abundant rocks of the metamorphic series. It is slaty, essentially
composed of mica and quartz, the mica sometimes appearing to constitute
the whole mass. Beds of pure quartz also occur in this formation. In
some districts, garnets in regular twelve-sided crystals form an
integrant part of mica-schist. This rock passes by insensible
gradations into clay-slate.
_Clay-slate—Argillaceous Schist—Argillite._—This rock sometimes
resembles an indurated clay or shale. It is for the most part extremely
fissile, often affording good roofing-slate. Occasionally it derives a
shining and silky lustre from the minute particles of mica or talc
which it contains. It varies from greenish or bluish-grey to a lead
colour; and it may be said of this, more than of any other schist, that
it is common to the metamorphic and fossiliferous series, for some
clay-slates taken from each division would not be distinguishable by
mineral characters alone. It is not uncommon to meet with an
argillaceous rock having the same composition, without the slaty
cleavage, which may be called argillite.
_Chlorite Schist_ is a green slaty rock, in which chlorite is abundant
in foliated plates, usually blended with minute grains of quartz, or
sometimes with feldspar or mica; often associated with, and graduating
into, gneiss and clay-slate.
_Quartzite,_ or _Quartz Rock,_ is an aggregate of grains of quartz
which are either in minute crystals, or in many cases slightly rounded,
occurring in regular strata, associated with gneiss or other
metamorphic rocks. Compact quartz, like that so frequently found in
veins, is also found together with granular quartzite. Both of these
alternate with gneiss or mica-schist, or pass into those rocks by the
addition of mica, or of feldspar and mica.
_Crystalline,_ or _Metamorphic Limestone._—This hypogene rock, called
by the earlier geologists _primary limestone,_ is sometimes a white
crystalline granular marble, which when in thick beds can be used in
sculpture; but more frequently it occurs in thin beds, forming a
foliated schist much resembling in colour and arrangement certain
varieties of gneiss and mica-schist. When it alternates with these
rocks, it often contains some crystals of mica, and occasionally
quartz, feldspar, hornblende, talc, chlorite, garnet, and other
minerals. It enters sparingly into the structure of the hypogene
districts of Norway, Sweden, and Scotland, but is largely developed in
the Alps.
Origin of the Metamorphic Strata.—Having said thus much of the mineral
composition of the metamorphic rocks, I may combine what remains to be
said of their structure and history with an account of the opinions
entertained of their probable origin. At the same time, it may be well
to forewarn the reader that we are here entering upon ground of
controversy, and soon reach the limits where positive induction ends,
and beyond which we can only indulge in speculations. It was once a
favourite doctrine, and is still maintained by many, that these rocks
owe their crystalline texture, their want of all signs of a mechanical
origin, or of fossil contents, to a peculiar and nascent condition of
the planet at the period of their formation. The arguments in
refutation of this hypothesis will be more fully considered when I
show, in Chapter XXXV, to how many different ages the metamorphic
formations are referable, and how gneiss, mica-schist, clay-slate, and
hypogene limestone (that of Carrara, for example) have been formed, not
only since the first introduction of organic beings into this planet,
but even long after many distinct races of plants and animals had
flourished and passed away in succession.
The doctrine respecting the crystalline strata implied in the name
metamorphic may properly be treated of in this place; and we must first
inquire whether these rocks are really entitled to be called stratified
in the strict sense of having been originally deposited as sediment
from water. The general adoption by geologists of the term stratified,
as applied to these rocks, sufficiently attests their division into
beds very analogous, at least in form, to ordinary fossiliferous
strata. This resemblance is by no means confined to the existence in
both occasionally of a laminated structure, but extends to every kind
of arrangement which is compatible with the absence of fossils, and of
sand, pebbles, ripple-mark, and other characters which the metamorphic
theory supposes to have been obliterated by Plutonic action. Thus, for
example, we behold alike in the crystalline and fossiliferous
formations an alternation of beds varying greatly in composition,
colour, and thickness. We observe, for instance, gneiss alternating
with layers of black hornblende-schist or of green chlorite-schist, or
with granular quartz or limestone; and the interchange of these
different strata may be repeated for an indefinite number of times. In
the like manner, mica-schist alternates with chlorite-schist, and with
beds of pure quartz or of granular limestone. We have already seen
that, near the immediate contact of granitic veins and volcanic dikes,
very extraordinary alterations in rocks have taken place, more
especially in the neighbourhood of granite. It will be useful here to
add other illustrations, showing that a texture undistinguishable from
that which characterises the more crystalline metamorphic formations
has actually been superinduced in strata once fossiliferous.
Fossiliferous Strata rendered metamorphic by intrusive Masses of
Granite.—In the southern extremity of Norway there is a large district,
on the west side of the fiord of Christiania, which I visited in 1837
with the late Professor Keilhau, in which syenitic granite protrudes in
mountain masses through fossiliferous strata, and usually sends veins
into them at the point of contact. The stratified rocks, replete with
shells and zoophytes, consist chiefly of shale, limestone, and some
sandstone, and all these are invariably altered near the granite for a
distance of from 50 to 400 yards. The aluminous shales are hardened,
and have become flinty. Sometimes they resemble jasper. Ribboned jasper
is produced by the hardening of alternate layers of green and
chocolate-coloured schist, each stripe faithfully representing the
original lines of stratification. Nearer the granite the schist often
contains crystals of hornblende, which are even met with in some places
for a distance of several hundred yards from the junction; and this
black hornblende is so abundant that eminent geologists, when passing
through the country, have confounded it with the ancient
hornblende-schist, subordinate to the great gneiss formation of Norway.
Frequently, between the granite and the hornblende-slate
above-mentioned, grains of mica and crystalline feldspar appear in the
schist, so that rocks resembling gneiss and mica-schist are produced.
Fossils can rarely be detected in these schists, and they are more
completely effaced in proportion to the more crystalline texture of the
beds, and their vicinity to the granite.
Fig. 623: Ground-plan of altered slate and limestone near granite.
Christiania. The arrows indicate the dip, and the oblique lines the
strike of the beds.
In some places the siliceous matter of the schist becomes a granular
quartz; and when hornblende and mica are added, the altered rock loses
its stratification, and passes into a kind of granite. The limestone,
which at points remote from the granite is of an earthy texture and
blue colour, and often abounds in corals, becomes a white granular
marble near the granite, sometimes siliceous, the granular structure
extending occasionally upward of 400 yards from the junction; the
corals being for the most part obliterated, though sometimes preserved,
even in the white marble. Both the altered limestone and hardened slate
contain garnets in many places, also ores of iron, lead, and copper,
with some silver. These alterations occur equally whether the granite
invades the strata in a line parallel to the general strike of the
fossiliferous beds, or in a line at right angles to their strike, both
of which modes of junction will be seen by the ground-plan in Fig.
623.[1]
The granite of Cornwall sends forth veins into a coarse
argillaceous-schist, provincially termed killas. This killas is
converted into hornblende-schist near the contact with the veins. These
appearances are well seen at the junction of the granite and killas, in
St. Michael’s Mount, a small island nearly 300 feet high, situated in
the bay, at a distance of about three miles from Penzance. The granite
of Dartmoor, in Devonshire, says Sir H. De la Beche, has intruded
itself into the Carboniferous slate and slaty sandstone, twisting and
contorting the strata, and sending veins into them. Hence some of the
slate rocks have become “micaceous; others more indurated, and with the
characters of mica-slate and gneiss; while others again appear
converted into a hard zoned rock strongly impregnated with
feldspar.”[2]
We learn from the investigation of M. Dufrenoy that in the eastern
Pyrenees there are mountain masses of granite posterior in date to the
formations called lias and chalk of that district, and that these
fossiliferous rocks are greatly altered in texture, and often charged
with iron-ore, in the neighbourhood of the granite. Thus in the
environs of St. Martin, near St. Paul de Fenouillet, the chalky
limestone becomes more crystalline and saccharoid as it approaches the
granite, and loses all trace of the fossils which it previously
contained in abundance. At some points, also, it becomes dolomitic, and
filled with small veins of carbonate of iron, and spots of red
iron-ore. At Rancie the lias nearest the granite is not only filled
with iron-ore, but charged with pyrites, tremolite, garnet, and a new
mineral somewhat allied to feldspar, called, from the place in the
Pyrenees where it occurs, “couzeranite.”
“Hornblende-schist,” says Dr. MacCulloch, “may at first have been mere
clay; for clay or shale is found altered by trap into Lydian stone, a
substance differing from hornblende-schist almost solely in compactness
and uniformity of texture.”[3] “In Shetland,” remarks the same author,
“argillaceous-schist (or clay-slate), when in contact with granite, is
sometimes converted into hornblende-schist, the schist becoming first
siliceous, and ultimately, at the contact, hornblende-schist.” In like
manner gneiss and mica-schist may be nothing more than altered
micaceous and argillaceous sandstones, granular quartz may have been
derived from siliceous sandstone, and compact quartz from the same
materials. Clay-slate may be altered shale, and granular marble may
have originated in the form of ordinary limestone, replete with shells
and corals, which have since been obliterated; and, lastly, calcareous
sands and marls may have been changed into impure crystalline
limestones.
The anthracite and plumbago associated with hypogene rocks may have
been coal; for not only is coal converted into anthracite in the
vicinity of some trap dikes, but we have seen that a like change has
taken place generally even far from the contact of igneous rocks, in
the disturbed region of the Appalachians. At Worcester, in the State of
Massachusetts, 45 miles due west of Boston, a bed of plumbago and
impure anthracite occurs, interstratified with mica-schist. It is about
two feet in thickness, and has been made use of both as fuel, and in
the manufacture of lead pencils. At the distance of 30 miles from the
plumbago, there occurs, on the borders of Rhode Island, an impure
anthracite in slates containing impressions of coal-plants of the
genera _Pecopteris, Neuropteris, Calamites,_ etc. This anthracite is
intermediate in character between that of Pennsylvania and the plumbago
of Worcester, in which last the gaseous or volatile matter (hydrogen,
oxygen, and nitrogen) is to the carbon only in the proportion of three
per cent. After traversing the country in various directions, I came to
the conclusion that the carboniferous shales or slates with anthracite
and plants, which in Rhode Island often pass into mica-schists, have at
Worcester assumed a perfectly crystalline and metamorphic texture; the
anthracite having been nearly transmuted into that state of pure carbon
which is called plumbago or graphite.[4]
Now the alterations above described as superinduced in rocks by
volcanic dikes and granite veins prove incontestably that powers exist
in nature capable of transforming fossiliferous into crystalline
strata, a very few simple elements constituting the component materials
common to both classes of rocks. These elements, which are enumerated
in the table at p. 499, may be made to form new combinations by what
has been termed Plutonic action, or those chemical changes which are no
doubt connected with the passage of heat, unusually heated steam and
waters, through the strata.
Hydrothermal Action, or the Influence of Steam and Gases in producing
Metamorphism.—The experiments of Gregory Watt, in fusing rocks in the
laboratory, and allowing them to consolidate by slow cooling, prove
distinctly that a rock need not be perfectly melted in order that a
re-arrangement of its component particles should take place, and a
partial crystallisation ensue.[5] We may easily suppose, therefore,
that all traces of shells and other organic remains may be destroyed,
and that new chemical combinations may arise, without the mass being so
fused as that the lines of stratification should be wholly obliterated.
