The Student's Elements of Geology 3
CHAPTER XXXIV.
METAMORPHIC ROCKS-- CONTINUED.
Definition of slaty Cleavage and Joints.
Supposed Causes of these Structures.
Crystalline Theory of Cleavage.
Mechanical Theory of Cleavage.
Condensation and Elongation of slate Rocks by lateral Pressure.
Lamination of some volcanic Rocks due to Motion.
Whether the Foliation of the crystalline Schists be usually parallel with the
original Planes of Stratification.
Examples in Norway and Scotland.
Causes of Irregularity in the Planes of Foliation.
CHAPTER XXXV.
ON THE DIFFERENT AGES OF THE METAMORPHIC ROCKS.
Difficulty of ascertaining the Age of metamorphic Strata.
Metamorphic Strata of Eocene date in the Alps of Switzerland and Savoy.
Limestone and Shale of Carrara.
Metamorphic Strata of older date than the Silurian and Cambrian Rocks.
Order of Succession in metamorphic Rocks.
Uniformity of mineral Character.
Supposed Azoic Period.
Connection between the Absence of Organic Remains and the Scarcity of calcareous
Matter in metamorphic Rocks.
CHAPTER XXXVI.
MINERAL VEINS.
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.
...
STUDENT'S ELEMENTS OF GEOLOGY.
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. (See "Principles of Geology" by the Author Index
"Nile" "Rivers" etc.)
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. (See Figure 7 Chapter 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. (Colonel R.J. Strachey found oolitic fossils 18,400
feet high in the Himalaya.)
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 Ardeche, 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 scoriae, 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 scoriae, 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 scoriae 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 can not, 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 scoriae, 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
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