manner a
stratification nearly undisturbed. The Table Mountain at the Cape of
Good Hope is another example of highly elevated yet perfectly
horizontal strata, no less than 3500 feet in thickness, and consisting
of sandstone of very ancient date.
Instead of imagining that such fossiliferous rocks were always at their
present level, and that the sea was once high enough to cover them, we
suppose them to have constituted the ancient bed of the ocean, and to
have been afterwards uplifted to their present height. This idea,
however startling it may at first appear, is quite in accordance, as
before stated, with the analogy of changes now going on in certain
regions of the globe. Thus, in parts of Sweden, and the shores and
islands of the Gulf of Bothnia, proofs have been obtained that the land
is experiencing, and has experienced for centuries, a slow upheaving
movement.[1]
It appears from the observations of Mr. Darwin and others, that very
extensive regions of the continent of South America have been
undergoing slow and gradual upheaval, by which the level plains of
Patagonia, covered with recent marine shells, and the Pampas of Buenos
Ayres, have been raised above the level of the sea. On the other hand,
the gradual sinking of the west coast of Greenland, for the space of
more than 600 miles from north to south, during the last four
centuries, has been established by the observations of a Danish
naturalist, Dr. Pingel. And while these proofs of continental elevation
and subsidence, by slow and insensible movements, have been recently
brought to light, the evidence has been daily strengthened of continued
changes of level effected by violent convulsions in countries where
earthquakes are frequent. There the rocks are rent from time to time,
and heaved up or thrown down several feet at once, and disturbed in
such a manner as to show how entirely the original position of strata
may be modified in the course of centuries.
Mr. Darwin has also inferred that, in those seas where circular coral
islands and barrier reefs abound, there is a slow and continued sinking
of the submarine mountains on which the masses of coral are based;
while there are other areas of the South Sea where the land is on the
rise, and where coral has been upheaved far above the sea-level.
Alternations of Marine and Fresh-water Strata.—It has been shown in the
third chapter that there is such a differencebetween land, fresh-water,
and marine fossils as to enable the geologist to determine whether
particular groups of strata were formed at the bottom of the ocean or
in estuaries, rivers, or lakes. If surprise was at first created by the
discovery of marine corals and shells at the height of several miles
above the sea-level, the imagination was afterwards not less startled
by observing that in the successive strata composing the earth’s crust,
especially if their total thickness amounted to thousands of feet, they
comprised in some parts formations of shallow-sea as well as of
deep-sea origin; also beds of brackish or even of purely fresh-water
formation, as well as vegetable matter or coal accumulated on ancient
land. In these cases we as frequently find fresh-water beds below a
marine set or shallow-water under those of deep-sea origin as the
reverse. Thus, if we bore an artesian well below London, we pass
through a marine clay, and there reach, at the depth of several hundred
feet, a shallow-water and fluviatile sand, beneath which comes the
white chalk originally formed in a deep sea. Or if we bore vertically
through the chalk of the North Downs, we come, after traversing marine
chalky strata, upon a fresh-water formation many hundreds of feet
thick, called the Wealden, such as is seen in Kent and Surrey, which is
known in its turn to rest on purely marine beds. In like manner, in
various parts of Great Britain we sink vertical shafts through marine
deposits of great thickness, and come upon coal which was formed by the
growth of plants on an ancient land-surface sometimes hundreds of
square miles in extent.
Vertical, Inclined, and Curved Strata.—It has been stated that marine
strata of different ages are sometimes found at a considerable height
above the sea, yet retaining their original horizontality; but this
state of things is quite exceptional. As a general rule, strata are
inclined or bent in such a manner as to imply that their original
position has been altered.
Fig. 54: Vertical conglomerate and sandstone.
The most unequivocal evidence of such a change is afforded by their
standing up vertically, showing their edges, which is by no means a
rare phenomenon, especially in mountainous countries. Thus we find in
Scotland, on the southern skirts of the Grampians, beds of
pudding-stone alternating with thin layers of fine sand, all placed
vertically to the horizon. When Saussure first observed certain
conglomerates in a similar position in the Swiss Alps, he remarked that
the pebbles, being for the most part of an oval shape, had their longer
axes parallel to the planes of stratification (see Fig. 54 on preceding
page). From this he inferred that such strata must, at first, have been
horizontal, each oval pebble having settled at the bottom of the water,
with its flatter side parallel to the horizon, for the same reason that
an egg will not stand on either end if unsupported. Some few, indeed,
of the rounded stones in a conglomerate occasionally afford an
exception to the above rule, for the same reason that in a river’s bed,
or on a shingle beach, some pebbles rest on their ends or edges; these
having been shoved against or between other stones by a wave or
current, so as to assume this position.