We must not, however, imagine that heat alone, such as may be applied
to a stone in the open air, can constitute all that is comprised in
Plutonic action. We know that volcanoes in eruption not only emit fluid
lava, but give off steam and other heated gases, which rush out in
enormous volume, for days, weeks, or years continuously, and are even
disengaged from lava during its consolidation.
We also know that long after volcanoes have spent their force, hot
springs continue for ages to flow out at various points in the same
area. In regions, also, subject to violent earthquakes such springs are
frequently observed issuing from rents, usually along lines of fault or
displacement of the rocks. These thermal waters are most commonly
charged with a variety of mineral ingredients, and they retain a
remarkable uniformity of temperature from century to century. A like
uniformity is also persistent in the nature of the earthy, metallic,
and gaseous substances with which they are impregnated. It is well
ascertained that springs, whether hot or cold, charged with carbonic
acid, especially with hydrofluoric acid, which is often present in
small quantities, are powerful causes of decomposition and chemical
reaction in rocks through which they percolate.
The changes which Daubrée has shown to have been produced by the
alkaline waters of Plombières in the Vosges, are more especially
instructive.[6] These waters have a heat of 160° F., or an excess of
109° above the average temperature of ordinary springs in that
district. They were conveyed by the Romans to baths through long
conduits or aqueducts. The foundations of some of their works consisted
of a bed of concrete made of lime, fragments of brick, and sandstone.
Through this and other masonry the hot waters have been percolating for
centuries, and have given rise to various zeolites—apophyllite and
chabazite among others; also to calcareous spar, arragonite, and fluor
spar, together with siliceous minerals, such as opal—all found in the
inter-spaces of the bricks and mortar, or constituting part of their
re-arranged materials. The quantity of heat brought into action in this
instance in the course of 2000 years has, no doubt, been enormous, but
the intensity of it developed at any one moment has been always
inconsiderable.
From these facts and from the experiments and observations of
Sénarmont, Daubrée, Delesse, Scheerer, Sorby, Sterry Hunt, and others,
we are led to infer that when in the bowels of the earth there are
large volumes of matter containing water and various acids intensely
heated under enormous pressure, these subterranean fluid masses will
gradually part with their heat by the escape of steam and various gases
through fissures, producing hot springs; or by the passage of the same
through the pores of the overlying and injected rocks. Even the most
compact rocks may be regarded, before they have been exposed to the air
and dried, in the light of sponges filled with water. According to the
experiments of Henry, water, under a hydrostatic pressure of 96 feet,
will absorb three times as much carbonic acid gas as it can under the
ordinary pressure of the atmosphere. There are other gases, as well as
the carbonic acid, which water absorbs, and more rapidly in proportion
to the amount of pressure. Although the gaseous matter first absorbed
would soon be condensed, and part with its heat, yet the continual
arrival of fresh supplies from below might, in the course of ages,
cause the temperature of the water, and with it that of the containing
rock, to be materially raised; the water acts not only as a vehicle of
heat, but also by its affinity for various silicates, which, when some
of the materials of the invaded rocks are decomposed, form quartz,
feldspar, mica, and other minerals. As for quartz, it can be produced
under the influence of heat by water holding alkaline silicates in
solution, as in the case of the Plombières springs. The quantity of
water required, according to Daubrée, to produce great transformations
in the mineral structure of rocks, is very small. As to the heat
required, silicates may be produced in the moist way at about incipient
red heat, whereas to form the same in the dry way would require a much
higher temperature.
M. Fournet, in his description of the metalliferous gneiss near
Clermont, in Auvergne, states that all the minute fissures of the rock
are quite saturated with free carbonic acid gas; which gas rises
plentifully from the soil there and in many parts of the surrounding
country. The various elements of the gneiss, with the exception of the
quartz, are all softened; and new combinations of the acid with lime,
iron, and manganese are continually in progress.[7]
The power of subterranean gases is well illustrated by the stufas of
St. Calogero in the Lipari Islands, where the horizontal strata of
tuffs, forming cliffs 200 feet high, have been discoloured in places by
the jets of steam often above the boiling point, called “stufas,”
issuing from the fissures; and similar instances are recorded by M.
Virlet of corrosion of rocks near Corinth, and by Dr. Daubeny of
decomposition of trachytic rocks by sulphureted hydrogen and muriatic
acid gases in the Solfatara, near Naples. In all these instances it is
clear that the gaseous fluids must have made their way through vast
thicknesses of porous or fissured rocks, and their modifying influence
may spread through the crust for thousands of yards in thickness.
It has been urged as an argument against the metamorphic theory, that
rocks have a small power of conducting heat, and it is true that when
dry, and in the air, they differ remarkably from metals in this
respect. The syenite of Norway, as we have seen (p. 558), has sometimes
altered fossiliferous strata both in the direction of their dip and
strike for a distance of a quarter of a mile, but the theory of gneiss
and mica-schist above proposed requires us to imagine that the same
influence has extended through strata miles in thickness. Professor
Bischof has shown what changes may be superinduced, on black marble and
other rocks, by the steam of a hot spring having a temperature of no
more than 133° to 167° Fahrenheit, and we are becoming more and more
acquainted with the prominent part which water is playing in
distributing the heat of the interior through mountain masses of
incumbent strata, and of introducing into them various mineral elements
in a fluid or gaseous state. Such facts may induce us to consider
whether many granites and other rocks of that class may not sometimes
represent merely the extreme of a similar slow metamorphism. But, on
the other hand, the heat of lava in a volcanic crater when it is white
and glowing like the sun must convince us that the temperature of a
column of such a fluid at the depth of many miles exceeds any heat
which can ever be witnessed at the surface. That large portions of the
Plutonic rocks had been formed under the influence of such intense heat
is in perfect accordance with their great volume, uniform composition,
and absence of stratification. The forcing also of veins into
contiguous stratified or schistose rocks is a natural consequence of
the hydrostatic pressure to which columns of molten matter many miles
in height must give rise.
Objections to the Metamorphic Theory considered.—It has been objected
to the metamorphic theory that the crystalline schists contain a
considerable proportion of potash and soda, whilst the sedimentary
strata out of which they are supposed to have been formed are usually
wanting in alkaline matter. But this reasoning proceeds on mistaken
data, for clay, marl, shale, and slate often contain a considerable
proportion of alkali, so much so as to make them frequently unfit to be
burnt into bricks or pottery, and the Old Red Sandstone in Forfarshire
and other parts of Scotland, derived from disintegration of granite,
contains much triturated feldspar rich in potash. In the common salt by
which strata are often largely impregnated, as in Patagonia, much soda
is present, and potash enters largely into the composition of fossil
sea-weeds, and recent analysis has also shown that the carboniferous
strata in England, the Upper and Lower Silurian in East Canada, and the
oldest clay-slates in Norway, all contain as much alkali as is
generally present in metamorphic rocks.
Another objection has been derived from the alternation of highly
crystalline strata with others less crystalline. The heat, it is said,
in its ascent from below, must have traversed the less altered schists
before it reached a higher and more crystalline bed. In answer to this,
it may be observed, that if a number of strata differing greatly in
composition from each other be subjected to equal quantities of heat,
or hydrothermal action, there is every probability that some will be
much more fusible or soluble than others. Some, for example, will
contain soda, potash, lime, or some other ingredient capable of acting
as a flux or solvent; while others may be destitute of the same
elements, and so refractory as to be very slightly affected by the same
causes. Nor should it be forgotten that, as a general rule, the less
crystalline rocks do really occur in the upper, and the more
crystalline in the lower part of each metamorphic series.
[1] Keilhau, Gæa Norvegica, pp. 61-63.
[2] Geol. Manual, p. 479.
[3] Syst. of Geol., vol. i, pp. 210, 211.
[4] See Lyell, Quart. Geol. Journ., vol. i, p. 199.
[5] Phil. Trans., 1804.
[6] Daubrée, Sur le Métamorphisme. Paris, 1860.
[7] See Principles, _Index,_ “Carbonated Springs,” etc.
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.
We have already seen that chemical forces of great intensity have
frequently acted upon sedimentary and fossiliferous strata long
subsequently to their consolidation, and we may next inquire whether
the component minerals of the altered rocks usually arrange themselves
in planes parallel to the original planes of stratification, or
whether, after crystallisation, they more commonly take up a different
position.
In order to estimate fairly the merits of this question, we must first
define what is meant by the terms cleavage and foliation. There are
four distinct forms of structure exhibited in rocks, namely,
stratification, joints, slaty cleavage, and foliation; and all these
must have different names, even though there be cases where it is
impossible, after carefully studying the appearances, to decide upon
the class to which they belong.
Slaty Cleavage.—Professor Sedgwick, whose essay “On the Structure of
large Mineral Masses” first cleared the way towards a better
understanding of this difficult subject, observes, that joints are
distinguishable from lines of slaty cleavage in this, that the rock
intervening between two joints has no tendency to cleave in a direction
parallel to the planes of the joints, whereas a rock is capable of
indefinite subdivision in the direction of its slaty cleavage. In cases
where the strata are curved, the planes of cleavage are still perfectly
parallel. This has been observed in the slate rocks of part of Wales
(see Fig. 624), which consists of a hard greenish slate. The true
bedding is there indicated by a number of parallel stripes, some of a
lighter and some of a darker colour than the general mass. Such stripes
are found to be parallel to the true planes of stratification, wherever
these are manifested by ripple-mark or by beds containing peculiar
organic remains. Some of the contorted strata are of a coarse
mechanical structure, alternating with fine-grained crystalline
chloritic slates, in which case the same slaty cleavage extends through
the coarser and finer beds, though it is brought out in greater
perfection in proportion as the materials of the rock are fine and
homogeneous. It is only when these are very coarse that the cleavage
planes entirely vanish. In the Welsh hills these planes are usually
inclined at a very considerable angle to the planes of the strata, the
average angle being as much as from 30° to 40°. Sometimes the cleavage
planes dip towards the same point of the compass as those of
stratification, but often to opposite points.[1] The cleavage, as
represented in Fig. 624, is generally constant over the whole of any
area affected by one great set of disturbances, as if the same lateral
pressure which caused the crumpling up of the rock along parallel,
anticlinal, and synclinal axes caused also the cleavage.
Fig. 624: Parallel planes of cleavage intersecting curved strata.
Fig. 625: Section in Lower Silurian slates of Cardiganshire, showing
the cleavage planes bent along the junction of the beds.
Mr. T. McK. Hughes remarks, that where a rough cleavage cuts
flag-stones at a considerable angle to the planes of stratification,
the rock often splits into large slabs, across which the lines of
bedding are frequently seen, but when the cleavage planes approach
within about 15° of stratification, the rock is apt to split along the
lines of bedding. He has also called my attention to the fact that
subsequent movements in a cleaved rock sometimes drag and bend the
cleavage planes along the junction of the beds in the manner indicated
in Fig. 625.