Anticlinal and Synclinal Curves.—Vertical strata, when they can be
traced continuously upward or downward for some depth, are almost
invariably seen to be parts of great curves, which may have a diameter
of a few yards, or of several miles. I shall first describe two curves
of considerable regularity, which occur in Forfarshire, extending over
a country twenty miles in breadth, from the foot of the Grampians to
the sea near Arbroath.
Fig. 55: Section of Forfarshire, from N.W. to S.E.
The mass of strata here shown may be 2000 feet in thickness, consisting
of red and white sandstone, and various coloured shales, the beds being
distinguishable into four principal groups, namely, No. 1, red marl or
shale; No. 2, red sandstone, used for building; No. 3, conglomerate;
and No. 4, grey paving-stone, and tile-stone, with green and reddish
shale, containing peculiar organic remains. A glance at the section
will show that each of the formations 2, 3, 4 are repeated thrice at
the surface, twice with a southerly, and once with a northerly
inclination or _dip_, and the beds in No. 1, which are nearly
horizontal, are still brought up twice by a slight curvature to the
surface, once on each side of A. Beginning at the north-west extremity,
the tile-stones and conglomerates, No. 4 and No. 3, are vertical, and
they generally form a ridge parallel to the southern skirts of the
Grampians. The superior strata, Nos. 2 and 1, become less and less
inclined on descending to the valley of Strathmore, where the strata,
having a concave bend, are said by geologists to lie in a “trough”
or“basin.” Through the centre of this valley runs an imaginary line A,
called technically a “synclinal line,” where the beds, which are tilted
in opposite directions, may be supposed to meet. It is most important
for the observer to mark such lines, for he will perceive by the
diagram that, in travelling from the north to the centre of the basin,
he is always passing from older to newer beds; whereas, after crossing
the line A, and pursuing his course in the same southerly direction, he
is continually leaving the newer, and advancing upon older strata. All
the deposits which he had before examined begin then to recur in
reversed order, until he arrives at the central axis of the Sidlaw
hills, where the strata are seen to form an arch, or _saddle_, having
an _anticlinal_ line, B, in the centre. On passing this line, and
continuing towards the S.E., the formations 4, 3, and 2, are again
repeated, in the same relative order of superposition, but with a
southerly dip. At Whiteness (see Fig. 55) it will be seen that the
inclined strata are covered by a newer deposit, _a_, in horizontal
beds. These are composed of red conglomerate and sand, and are newer
than any of the groups, 1, 2, 3, 4, before described, and rest
_unconformably_ upon strata of the sandstone group, No. 2.
An example of curved strata, in which the bends or convolutions of the
rock are sharper and far more numerous within an equal space, has been
well described by Sir James Hall.[2] It occurs near St. Abb’s Head, on
the east coast of Scotland, where the rocks consist principally of a
bluish slate, having frequently a ripple-marked surface. The
undulations of the beds reach from the top to the bottom of cliffs from
200 to 300 feet in height, and there are sixteen distinct bendings in
the course of about six miles, the curvatures being alternately concave
and convex upward.
Folding by Lateral Movement.—An experiment was made by Sir James Hall,
with a view of illustrating the manner in which such strata, assuming
them to have been originally horizontal, may have been forced into
their present position. A set of layers of clay were placed under a
weight, and their opposite ends pressed towards each other with such
force as to cause them to approach more nearly together. On the removal
of the weight, the layers of clay were found to be curved and folded,
so as to bear a miniature resemblance to the strata in the cliffs. We
must, however, bear in mind that in the natural section or sea-cliff we
only see the foldings imperfectly, one part being invisible beneath the
sea, and the other, or upper portion, being supposed to have been
carried away by _denudation_, or that action of water which will be
explained in the next chapter. The dark lines in the plan (Fig. 57)
represent what is actually seen of the strata in the line of cliff
alluded to; the fainter lines, that portion which is concealed beneath
the sea-level, as also that which is supposed to have once existed
above the present surface.