Jointed Structure.—In regard to joints, they are natural fissures which
often traverse rocks in straight and well-determined lines. They afford
to the quarryman, as Sir R. Murchison observes, when speaking of the
phenomenon, as exhibited in Shropshire and the neighbouring counties,
the greatest aid in the extraction of blocks of stone; and, if a
sufficient number cross each other, the whole mass of rock is split
into symmetrical blocks. The faces of the joints are for the most part
smoother and more regular than the surfaces of true strata. The joints
are straight-cut chinks, sometimes slightly open, and often passing,
not only through layers of successive deposition, but also through
balls of limestone or other matter which have been formed by
concretionary action since the original accumulation of the strata.
Such joints, therefore, must often have resulted from one of the last
changes superinduced upon sedimentary deposits.[2]
Fig. 626: Stratification, joints, and cleavage.
In Fig. 626 the flat-surfaces of rock, A, B, C, represent exposed faces
of joints, to which the walls of other joints, J J, are parallel. S S
are the lines of stratification; D D are lines of slaty cleavage, which
intersect the rock at a considerable angle to the planes of
stratification.
In the Swiss and Savoy Alps, as Mr. Bakewell has remarked, enormous
masses of limestone are cut through so regularly by nearly vertical
partings, and these joints are often so much more conspicuous than the
seams of stratification, that an inexperienced observer will almost
inevitably confound them, and suppose the strata to be perpendicular in
places where in fact they are almost horizontal.[3]
Now such joints are supposed to be analogous to the partings which
separate volcanic and Plutonic rocks into cuboidal and prismatic
masses. On a small scale we see clay and starch when dry split into
similar shapes; this is often caused by simple contraction, whether the
shrinking be due to the evaporation of water, or to a change of
temperature. It is well known that many sandstones and other rocks
expand by the application of moderate degrees of heat, and then
contract again on cooling; and there can be no doubt that large
portions of the earth’s crust have, in the course of past ages, been
subjected again and again to very different degrees of heat and cold.
These alternations of temperature have probably contributed largely to
the production of joints in rocks.
In many countries where masses of basalt rest on sandstone, the aqueous
rock has, for the distance of several feet from the point of junction,
assumed a columnar structure similar to that of the trap. In like
manner some hearth-stones, after exposure to the heat of a furnace
without being melted, have become prismatic. Certain crystals also
acquire by the application of heat a new internal arrangement, so as to
break in a new direction, their external form remaining unaltered.
Crystalline Theory of Cleavage.—Professor Sedgwick, speaking of the
planes of slaty cleavage, where they are decidedly distinct from those
of sedimentary deposition, declared, in the essay before alluded to,
his opinion that no retreat of parts, no contraction in the dimensions
of rocks in passing to a solid state, can account for the phenomenon.
He accordingly referred it to crystalline or polar forces acting
simultaneously, and somewhat uniformly, in given directions, on large
masses having a homogeneous composition.
Sir John Herschel, in allusion to slaty cleavage, has suggested that
“if rocks have been so heated as to allow a commencement of
crystallisation—that is to say, if they have been heated to a point at
which the particles can begin to move among themselves, or at least on
their own axes, some general law must then determine the position in
which these particles will rest on cooling. Probably, that position
will have some relation to the direction in which the heat escapes.
Now, when all, or a majority of particles of the same nature have a
general tendency to one position, that must of course determine a
cleavage-plane. Thus we see the infinitesimal crystals of
fresh-precipitated sulphate of barytes, and some other such bodies,
arrange themselves alike in the fluid in which they float; so as, when
stirred, all to glance with one light, and give the appearance of silky
filaments. Some sorts of soap, in which insoluble margarates[4] exist,
exhibit the same phenomenon when mixed with water; and what occurs in
our experiments on a minute scale may occur in nature on a great
one.”[5]
Mechanical Theory of Cleavage.—Professor Phillips has remarked that in
some slaty rocks the form of the outline of fossil shells and
trilobites has been much changed by distortion, which has taken place
in a longitudinal, transverse, or oblique direction. This change, he
adds, seems to be the result of a “creeping movement” of the particles
of the rock along the planes of cleavage, its direction being always
uniform over the same tract of country, and its amount in space being
sometimes measurable, and being as much as a quarter or even half an
inch. The hard shells are not affected, but only those which are
thin.[6] Mr. D. Sharpe, following up the same line of inquiry, came to
the conclusion that the present distorted forms of the shells in
certain British slate rocks may be accounted for by supposing that the
rocks in which they are imbedded have undergone compression in a
direction perpendicular to the planes of cleavage, and a corresponding
expansion in the direction of the dip of the cleavage.[7]
Subsequently (1853) Mr. Sorby demonstrated the great extent to which
this mechanical theory is applicable to the slate rocks of North Wales
and Devonshire,[8] districts where the amount of change in dimensions
can be tested and measured by comparing the different effects exerted
by lateral pressure on alternating beds of finer and coarser materials.
Thus, for example, in Fig. 627 it will be seen that the sandy bed _d
f,_ which has offered greater resistance, has been sharply contorted,
while the fine-grained strata, _a, b, c,_ have remained comparatively
unbent. The points _d_ and _f_ in the stratum _d f_ must have been
originally four times as far apart as they are now. They have been
forced so much nearer to each other, partly by bending, and partly by
becoming elongated in the direction of what may be called the longer
axes of their contortions, and lastly, to a certain small amount, by
condensation. The chief result has obviously been due to the bending;
but, in proof of elongation, it will be observed that the thickness of
the bed _d f_ is now about four times greater in those parts lying in
the main direction of the flexures than in a plane perpendicular to
them; and the same bed exhibits cleavage planes in the direction of the
greatest movement, although they are much fewer than in the slaty
strata above and below.
Fig. 627: Vertical section of slate rock in the cliffs near Ilfracombe,
North Devon.
Above the sandy bed _d f,_ the stratum _c_ is somewhat disturbed, while
the next bed, _b,_ is much less so, and a not at all; yet all these
beds, _c, b,_ and _a,_ must have undergone an equal amount of pressure
with _d,_ the points a and g having approximated as much towards each
other as have _ d_ and _f._ The same phenomena are also repeated in the
beds below _d,_ and might have been shown, had the section been
extended downward. Hence it appears that the finer beds have been
squeezed into a fourth of the space they previously occupied, partly by
condensation, or the closer packing of their ultimate particles (which
has given rise to the great specific gravity of such slates), and
partly by elongation in the line of the dip of the cleavage, of which
the general direction is perpendicular to that of the pressure. “These
and numerous other cases in North Devon are analogous,” says Mr. Sorby,
“to what would occur if a strip of paper were included in a mass of
some soft plastic material which would readily change its dimensions.
If the whole were then compressed in the direction of the length of the
strip of paper, it would be bent and puckered up into contortions,
while the plastic material would readily change its dimensions without
undergoing such contortions; and the difference in distance of the ends
of the paper, as measured in a direct line or along it, would indicate
the change in the dimensions of the plastic material.”
By microscopic examination of minute crystals, and by other
observations, Mr. Sorby has come to the conclusion that the absolute
condensation of the slate rocks amounts upon an average to about one
half their original volume. Most of the scales of mica occurring in
certain slates examined by Mr. Sorby lie in the plane of cleavage;
whereas in a similar rock not exhibiting cleavage they lie with their
longer axes in all directions. May not their position in the slates
have been determined by the movement of elongation before alluded to?
To illustrate this theory some scales of oxide of iron were mixed with
soft pipe-clay in such a manner that they inclined in all directions.
The dimensions of the mass were then changed artificially to a similar
extent to what has occurred in slate rocks, and the pipe-clay was then
dried and baked. When it was afterwards rubbed to a flat surface
perpendicular to the pressure and in the line of elongation, or in a
plane corresponding to that of the dip of cleavage, the particles were
found to have become arranged in the same manner as in natural slates,
and the mass admitted of easy fracture into thin flat pieces in the
plane alluded to, whereas it would not yield in that perpendicular to
the cleavage.[9]
Dr. Tyndall, when commenting in 1856 on Mr. Sorby’s experiments,
observed that pressure alone is sufficient to produce cleavage, and
that the intervention of plates of mica or scales of oxide of iron, or
any other substances having flat surfaces, is quite unnecessary. In
proof of this he showed experimentally that a mass of “pure white wax,
after having been submitted to great pressure, exhibited a cleavage
more clean than that of any slate-rock, splitting into laminæ of
surpassing tenuity.”[10] He remarks that every mass of clay or mud is
divided and subdivided by surfaces among which the cohesion is
comparatively small. On being subjected to pressure, such masses yield
and spread out in the direction of least resistance, small nodules
become converted into laminæ separated from each other by surfaces of
weak cohesion, and the result is that the mass cleaves at right angles
to the line in which the pressure is exerted. In further illustration
of this, Mr. Hughes remarks that “concretions which in the undisturbed
beds have their longer axes parallel to the bedding are, where the rock
is much cleaved, frequently found flattened laterally, so as to have
their longer axes parallel to the cleavage planes, and at a
considerable angle, even right angles, to their former position.”
Mr. Darwin attributes the lamination and fissile structure of volcanic
rocks of the trachytic series, including some obsidians in Ascension,
Mexico, and elsewhere, to their having moved when liquid in the
direction of the laminæ. The zones consist sometimes of layers of
air-cells drawn out and lengthened in the supposed direction of the
moving mass.[11]
Foliation of Crystalline Schists.—After studying, in 1835, the
crystalline rocks of South America, Mr. Darwin proposed the term
_foliation_ for the laminæ or plates into which gneiss, mica-schist,
and other crystalline rocks are divided. Cleavage, he observes, may be
applied to those divisional planes which render a rock fissile,
although it may appear to the eye quite or nearly homogeneous.
Foliation may be used for those alternating layers or plates of
different mineralogical nature of which gneiss and other metamorphic
schists are composed.
That the planes of foliation of the crystalline schists in Norway
accord very generally with those of original stratification is a
conclusion long since espoused by Keilhau.[12] Numerous observations
made by Mr. David Forbes in the same country (the best probably in
Europe for studying such phenomena on a grand scale) confirm Keilhau’s
opinion. In Scotland, also, Mr. D. Forbes has pointed out a striking
case where the foliation is identical with the lines of stratification
in rocks well seen near Crianlorich on the road to Tyndrum, about eight
miles from Inverarnon, in Perthshire. There is in that locality a blue
limestone foliated by the intercalation of small plates of white mica,
so that the rock is often scarcely distinguishable in aspect from
gneiss or mica-schist. The stratification is shown by the large beds
and coloured bands of limestone all dipping, like the folia, at an
angle of 32° N.E.[13] In stratified formations of every age we see
layers of siliceous sand with or without mica, alternating with clay,
with fragments of shells or corals, or with seams of vegetable matter,
and we should expect the mutual attraction of like particles to favour
the crystallisation of the quartz, or mica, or feldspar, or carbonate
of lime, along the planes of original deposition, rather than in planes
placed at angles of 20 or 40 degrees to those of stratification.
We have seen how much the original planes of stratification may be
interfered with or even obliterated by concretionary action in deposits
still retaining their fossils, as in the case of the magnesian
limestone (see p. 63). Hence we must expect to be frequently baffled
when we attempt to decide whether the foliation does or does not accord
with that arrangement which gravitation, combined with current-action,
imparted to a deposit from water. Moreover, when we look for
stratification in crystalline rocks, we must be on our guard not to
expect too much regularity. The occurrence of wedge-shaped masses, such
as belong to coarse sand and pebbles—diagonal lamination (p.
42)—ripple-marked, unconformable stratification,—the fantastic folds
produced by lateral pressure—faults of various width—intrusive dikes of
trap—organic bodies of diversified shapes, and other causes of
unevenness in the planes of deposition, both on the small and on the
large scale, will interfere with parallelism. If complex and
enigmatical appearances did not present themselves, it would be a
serious objection to the metamorphic theory. Mr. Sorby has shown that
the peculiar structure belonging to ripple-marked sands, or that which
is generated when ripples are formed during the deposition of the
materials, is distinctly recognisable in many varieties of mica-schists
in Scotland.[14]
Fig. 628: Lamination of clay-stone. Montagne de Seguinat, near
Gavarnie, in the Pyrenees.
In Fig. 628 I have represented carefully the lamination of a coarse
argillaceous schist which I examined in 1830 in the Pyrenees. In part
it approaches in character to a green and blue roofing-slate, while
part is extremely quartzose, the whole mass passing downward into
micaceous schist. The vertical section here exhibited is about three
feet in height, and the layers are sometimes so thin that fifty may be
counted in the thickness of an inch. Some of them consist of pure
quartz. There is a resemblance in such cases to the diagonal lamination
which we see in sedimentary rocks, even though the layers of quartz and
of mica, or of feldspar and other minerals, may be more distinct in
alternating folia than they were originally.
[1] Geol. Trans., 2nd series, vol. iii, p. 461.
[2] Silurian System, p. 246.
[3] Introduction to Geology, chap. iv.
[4] Margaric acid is an oleaginous acid, formed from different animal
and vegetable fatty substances. A margarate is a compound of this acid
with soda, potash, or some other base, and is so named from its pearly
lustre.
[5] Letter to the author, dated Cape of Good Hope, Feb. 20, 1836.
[6] Report, Brit. Assoc., Cork, 1843, Sect. p. 60.
[7] Quart. Geol. Journ., vol. iii, p. 87, 1847.
[8] On the Origin of Slaty Cleavage, by H. C. Sorby, Edin. New Phil.
Journ., 1853, vol. lv, p. 137.
[9] Sorby, as cited above, p. 741, note.
[10] Tyndall, View of the Cleavage of Crystals and Slate rocks.
[11] Darwin, Volcanic Islands, pp. 69, 70.
[12] Norske Mag. Naturvidsk., vol. i, p. 71.
[13] Memoir read before the Geol. Soc. London, Jan. 31, 1855.
[14] H. C. Sorby, Quart. Geol. Journ., vol. xix., p. 401.
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.
According to the theory adopted in the last chapter, the metamorphic
strata have been deposited at one period, and have become crystalline
at another. We can rarely hope to define with exactness the date of
both these periods, the fossils having been destroyed by Plutonic
action, and the mineral characters being the same, whatever the age.
Superposition itself is an ambiguous test, especially when we desire to
determine the period of crystallisation. Suppose, for example, we are
convinced that certain metamorphic strata in the Alps, which are
covered by cretaceous beds, are altered lias; this lias may have
assumed its crystalline texture in the cretaceous or in some tertiary
period, the Eocene for example.
When discussing the ages of the Plutonic rocks, we have seen that
examples occur of various primary, secondary, and tertiary deposits
converted into metamorphic strata near their contact with granite.
There can be no doubt in these cases that strata once composed of mud,
sand, and gravel, or of clay, marl, and shelly limestone, have for the
distance of several yards, and in some instances several hundred feet,
been turned into gneiss, mica-schist, hornblende-schist,
chlorite-schist, quartz rock, statuary marble, and the rest. (See the
two preceding chapters.) It may be easy to prove the identity of two
different parts of the same stratum; one, where the rock has been in
contact with a volcanic or Plutonic mass, and has been changed into
marble or hornblende-schist, and another not far distant, where the
same bed remains unaltered and fossiliferous; but when hydrothermal
action, as described in Chapter XXXIII, has operated gradually on a
more extensive scale, it may have finally destroyed all monuments of
the date of its development throughout a whole mountain chain, and all
the labour and skill of the most practised observers are required, and
may sometimes be at fault. I shall mention one or two examples of
alteration on a grand scale, in order to explain to the student the
kind of reasoning by which we are led to infer that dense masses of
fossiliferous strata have been converted into crystalline rocks.
Eocene Strata rendered metamorphic in the Alps.—In the eastern part of
the Alps, some of the Palæozoic strata, as well as the older Mesozoic
formations, including the oolitic and cretaceous rocks, are distinctly
recognisable. Tertiary deposits also appear in a less elevated position
on the flanks of the Eastern Alps; but in the Central or Swiss Alps,
the Palæozoic and older Mesozoic formations disappear, and the
Cretaceous, Oolitic, Liassic, and at some points even the Eocene
strata, graduate insensibly into metamorphic rocks, consisting of
granular limestone, talc-schist, talcose-gneiss, micaceous schist, and
other varieties.
As an illustration of the partial conversion into gneiss of portions of
a highly inclined set of beds, I may cite Sir R. Murchison’s memoir on
the structure of the Alps. Slates provincially termed “flysch” (see p.
278), overlying the nummulite limestone of Eocene date, and comprising
some arenaceous and some calcareous layers, are seen to alternate
several times with bands of granitoid rock, answering in character to
gneiss. In this case heat, vapour, or water at a high temperature may
have traversed the more permeable beds, and altered them so far as to
admit of an internal movement and re-arrangement of the molecules,
while the adjoining strata did not give passage to the same heated
gases or water, or, if so, remained unchanged because they were
composed of less fusible or decomposable materials. Whatever hypothesis
we adopt, the phenomena establish beyond a doubt the possibility of the
development of the metamorphic structure in a tertiary deposit in
planes parallel to those of stratification. The strata appear clearly
to have been affected, though in a less intense degree, by that same
Plutonic action which has entirely altered and rendered metamorphic so
many of the subjacent formations; for in the Alps this action has by no
means been confined to the immediate vicinity of granite. Granite,
indeed, and other Plutonic rocks, rarely make their appearance at the
surface, notwithstanding the deep ravines which lay open to view the
internal structure of these mountains. That they exist below at no
great depth we cannot doubt, for at some points, as in the Valorsine,
near Mont Blanc, granite and granitic veins are observable, piercing
through talcose gneiss, which passes insensibly upward into secondary
strata.
It is certainly in the Alps of Switzerland and Savoy, more than in any
other district in Europe, that the geologist is prepared to meet with
the signs of an intense development of Plutonic action; for here strata
thousands of feet thick have been bent, folded, and overturned, and
marine secondary formations of a comparatively modern date, such as the
Oolitic and Cretaceous, have been upheaved to the height of 12,000, and
some Eocene strata to elevations of 10,000 feet above the level of the
sea; and even deposits of the Miocene era have been raised 4000 or 5000
feet, so as to rival in height the loftiest mountains in Great Britain.
In one of the sections described by M. Studer in the highest of the
Bernese Alps, namely in the Roththal, a valley bordering the line of
perpetual snow on the northern side of the Jungfrau, there occurs a
mass of gneiss 1000 feet thick, and 15,000 feet long, which I examined,
not only resting upon, but also again covered by strata containing
oolitic fossils. These anomalous appearances may partly be explained by
supposing great solid wedges of intrusive gneiss to have been forced in
laterally between strata to which I found them to be in many sections
unconformable. The superposition, also, of the gneiss to the oolite
may, in some cases, be due to a reversal of the original position of
the beds in a region where the convulsions have been on so stupendous a
scale.
Northern Apennines.—Carrara.—The celebrated marble of Carrara, used in
sculpture, was once regarded as a type of primitive limestone. It
abounds in the mountains of Massa Carrara, or the “Apuan Alps,” as they
have been called, the highest peaks of which are nearly 6000 feet high.
Its great antiquity was inferred from its mineral texture, from the
absence of fossils, and its passage downward into talc-schist and
garnetiferous mica-schist; these rocks again graduating downward into
gneiss, which is penetrated, at Forno, by granite veins. But the
researches of MM. Savi, Boué, Pareto, Guidoni, De la Beche, Hoffman,
and Pilla demonstrated that this marble, once supposed to be formed
before the existence of organic beings, is, in fact, an altered
limestone of the Oolitic period, and the underlying crystalline schists
are secondary sandstones and shales, modified by Plutonic action. In
order to establish these conclusions it was first pointed out that the
calcareous rocks bordering the Gulf of Spezia, and abounding in Oolitic
fossils, assume a texture like that of Carrara marble, in proportion as
they are more and more invaded by certain trappean and Plutonic rocks,
such as diorite, serpentine, and granite, occurring in the same
country.
It was then observed that, in places where the secondary formations are
unaltered, the uppermost consist of common Apennine limestone with
nodules of flint, below which are shales, and at the base of all,
argillaceous and siliceous sandstones. In the limestone fossils are
frequent, but very rare in the underlying shale and sandstone. Then a
gradation was traced laterally from these rocks into another and
corresponding series, which is completely metamorphic; for at the top
of this we find a white granular marble, wholly devoid of fossils, and
almost without stratification, in which there are no nodules of flint,
but in its place siliceous matter disseminated through the mass in the
form of prisms of quartz. Below this, and in place of the shales, are
talc-schists, jasper, and hornstone; and at the bottom, instead of the
siliceous and argillaceous sandstones, are quartzite and gneiss.[1] Had
these secondary strata of the Apennines undergone universally as great
an amount of transmutation, it would have been impossible to form a
conjecture respecting their true age; and then, according to the method
of classification adopted by the earlier geologists, they would have
ranked as primary rocks. In that case the date of their origin would
have been thrown back to an era antecedent to the deposition of the
Lower Silurian or Cambrian strata, although in reality they were formed
in the Oolitic period, and altered at some subsequent and perhaps much
later epoch.
Metamorphic Strata of older date than the Silurian and Cambrian
Rocks.—It was remarked (Fig. 617) that as the hypogene rocks, both
stratified and unstratified, crystallise originally at a certain depth
beneath the surface, they must always, before they are upraised and
exposed at the surface, be of considerable antiquity, relatively to a
large portion of the fossiliferous and volcanic rocks. They may be
forming at all periods; but before any of them can become visible, they
must be raised above the level of the sea, and some of the rocks which
previously concealed them must have been removed by denudation.
In Canada, as we have seen (p. 491), the Lower Laurentian gneiss,
quartzite, and limestone may be regarded as metamorphic, because, among
other reasons, organic remains (_Eozoon Canadense_) have been detected
in a part of one of the calcareous masses. The Upper Laurentian or
Labrador series lies unconformably upon the Lower, and differs from it
chiefly in having as yet yielded no fossils. It consists of gneiss with
Labrador-feldspar and feldstones, in all 10,000 feet thick, and both
its composition and structure lead us to suppose that, like the Lower
Laurentian, it was originally of sedimentary origin and owes its
crystalline condition to metamorphic action. The remote date of the
period when some of these old Laurentian strata of Canada were
converted into gneiss may be inferred from the fact that pebbles of
that rock are found in the overlying Huronian formation, which is
probably of Cambrian age (p. 490).