Fig. 56: Curved strata of slate near St. Abb’s Head, Berwickshire.
Fig. 57
We may still more easily illustrate the effects which a lateral thrust
might produce on flexible strata, by placing several pieces of
differently coloured cloths upon a table, and when they are spread out
horizontally, cover them with a book. Then apply other books to each
end, and force them towards each other. The folding of the cloths (see
Fig. 58) will imitate those of the bent strata; the incumbent book
being slightly lifted up, and no longer touching the two volumes on
which it rested before, because it is supported by the tops of the
anticlinal ridges formed by the curved cloths. In like manner there can
be no doubt that the squeezed strata, although laterally condensed and
more closely packed, are yet elongated and made to rise upward, in a
direction perpendicular to the pressure.
Fig. 58
Whether the analogous flexures in stratified rocks have really been due
to similar sideway movements is a question which we can not decide by
reference to our own observation. Our inability to explain the nature
of the process is, perhaps, not simply owing to the inaccessibility of
the subterranean regions where the mechanical force is exerted, but to
the extreme slowness of the movement. The changes may sometimes be due
to variation in the temperature of mountain masses of rock causing
them, while still solid, to expand or contract; or melting them, and
then again cooling them and allowing them to crystallise. If such be
the case, we have scarcely more reason to expect to witness the
operation of the process within the limited periods of our scientific
observation than to see the swelling of the roots of a tree, by which,
in the course of years, a wall of solid masonry may be lifted up, rent
or thrown down. In both instances the force may be irresistible, but
though adequate, it need not be visible by us, provided the time
required for its development be very great. The lateral pressure
arising from the unequal expansion of rocks by heat may cause one mass
lying in the same horizontal plane gradually to occupy a larger space,
so as to press upon another rock, which, if flexible, may be squeezed
into a bent and folded form. It will also appear, when the volcanic and
granitic rocks are described, that some of them have, when melted in
the interior of the earth’s crust, been injected forcibly into
fissures, and after the solidification of such intruded matter, other
sets of rents, crossing the first, have been formed and in their turn
filled by melted rock. Such repeated injections imply a stretching, and
often upheaval, of the whole mass.
We also know, especially by the study of regions liable to earthquakes,
that there are causes at work in the interior of the earth capable of
producing a sinking in of the ground, sometimes very local, but often
extending over a wide area. The continuance of such a downward
movement, especially if partial and confined to linear areas, may
produce regular folds in the strata.
Creeps in Coal-mines.—The “creeps,” as they are called in coal-mines,
afford an excellent illustration of this fact.—First, it may be stated
generally, that the excavation of coal at a considerable depth causes
the mass of overlying strata to sink down bodily, even when props are
left to support the roof of the mine. “In Yorkshire,” says Mr. Buddle,
“three distinct subsidences were perceptible at the surface, after the
clearing out of three seams of coal below, and innumerable vertical
cracks were caused in the incumbent mass of sandstone and shale which
thus settled down.”[3] The exact amount of depression in these cases
can only be accurately measured where water accumulates on the surface,
or a railway traverses a coal-field.
When a bed of coal is worked out, pillars or rectangular masses of coal
are left at intervals as props to support the roof, and protect the
colliers. Thus in Fig. 59, representing a section at Wallsend,
Newcastle, the galleries which have been excavated are represented by
the white spaces _a, b,_ while the adjoining dark portions are parts of
the original coal seam left as props, beds of sandy clay or shale
constituting the floor of the mine. When the props have been reduced in
size, they are pressed down by the weight of overlying rocks (no less
than 630 feet thick) upon the shale below, which is thereby squeezed
and forced up into the open spaces.
Now it might have been expected that, instead of the floor rising up,
the ceiling would sink down, and this effect, called a “thrust,” does,
in fact, take place where the pavement is more solid than the roof. But
it usually happens, in coal-mines, that the roof is composed of hard
shale, or occasionally of sandstone, more unyielding than the
foundation, which often consists of clay. Even where the argillaceous
substrata are hard at first, they soon become softened and reduced to a
plastic state when exposed to the contact of air and water in the floor
of a mine.
Fig. 59: Section of carboniferous strata at Wallsend showing ‘creeps’.