The oldest stratified rock of Scotland is the hornblendic gneiss of
Lewis, in the Hebrides, and that of the north-west coast of Ross-shire,
represented at the base of the section given at Fig. 82. It is the same
as that intersected by numerous granite veins which forms the cliffs of
Cape Wrath, in Sutherlandshire (see Fig. 613), and is conjectured to be
of Laurentian age. Above it, as shown in the section (Fig. 82), lie
unconformable beds of a reddish or purple sandstone and conglomerate,
nearly horizontal, and between 3000 and 4000 feet thick. In these
ancient grits no fossils have been found, but they are supposed to be
of Cambrian date, for Sir R. Murchison found Lower Silurian strata
resting unconformably upon them. These strata consist of quartzite with
annelid burrows already alluded to (p. 112), and limestone in which Mr.
Charles Peach was the first to find, in 1854, three or four species of
_Orthoceras,_ also the genera _Cyrtoceras_ and _ Lituites,_ two species
of _Murchisonia,_ a _ Pleurotomaria,_ a species of _Maclurea,_ one of _
Euomphalus,_ and an _Orthis._ Several of the species are believed by
Mr. Salter to be identical with Lower Silurian fossils of Canada and
the United States.
The discovery of the true age of these fossiliferous rocks was one of
the most important steps made of late years in the progress of British
Geology, for it led to the unexpected conclusion that all the Scotch
crystalline strata to the eastward, once called primitive, which
overlie the limestone and quartzite in question, are referable to some
part of the Silurian series.
These Scotch metamorphic strata are of gneiss, mica-schist, and
clay-slate of vast thickness, and having a strike from north-east to
south-west almost at right angles to that of the older Laurentian
gneiss before mentioned. The newer crystalline series, comprising the
crystalline rocks of Aberdeenshire, Perthshire, and Forfarshire, were
inferred by Sir R. Murchison to be altered Silurian strata; and his
opinion has been since confirmed by the observations of three able
geologists, Messrs. Ramsay, Harkness, and Geikie. The newest of the
series is a clay-slate, on which, along the southern borders of the
Grampians, the Lower Old Red, containing _Cephalaspis Lyelli,
Pterygotus Anglicus,_ and _Parka decipiens,_ rests unconformably.
Order of Succession in Metamorphic Rocks.—There is no universal and
invariable order of superposition in metamorphic rocks, although a
particular arrangement may prevail throughout countries of great
extent, for the same reason that it is traceable in those sedimentary
formations from which crystalline strata are derived. Thus, for
example, we have seen that in the Apennines, near Carrara, the
descending series, where it is metamorphic, consists of, first,
saccharine marble; second, talcose-schist; and third, of quartz-rock
and gneiss: where unaltered, of, first, fossiliferous limestone;
second, shale; and third, sandstone.
But if we investigate different mountain chains, we find gneiss,
mica-schist, hornblende-schist, chlorite-schist, hypogene limestone,
and other rocks, succeeding each other, and alternating with each other
in every possible order. It is, indeed, more common to meet with some
variety of clay-slate forming the uppermost member of a metamorphic
series than any other rock; but this fact by no means implies, as some
have imagined, that all clay-slates were formed at the close of an
imaginary period when the deposition of the crystalline strata gave way
to that of ordinary sedimentary deposits. Such clay-slates, in fact,
are variable in composition, and sometimes alternate with fossiliferous
strata, so that they may be said to belong almost equally to the
sedimentary and metamorphic order of rocks. It is probable that, had
they been subjected to more intense Plutonic action, they would have
been transformed into hornblende-schist, foliated chlorite-schist,
scaly talcose-schist, mica-schist, or other more perfectly crystalline
rocks, such as are usually associated with gneiss.
_Uniformity of Mineral Character in Hypogene Rocks._—It is true, as
Humboldt has happily remarked, that when we pass to another hemisphere,
we see new forms of animals and plants, and even new constellations in
the heavens; but in the rocks we still recognise our old
acquaintances—the same granite, the same gneiss, the same micaceous
schist, quartz-rock, and the rest. There is certainly a great and
striking general resemblance in the principal kinds of hypogene rocks
in all countries, however different their ages; but each of them, as we
have seen, must be regarded as geological families of rocks, and not as
definite mineral compounds. They are more uniform in aspect than
sedimentary strata, because these last are often composed of fragments
varying greatly in form, size, and colour, and contain fossils of
different shapes and mineral composition, and acquire a variety of
tints from the mixture of various kinds of sediment. The materials of
such strata, if they underwent metamorphism, would be subject to
chemical laws, simple and uniform in their action, the same in every
climate, and wholly undisturbed by mechanical and organic causes. It
would, however, be a great error to assume, as some have done, that the
hypogene rocks, considered as aggregates of simple minerals, are really
more homogeneous in their composition than the several members of the
sedimentary series. Not only do the proportional quantities of
feldspar, quartz, mica, hornblende, and other minerals, vary in
hypogene rocks bearing the same name; but what is still more important,
the ingredients, as we have seen, of the same simple mineral are not
always constant (see p. 503 and table, p. 499).
Supposed Azoic Period.—The total absence of any trace of fossils has
inclined many geologists to attribute the origin of the most ancient
strata to an azoic period, or one antecedent to the existence of
organic beings. Admitting, they say, the obliteration, in some cases,
of fossils by Plutonic action, we might still expect that traces of
them would oftener be found in certain ancient systems of slate which
can scarcely be said to have assumed a crystalline structure. But in
urging this argument it seems to have been forgotten that there are
stratified formations of enormous thickness, and of various ages, some
of them even of Tertiary date, and which we know were formed after the
earth had become the abode of living creatures, which are,
nevertheless, in some districts, entirely destitute of all vestiges of
organic bodies. In some, the traces of fossils may have been effaced by
water and acids, at many successive periods; indeed the removal of the
calcareous matter of fossil shells is proved by the fact of such
organic remains being often replaced by silex or other minerals, and
sometimes by the space once occupied by the fossil being left empty, or
only marked by a faint impression.
Those who believed the hypogene rocks to have originated antecedently
to the creation of organic beings, imputed the absence of lime, so
remarkable in metamorphic strata, to the non-existence of those
mollusca and zoophytes by which shells and corals are secreted; but
when we ascribe the crystalline formations to Plutonic action, it is
natural to inquire whether this action itself may not tend to expel
carbonic acid and lime from the materials which it reduces to fusion or
semi-fusion. Not only carbonate of lime, but also free carbonic acid
gas, is given off plentifully from the soil and crevices of rocks in
regions of active and spent volcanoes, as near Naples and in Auvergne.
By this process, fossil shells or corals may often lose their carbonic
acid, and the residual lime may enter into the composition of augite,
hornblende, garnet, and other hypogene minerals. Although we cannot
descend into the subterranean regions where volcanic heat is developed,
we can observe in regions of extinct volcanoes, such as Auvergne and
Tuscany, hundreds of springs, both cold and thermal, flowing out from
granite and other rocks, and having their waters plentifully charged
with carbonate of lime.
If all the calcareous matter transferred in the course of ages by these
and thousands of other springs from the lower part of the earth’s crust
to the atmosphere could be presented to us in a solid form, we should
find that its volume was comparable to that of many a chain of hills.
Calcareous matter is poured into lakes and the ocean by a thousand
springs and rivers; so that part of almost every new calcareous rock
chemically precipitated, and of many reefs of shelly and coralline
stone, must be derived from mineral matter subtracted by Plutonic
agency, and driven up by gas and steam from fused and heated rocks in
the bowels of the earth.
The scarcity of limestone in many extensive regions of metamorphic
rocks, as in the Eastern and Southern Grampians of Scotland, may have
been the result of some action of this kind; and if the limestones of
the Lower Laurentian in Canada afford a remarkable exception to the
general rule, we must not forget that it is precisely in this most
ancient formation that the _Eozoon Canadense_ has been found. The fact
that some distinct bands of limestone from 700 to 1500 feet thick occur
here, may be connected with the escape from destruction of some few
traces of organic life, even in a rock in which metamorphic action has
gone so far as to produce serpentine, augite, and other minerals found
largely intermixed with the carbonate of lime.
[1] See notices of Savi, Hoffman, and others, referred to by Boué,
Bull. de la Soc. Géol. de France, tome v, p. 317 and tome iii, p. 44;
also Pilla, cited by Murchison, Quart. Geol. Journ., vol. v, p. 266.
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.
The manner in which metallic substances are distributed through the
earth’s crust, and more especially the phenomena of those more or less
connected masses of ore called mineral veins, from which the larger
part of the precious metals used by man are obtained, are subjects of
the highest practical importance to the miner, and of no less
theoretical interest to the geologist.
On different Kinds of Mineral Veins.—The mineral veins with which we
are most familiarly acquainted are those of quartz and carbonate of
lime, which are often observed to form lenticular masses of limited
extent traversing both hypogene strata and fossiliferous rocks. Such
veins appear to have once been chinks or small cavities, caused, like
cracks in clay, by the shrinking of the mass, during desiccation, or in
passing from a higher to a lower temperature. Siliceous, calcareous,
and occasionally metallic matters have sometimes found their way
simultaneously into such empty spaces, by infiltration from the
surrounding rocks. Mixed with hot water and steam, metallic ores may
have permeated the mass until they reached those receptacles formed by
shrinkage, and thus gave rise to that irregular assemblage of veins,
called by the Germans a “stockwerk,” in allusion to the different
floors on which the mining operations are in such cases carried on.
The more ordinary or regular veins are usually worked in vertical
shafts, and have evidently been fissures produced by mechanical
violence. They traverse all kinds of rocks, both hypogene and
fossiliferous, and extend downward to indefinite or unknown depths. We
may assume that they correspond with such rents as we see caused from
time to time by the shock of an earthquake. Metalliferous veins
referable to such agency are occasionally a few inches wide, but more
commonly three or four feet. They hold their course continuously in a
certain prevailing direction for miles or leagues, passing through
rocks varying in mineral composition.
That Metalliferous Veins were Fissures.—As some intelligent miners,
after an attentive study of metalliferous veins, have been unable to
reconcile many of their characteristics with the hypothesis of
fissures, I shall begin by stating the evidence in its favour. The most
striking fact, perhaps, which can be adduced in its support is, the
coincidence of a considerable proportion of mineral veins with
_faults,_ or those dislocations of rocks which are indisputably due to
mechanical force, as above explained (p. 87). There are even proofs in
almost every mining district of a succession of faults, by which the
opposite walls of rents, now the receptacles of metallic substances,
have suffered displacement. Thus, for example, suppose _a a,_ Fig. 629,
to be a tin lode in Cornwall, the term _lode_ being applied to veins
containing metallic ores. This lode, running east and west, is a yard
wide, and is shifted by a copper lode (_b b_) of similar width. The
first fissure (_a a_) has been filled with various materials, partly of
chemical origin, such as quartz, fluor-spar, peroxide of tin, sulphuret
of copper, arsenical pyrites, bismuth, and sulphuret of nickel, and
partly of mechanical origin, comprising clay and angular fragments or
detritus of the intersected rocks. The plates of quartz and the ores
are, in some places, parallel to the vertical sides or walls of the
vein, being divided from each other by alternating layers of clay or
other earthy matter. Occasionally the metallic ores are disseminated in
detached masses among the vein-stones.
It is clear that, after the gradual introduction of the tin and other
substances, the second rent (_b b_) was produced by another fracture
accompanied by a displacement of the rocks along the plane of _b b._
This new opening was then filled with minerals, some of them resembling
those in _a a,_ as fluor-spar (or fluate of lime) and quartz; others
different, the copper being plentiful and the tin wanting or very
scarce. We must next suppose a third movement to occur, breaking
asunder all the rocks along the line _c c,_ Fig. 630; the fissure, in
this instance, being only six inches wide, and simply filled with clay,
derived, probably, from the friction of the walls of the rent, or
partly, perhaps, washed in from above. This new movement has displaced
the rock in such a manner as to interrupt the continuity of the copper
vein (_b b_), and, at the same time, to shift or heave laterally in the
same direction a portion of the tin vein which had not previously been
broken.
Vertical sections of the mine at Huel Peever, Redruth, Cornwall. Fig.
629: Tin; Fig. 630: Copper; Fig. 631: Clay.
Again, in Fig. 631 we see evidence of a fourth fissure (_d d_), also
filled with clay, which has cut through the tin vein (_a a_), and has
lifted it slightly upward towards the south. The various changes here
represented are not ideal, but are exhibited in a section obtained in
working an old Cornish mine, long since abandoned, in the parish of
Redruth, called Huel Peever, and described both by Mr. Williams and Mr.
Carne.[1] The principal movement here referred to, or that of _c c,_
Fig. 631, extends through a space of no less than 84 feet; but in this,
as in the case of the other three, it will be seen that the outline of
the country above, _d, c, b, a,_ etc., or the geographical features of
Cornwall, are not affected by any of the dislocations, a powerful
denuding force having clearly been exerted subsequently to all the
faults. (See p. 93.) It is commonly said in Cornwall, that there are
eight distinct systems of veins, which can in like manner be referred
to as many successive movements or fractures; and the German miners of
the Hartz Mountains speak also of eight systems of veins, referable to
as many periods.
Besides the proofs of mechanical action already explained, the opposite
walls of veins are often beautifully polished, as if glazed, and are
not unfrequently striated or scored with parallel furrows and ridges,
such as would be produced by the continued rubbing together of surfaces
of unequal hardness. These smoothed surfaces resemble the rocky floor
over which a glacier has passed (see Fig. 106). They are common even in
cases where there has been no shift, and occur equally in
non-metalliferous fissures. They are called by miners “slicken-sides,”
from the German _schlichten,_ to plane, and _seite,_ side. It is
supposed that the lines of the striæ indicate the direction in which
the rocks were moved.
In some of the veins in the mountain limestone of Derbyshire,
containing lead, the vein-stuff, which is nearly compact, is
occasionally traversed by what may be called a vertical crack passing
down the middle of the vein. The two faces in contact are
slicken-sides, well polished and fluted, and sometimes covered by a
thin coating of lead-ore. When one side of the vein-stuff is removed,
the other side cracks, especially if small holes be made in it, and
fragments fly off with loud explosions, and continue to do so for some
days. The miner, availing himself of this circumstance, makes with his
pick small holes about six inches apart, and four inches deep, and on
his return in a few hours finds every part ready broken to his hand.[2]
That a great many veins communicated originally with the surface of the
country above, or with the bed of the sea, is proved by the occurrence
in them of well-rounded pebbles, agreeing with those in superficial
alluviums, as in Auvergne and Saxony. Marine fossil shells, also, have
been found at great depths, having probably been ingulfed during
submarine earthquakes. Thus, a gryphæa is stated by M. Virlet to have
been met with in a lead-mine near Semur, in France, and a madrepore in
a compact vein of cinnabar in Hungary.[3] In Bohemia, similar pebbles
have been met with at the depth of 180 fathoms; and in Cornwall, Mr.
Carne mentions true pebbles of quartz and slate in a tin lode of the
Relistran Mine, at the depth of 600 feet below the surface. They were
cemented by oxide of tin and bisulphuret of copper, and were traced
over a space more than twelve feet long and as many wide.[4] When
different sets or systems of veins occur in the same country, those
which are supposed to be of contemporaneous origin, and which are
filled with the same kind of metals, often maintain a general
parallelism of direction. Thus, for example, both the tin and copper
veins in Cornwall run nearly east and west, while the lead veins run
north and south; but there is no general law of direction common to
different mining districts. The parallelism of the veins is another
reason for regarding them as ordinary fissures, for we observe that
faults and trap dikes, admitted by all to be masses of melted matter
which have filled rents, are often parallel.
_Fracture, Re-opening and Successive Formation of Veins._—Assuming,
then, that veins are simply fissures in which chemical and mechanical
deposits have accumulated, we may next consider the proofs of their
having been filled gradually and often during successive enlargements.
Werner observed, in a vein near Gersdorff, in Saxony, no less than
thirteen beds of different minerals, arranged with the utmost
regularity on each side of the central layer. This layer was formed of
two plates of calcareous spar, which had evidently lined the opposite
walls of a vertical cavity. The thirteen beds followed each other in
corresponding order, consisting of fluor-spar, heavy spar, galena, etc.
In these cases the central mass has been last formed, and the two
plates which coat the walls of the rent on each side are the oldest of
all. If they consist of crystalline precipitates, they may be explained
by supposing the fissure to have remained unaltered in its dimensions,
while a series of changes occurred in the nature of the solutions which
rose up from below: but such a mode of deposition, in the case of many
successive and parallel layers, appears to be exceptional.
If a vein-stone consist of crystalline matter, the points of the
crystals are always turned inward, or towards the centre of the vein;
in other words, they point in the direction where there was space for
the development of the crystals. Thus each new layer receives the
impression of the crystals of the preceding layer, and imprints its
crystals on the one which follows, until at length the whole of the
vein is filled: the two layers which meet dovetail the points of their
crystals the one into the other. But in Cornwall, some lodes occur
where the vertical plates, or _combs,_ as they are there called,
exhibit crystals so dovetailed as to prove that the same fissure has
been often enlarged. Sir H. De la Beche gives the following curious and
instructive example (Fig. 632), from a copper-mine in granite, near
Redruth.[5] Each of the plates or combs (_a, b, c, d, e, f_) is double,
having the points of their crystals turned inward along the axis of the
comb. The sides or walls (2, 3, 4, 5 and 6) are parted by a thin
covering of ochreous clay, so that each comb is readily separable from
another by a moderate blow of the hammer. The breadth of each
represents the whole width of the fissure at six successive periods,
and the outer walls of the vein, where the first narrow rent was
formed, consisted of the granitic surfaces 1 and 7.
Fig. 632: Copper lode, near Redruth, enlarged at six successive
periods.
A somewhat analogous interpretation is applicable to many other cases,
where clay, sand, or angular detritus, alternate with ores and
vein-stones. Thus, we may imagine the sides of a fissure to be
incrusted with siliceous matter, as Von Buch observed, in Lancerote,
the walls of a volcanic crater formed in 1731 to be traversed by an
open rent in which hot vapours had deposited hydrate of silica, the
incrustation nearly extending to the middle.[6] Such a vein may then be
filled with clay or sand, and afterwards re-opened, the new rent
dividing the argillaceous deposit, and allowing a quantity of rubbish
to fall down. Various metals and spars may then be precipitated from
aqueous solutions among the interstices of this heterogeneous mass.
That such changes have repeatedly occurred, is demonstrated by
occasional cross-veins, implying the oblique fracture of previously
formed chemical and mechanical deposits. Thus, for example, M. Fournet,
in his description of some mines in Auvergne worked under his
superintendence, observes that the granite of that country was first
penetrated by veins of granite, and then dislocated, so that open rents
crossed both the granite and the granitic veins. Into such openings,
quartz, accompanied by sulphurets of iron and arsenical pyrites, was
introduced. Another convulsion then burst open the rocks along the old
line of fracture, and the first set of deposits were cracked and often
shattered, so that the new rent was filled, not only with angular
fragments of the adjoining rocks, but with pieces of the older
vein-stones. Polished and striated surfaces on the sides or in the
contents of the vein also attest the reality of these movements. A new
period of repose then ensued, during which various sulphurets were
introduced, together with hornstone quartz, by which angular fragments
of the older quartz before mentioned were cemented into a breccia. This
period was followed by other dilatations of the same veins, and the
introduction of other sets of mineral deposits, as well as of pebbles
of the basaltic lavas of Auvergne, derived from superficial alluviums,
probably of Miocene or even Older Pliocene date. Such repeated
enlargement and re-opening of veins might have been anticipated, if we
adopt the theory of fissures, and reflect how few of them have ever
been sealed up entirely, and that a country with fissures only
partially filled must naturally offer much feebler resistance along the
old lines of fracture than anywhere else.
Cause of alternate Contraction and Swelling of Veins.—A large
proportion of metalliferous veins have their opposite walls nearly
parallel, and sometimes over a wide extent of country. There is a fine
example of this in the celebrated vein of Andreasburg in the Hartz,
which has been worked for a depth of 500 yards perpendicularly, and 200
horizontally, retaining almost everywhere a width of three feet. But
many lodes in Cornwall and elsewhere are extremely variable in size,
being one or two inches in one part, and then eight or ten feet in
another, at the distance of a few fathoms, and then again narrowing as
before. Such alternate swelling and contraction is so often
characteristic as to require explanation. The walls of fissures in
general, observes Sir H. De la Beche, are rarely perfect planes
throughout their entire course, nor could we well expect them to be so,
since they commonly pass through rocks of unequal hardness and
different mineral composition. If, therefore, the opposite sides of
such irregular fissures slide upon each other, that is to say, if there
be a fault, as in the case of so many mineral veins, the parallelism of
the opposite walls is at once entirely destroyed, as will be readily
seen by studying Figs. 633 to 635.
Let _a b,_ Fig. 633, be a line of fracture traversing a rock, and let
_a b,_ Fig. 634, represent the same line. Now, if we cut in two a piece
of paper representing this line, and then move the lower portion of
this cut paper sideways from _a_ to _a′_, taking care that the two
pieces of paper still touch each other at the points 1, 2, 3, 4, 5, we
obtain an irregular aperture at _c,_ and isolated cavities at _d, d,
d,_ and when we compare such figures with nature we find that, with
certain modifications, they represent the interior of faults and
mineral veins. If, instead of sliding the cut paper to the right hand,
we move the lower part towards the left, about the same distance that
it was previously slid to the right, we obtain considerable variation
in the cavities so produced, two long irregular open spaces, _f, f,_
Fig. 635, being then formed. This will serve to show to what slight
circumstances considerable variations in the character of the openings
between unevenly fractured surfaces may be due, such surfaces being
moved upon each other, so as to have numerous points of contact.
Figs. 633, 634, 635: Lines of fracture traversing a rock.
Fig. 636: Nipped ores where the course of a vein departs from
verticality.
Most lodes are perpendicular to the horizon, or nearly so; but some of
them have a considerable inclination or “hade,” as it is termed, the
angles of dip being very various. The course of a vein is frequently
very straight; but if tortuous, it is found to be choked up with clay,
stones, and pebbles, at points where it departs most widely from
verticality. Hence at places, such as _a,_ Fig. 636, the miner
complains that the ores are “nipped,” or greatly reduced in quantity,
the space for their free deposition having been interfered with in
consequence of the pre-occupancy of the lode by earthy materials. When
lodes are many fathoms wide, they are usually filled for the most part
with earthy matter, and fragments of rock, through which the ores are
disseminated. The metallic substances frequently coat or encircle
detached pieces of rock, which our miners call “horses” or “riders.”
That we should find some mineral veins which split into branches is
also natural, for we observe the same in regard to open fissures.
Chemical Deposits in Veins.—If we now turn from the mechanical to the
chemical agencies which have been instrumental in the production of
mineral veins, it may be remarked that those parts of fissures which
were choked up with the ruins of fractured rocks must always have been
filled with water; and almost every vein has probably been the channel
by which hot springs, so common in countries of volcanoes and
earthquakes, have made their way to the surface. For we know that the
rents in which ores abound extend downward to vast depths, where the
temperature of the interior of the earth is more elevated. We also know
that mineral veins are most metalliferous near the contact of Plutonic
and stratified formations, especially where the former send veins into
the latter, a circumstance which indicates an original proximity of
veins at their inferior extremity to igneous and heated rocks. It is
moreover acknowledged that even those mineral and thermal springs
which, in the present state of the globe, are far from volcanoes, are
nevertheless observed to burst out along great lines of upheaval and
dislocation of rocks.[7] It is also ascertained that all the substances
with which hot springs are impregnated agree with those discharged in a
gaseous form from volcanoes. Many of these bodies occur as vein-stones;
such as silex, carbonate of lime, sulphur, fluor-spar, sulphate of
barytes, magnesia, oxide of iron, and others. I may add that, if veins
have been filled with gaseous emanations from masses of melted matter,
slowly cooling in the subterranean regions, the contraction of such
masses as they pass from a plastic to a solid state would, according to
the experiments of Deville on granite (a rock which may be taken as a
standard), produce a reduction in volume amounting to 10 per cent. The
slow crystallisation, therefore, of such Plutonic rocks supplies us
with a force not only capable of rending open the incumbent rocks by
causing a failure of support, but also of giving rise to faults
whenever one portion of the earth’s crust subsides slowly while another
contiguous to it happens to rest on a different foundation, so as to
remain unmoved.
Although we are led to infer, from the foregoing reasoning, that there
has often been an intimate connection between metalliferous veins and
hot springs holding mineral matter in solution, yet we must not on that
account expect that the contents of hot springs and mineral veins would
be identical. On the contrary, M. E. de Beaumont has judiciously
observed that we ought to find in veins those substances which, being
least soluble, are not discharged by hot springs—or that class of
simple and compound bodies which the thermal waters ascending from
below would first precipitate on the walls of a fissure, as soon as
their temperature began slightly to diminish. The higher they mount
towards the surface, the more will they cool, till they acquire the
average temperature of springs, being in that case chiefly charged with
the most soluble substances, such as the alkalies, soda and potash.
These are not met with in veins, although they enter so largely into
the composition of granitic rocks.[8]
To a certain extent, therefore, the arrangement and distribution of
metallic matter in veins may be referred to ordinary chemical action,
or to those variations in temperature which waters holding the ores in
solution must undergo, as they rise upward from great depths in the
earth. But there are other phenomena which do not admit of the same
simple explanation. Thus, for example, in Derbyshire, veins containing
ores of lead, zinc, and copper, but chiefly lead, traverse alternate
beds of limestone and greenstone. The ore is plentiful where the walls
of the rent consist of limestone, but is reduced to a mere string when
they are formed of greenstone, or “toad-stone,” as it is called
provincially. Not that the original fissure is narrower where the
greenstone occurs, but because more of the space is there filled with
vein-stones, and the waters at such points have not parted so freely
with their metallic contents.
“Lodes in Cornwall,” says Mr. Robert W. Fox, “are very much influenced
in their metallic riches by the nature of the rock which they traverse,
and they often change in this respect very suddenly, in passing from
one rock to another. Thus many lodes which yield abundance of ore in
granite, are unproductive in clay-slate, or killas and _vice versa._
Supposed relative Age of the different Metals.—After duly reflecting on
the facts above described, we cannot doubt that mineral veins, like
eruptions of granite or trap, are referable to many distinct periods of
the earth’s history, although it may be more difficult to determine the
precise age of veins; because they have often remained open for ages,
and because, as we have seen, the same fissure, after having been once
filled, has frequently been re-opened or enlarged. But besides this
diversity of age, it has been supposed by some geologists that certain
metals have been produced exclusively in earlier, others in more modern
times; that tin, for example, is of higher antiquity than copper,
copper than lead or silver, and all of them more ancient than gold. I
shall first point out that the facts once relied upon in support of
some of these views are contradicted by later experience, and then
consider how far any chronological order of arrangement can be
recognised in the position of the precious and other metals in the
earth’s crust.
In the first place, it is not true that veins in which tin abounds are
the oldest lodes worked in Great Britain. The government survey of
Ireland has demonstrated that in Wexford veins of copper and lead (the
latter as usual being argentiferous) are much older than the tin of
Cornwall. In each of the two countries a very similar series of
geological changes has occurred at two distinct epochs—in Wexford,
before the Devonian strata were deposited; in Cornwall, after the
Carboniferous epoch. To begin with the Irish mining district: We have
granite in Wexford traversed by granite veins, which veins also intrude
themselves into the Silurian strata, the same Silurian rocks as well as
the veins having been denuded before the Devonian beds were
superimposed. Next we find, in the same county, that elvans, or
straight dikes of porphyritic granite, have cut through the granite and
the veins before mentioned, but have not penetrated the Devonian rocks.
Subsequently to these elvans, veins of copper and lead were produced,
being of a date certainly posterior to the Silurian, and anterior to
the Devonian; for they do not enter the latter, and, what is still more
decisive, streaks or layers of derivative copper have been found near
Wexford in the Devonian, not far from points where mines of copper are
worked in the Silurian strata.
Although the precise age of such copper lodes cannot be defined, we may
safely affirm that they were either filled at the close of the Silurian
or commencement of the Devonian period. Besides copper, lead, and
silver, there is some gold in these ancient or primary metalliferous
veins. A few fragments also of tin found in Wicklow in the drift are
supposed to have been derived from veins of the same age.[9]
Next, if we turn to Cornwall, we find there also the monuments of a
very analogous sequence of events. First, the granite was formed; then,
about the same period, veins of fine-grained granite, often tortuous
(see Fig. 614), penetrating both the outer crust of granite and the
adjoining fossiliferous or primary rocks, including the coal-measures;
thirdly, elvans, holding their course straight through granite,
granitic veins, and fossiliferous slates; fourthly, veins of tin also
containing copper, the first of those eight systems of fissures of
different ages already alluded to, p. 607. Here, then, the tin lodes
are newer than the elvans. It has, indeed, been stated by some Cornish
miners that the elvans are in some instances posterior to the oldest
tin-bearing lodes, but the observations of Sir H. de la Beche during
the survey led him to an opposite conclusion, and he has shown how the
cases referred to in corroboration can be otherwise interpreted.[10] We
may, therefore, assert that the most ancient Cornish lodes are younger
than the coal-measures of that part of England, and it follows that
they are of a much later date than the Irish copper and lead of Wexford
and some adjoining counties. How much later, it is not so easy to
declare, although probably they are not newer than the beginning of the
Permian period, as no tin lodes have been discovered in any red
sandstone which overlies the coal in the south-west of England.
There are lead veins in Glamorganshire which enter the lias, and others
near Frome, in Somersetshire, which have been traced into the Inferior
Oolite. In Bohemia, the rich veins of silver of Joachimsthal cut
through basalt containing olivine, which overlies tertiary lignite, in
which are leaves of dicotyledonous trees. This silver, therefore, is
decidedly a tertiary formation. In regard to the age of the gold of the
Ural mountains, in Russia, which, like that of California, is obtained
chiefly from auriferous alluvium, it occurs in veins of quartz in the
schistose and granitic rocks of that chain, and is supposed by Sir R.
Murchison, MM. Deverneuil and Keyserling to be newer than the syenitic
granite of the Ural—perhaps of tertiary date. They observe that no gold
has yet been found in the Permian conglomerates which lie at the base
of the Ural Mountains, although large quantities of iron and copper
detritus are mixed with the pebbles of those Permian strata. Hence it
seems that the Uralian quartz veins, containing gold and platinum, were
not formed, or certainly not exposed to aqueous denudation, during the
Permian era.
In the auriferous alluvium of Russia, California, and Australia, the
bones of extinct land-quadrupeds have been met with, those of the
mammoth being common in the gravel at the foot of the Ural Mountains,
while in Australia they consist of huge marsupials, some of them of the
size of the rhinoceros and allied to the living wombat. They belong to
the genera Diprotodon and Nototherium of Professor Owen. The gold of
Northern Chili is associated in the mines of Los Hornos with copper
pyrites, in veins traversing the cretaceo-oolitic formations, so-called
because its fossils have the character partly of the cretaceous and
partly of the oolitic fauna of Europe.[11] The gold found in the United
States, in the mountainous parts of Virginia, North and South Carolina,
and Georgia, occurs in metamorphic Silurian strata, as well as in
auriferous gravel derived from the same.
Gold has now been detected in almost every kind of rock, in slate,
quartzite, sandstone, limestone, granite, and serpentine, both in veins
and in the rocks themselves at short distances from the veins. In
Australia it has been worked successfully not only in alluvium, but in
vein-stones in the native rock, generally consisting of Silurian shales
and slates. It has been traced on that continent over more than nine
degrees of latitude (between the parallels of 30° and 39° S.), and over
twelve of longitude, and yielded in 1853 an annual supply equal, if not
superior, to that of California; nor is there any apparent prospect of
this supply diminishing, still less of the exhaustion of the
gold-fields.
_Origin of Gold in California._—Mr. J. Arthur Phillips,[12] in his
treatise “On the Gold Fields of California,” has shown that the ore in
the gold workings is derived from drifts, or gravel clay, and sand, of
two distinct geological ages, both comparatively modern, but belonging
to different river-systems, the older of which is so ancient as to be
capped by a thick sheet of lava divided by basaltic columns. The
auriferous quartz of these drifts is derived from veins apparently due
to hydrothermal agency, proceeding from granite and penetrating strata
supposed to be of Jurassic and Triassic date. The fossil wood of the
drift is sometimes beautifully silicified, and occasionally the trunks
of trees are replaced by iron pyrites, but gold seems not to have been
found as in the pyrites of similarly petrified trees in the drift of
Australia.
The formation of recent metalliferous veins is now going on, according
to Mr. Phillips, in various parts of the Pacific coast. Thus, for
example, there are fissures at the foot of the eastern declivity of the
Sierra Nevada in the state of that name, from which boiling water and
steam escape, forming siliceous incrustations on the sides of the
fissures. In one case, where the fissure is partially filled up with
silica inclosing iron and copper pyrites, gold has also been found in
the vein-stone.
It has been remarked by M. de Beaumont, that lead and some other metals
are found in dikes of basalt and greenstone, as well as in mineral
veins connected with trap-rock, whereas tin is met with in granite and
in veins associated with the Plutonic series. If this rule hold true
generally, the geological position of tin accessible to the miner will
belong, for the most part, to rocks older than those bearing lead. The
tin veins will be of higher relative antiquity for the same reason that
the “underlying” igneous formations or granites which are visible to
man are older, on the whole, than the overlying or trappean formations.
If different sets of fissures, originating simultaneously at different
levels in the earth’s crust, and communicating, some of them with
volcanic, others with heated Plutonic masses, be filled with different
metals, it will follow that those formed farthest from the surface will
usually require the longest time before they can be exposed
superficially. In order to bring them into view, or within reach of the
miner, a greater amount of upheaval and denudation must take place in
proportion as they have lain deeper when first formed and filled. A
considerable series of geological revolutions must intervene before any
part of the fissure which has been for ages in the proximity of the
Plutonic rock, so as to receive the gases discharged from it when it
was cooling, can emerge into the atmosphere. But I need not enlarge on
this subject, as the reader will remember what was said in the 30th,
32nd, and 35th chapters on the chronology of the volcanic and hypogene
formations.
[1] Geol. Trans., vol. iv, p. 139; Trans. Royal Geol. Society,
Cornwall, vol. ii, p. 90
[2] Conybeare and Phil. Geol., p. 401, and Farey’s Derbyshire, p. 243.
[3] Fournet, Études sur les Dépôts Métallifères.
[4] Carne, Trans. Geol. Soc., Cornwall, vol. iii, p. 238.
[5] Geol. Rep. on Cornwall, p. 340.
[6] Principles, chap. xxvii, 8th edit., p. 422.
[7] See Dr. Daubeny’s Volcanoes.
[8] Bulletin, iv, p. 1278.
[9] Sir H. De la Beche, MS. Notes on Irish Survey.
[10] Report on Geology of Cornwall, p. 310.
[11] Darwin’s South America, p. 209, etc.
[12] Proc. Royal Soc., 1868, p. 294.
INDEX.
——::——
_The Fossils, the names of which appear in Italics, are figured in the
Text._
ABBEVILLE, flint tools of, 152
Aberdeenshire, granite of, 558
Abich, M., on trachytic rocks, 504
_Acer trilobatum,_ Miocene, 220, 221
_Acrodus nobilis,_ Lias, 359
Acrogens, term explained, 303
_Acrolepis Sedgwickii,_ Permian, 390
_Actæon acutus,_ Great Oolite, 345
_Actinocyclas,_ in Atlantic mud, 288
Actinolite, 499, 502
—— schist, 578
_Æchmodus Leachii,_ Lias, 358
_Adiantites Hibernica,_ Old Red, 441
Agassiz on fish of Sheppey, 267
—— on fish of the Brown-Coal, 540
—— on fish of Monte Bolca, 544
—— on Old Red fossil fish, 443, 447
—— on Silurian fish, 460
Age of metamorphic rocks, 597
—— of Plutonic rocks, 564
—— of strata, tests of, 123
—— of volcanic rocks, 520
Agglomerate described, 509
_Agnostus integer. A. Rex_, 488
Air-breathers of the Coal, 413
Aix-la-Chapelle, Cretaceous flora of, 302
Alabaster defined, 39
Alberti on Keuper, 376
Albite, 499, 500
Aldeby and Chillesford beds, 192
Alkali, present in the Palæozoic strata, 587
Alpine blocks on the Jura, 169
Alps, age of metamorphic rocks in, 599
——, nummulitic limestone and flysch of, 77
Alum schists of Norway and Sweden, 489
Alluvial deposits, Recent and Post-pliocene, 151
Alluvium, term explained, 99
—— in Auvergne, 100
Alternations of marine and fresh-water strata, 72
Alum Bay beds, plants of the, 262
Amblyrhynchus cristatus, a living marine saurian, 362
America. _See_ United States, Canada, Nova Scotia.
——, North, Glacial formations of, 182
——, South, gradual rise of land in, 72
——, Silurian strata of, 478
American character of Lower Miocene flora, 238
—— forms in Swiss Miocene flora, 223
Amiens, flint tools of, 152
_Ammonites bifrons,_ Lias, 356
—— _Braikenridgii,_ Oolite, 351
—— _Bucklandi,_ Lias, 356
—— _Deshayesii,_ Neocomian, 311
—— _Humphresianus,_ Inferior Oolite, 351
—— _Jason,_ Oxford Clay, 340
—— _Noricus,_ Speeton, 312
—— _macrocephalus,_ Oolite, 352
—— _margaritatus,_ Lias, 357
—— _planorbis,_ Lias, 356
—— _rhotomagensis,_ Chalk marl, 298
Amphibole group of minerals, 499, 502
_Amphistegina Hauerina,_ Vienna basin, 225
_Amphitherium Broderipii,_ in Stonesfield, 348
—— _Prevostii,_ Stonesfield slate, 347
_Ampullaria glauca_, 56
_Amygdaloid_, 507
Analcime, 500
Anamesite, a variety of basalt, 504
_Ananchytes ovatus,_ White chalk, 293
——, with crania attached, 49
_Ancillaria subulata,_ Eocene, 57
_Ancyloceras gigas_, 309
—— _spinigerum,_ Gault, 301
—— _Duvallei,_ Neocomian, 312
_Ancylus velletia (A. elegans)_, 55
Andalusite, 500
Andes, Plutonic rocks of the, 569
Andreasburg, metalliferous vein of, 611
Angelin, on Cambrian of Sweden, 489
Angiosperms, 303
—— of the Coal, 429
Anglesea, dike cutting through shale in, 514
_Anodonta Cordierii_, 54
—— _Jukesii,_ Upper Old Red, 441
—— _latimarginata_, 54
_Anoplotherium commune,_ Binstead, 254
—— _gracile,_ Paris basin, 271
Anorthite, 499, 501
_Annularia sphenophylloides,_ Coal, 425
_Antholithes,_ coal-measures, 429
Anthracite, conversion of coal into, 408
Anticlinal and synclinal curves, 74, 85
Antrim, Chalk altered by a dike in, 516
——, Lower Miocene, volcanic rocks of, 539
Antwerp Crag, 204
Apateon pedestris, a carboniferous reptile, 406
Apatite, 500
Apennines, Northern, metamorphic rocks of, 599
Apes, fossil of the Upper Miocene, 215
_Apiocrinites rotundus,_ Bradford, 343
Appalachians, long lines of flexures in, 92, 93
——, vast thickness of successive strata in, 110
_Aptychus,_ part of ammonite, 336
Aqueous rocks defined, 27, 35
_Araucaria sphærocarpa,_ Inferior Oolite, 348
Arbroath, section of Old Red at, 74
_Archæopteryx macrura,_ Solenhofen, 338
_Archegosaurus minor and A. medius,_ coal measures, 406, 407
Archiac, M. de, on nummulites, 277
——, on chalk of France, 306
Arctic Miocene Flora, 239
Area of the Wealden, 319
Areas, permanence of continental, 117
Arenaceous rocks described, 35
_Arenicolites linearis,_ Arenig beds, 475
Arenig or Stiper-Stones group, 474
——, volcanic formations of, 549
Argile plastique, 276
Argillaceous rocks described, 36
Argillite, Argillaceous schist, 579
Argyll, Duke of, on Isle of Mull leaf-beds, 247
Armagh, bone-beds in Mountain Limestone at, 437
Arran, amygdaloid filled with spar near, 518
——, erect trees in volcanic ash of, 546
——, Greenstone dike in, 514
Arthur’s seat, trap rocks of, 545
_Arvicola,_ tooth of, 165
_Asaphus caudatus,_ Silurian, 467
—— _tyrannus, A. Buchii_, 474
Ascension, lamination of volcanic rocks in, 595
Ash, Mr., on fossils of Tremadoc beds, 483
Ashby-de-la-Zouch, fault in coal field of, 91
_Aspidura loricata,_ Muschelkalk, 379
_Astarte borealis_ (=_A. arctica=A. compressa_), 176
—— _Omalii,_ Crag, 199
_Asterophyllites foliosus,_ Coal, 425
_Astrangia lineata (Anthophyllum lineatum)_, 229
_Astræa basaltiforme,_ Carboniferous, 432
_Astropecten crispatus,_ London clay, 266
Atherfield clay, 309
Atlantic mud, composition of, 287
_Atrypa reticularis,_ Aymestry, 462
_Aturia ziczac (Nautilus ziczac)_, 266
Augite, 499, 502
_Auricula,_ recent, 55
Austen, Mr. Godwin, on marine deposit of Selsea Bill, 182
——, on boulders in chalk, 292
Australian cave breccias, 158
Australia, auriferous gravel of, 617
Auvergne, alluvium in, 100
——, chain of extinct volcanoes in, 495
——, granite veins in, 610
——, Lower Miocene of, 233
——, Miocene volcanic rocks of, 540
——, Post-pliocene volcanic eruptions in, 527
——, springs from spent volcanoes in, 604
Aveline Mr., on Tarannon shales, 468
_Avicula contorta,_ Rhætic beds, 366
—— _cygnipes,_ Lias, 355
—— _inæquivalvis,_ Lias, 355
—— _socialis,_ Muschelkalk, 379
_Aviculopecten papyraceus,_ coal measures, 405
—— _sublobatus,_ mountain limestone, 434
Aymestry Limestone, 461
Azoic period, supposed, 603
Azores, Miocene lavas with shells, 539
_BACILLARIA paradoxa_, 51
_Baculites anceps,_ Lower Chalk, 298
—— _Fauiasii,_ chalk, 286
Baffin’s Bay, formation of drift in, 171, 173
Bagshot sands, 258, 259, 262
Baiæ, Bay of, subterranean igneous action in, 569
Bakewell, Mr., on cleavage in Swiss Alps, 590
Bala and Caradoc beds, 470
_Balistidæ,_ defensive spine of, 261
Bangor, or Longmynd group, 485
_Banksia,_ seed and fruit of, Lower Miocene, 238
Barmouth sandstones, 486
Barnes, Mr. J., on insects in American coal, 416
Barnstaple, Upper Devonian of, 450
Barrande, M. Joachim, his “Primordial Zone,” 471, 482, 487
——, on metamorphosis of trilobites, 471
Barrett, Mr., on bird in Blackdown beds, 299
Barton series sands and clays, 258
—— shells, percentage of, common to London clay, 258
Basalt, columnar, 511
——, composition of, 504
Basaltic rocks, poor in silica, 504
——, specific gravity of minerals in, 504
_Basilosaurus,_ Eocene, United States, 280
Basset, term explained, 83
Basterot, M. de, on Bordeaux tertiary strata, 141
Bath Oolite, 342
Batrachian reptiles in coal, 406
Bay of Fundy, denudation in coalfield in, 418
Bean, Mr., on Yorkshire Oolite, 350
Bear Island carboniferous flora, 441
Beaumont, M. E. de, on island in Cretaceous sea, 305
——, on mineral veins, 613
——, on Jurassic plutonic rocks, 571
——, on formation of granite, 553
Beckles, Mr. S. H., on footprints in Hastings sands, 315, 330
—— on Mammalia of Purbeck, 326
_Belemnitella mucronata,_ Chalk, 283
_Belemnites hastatus,_ Oxford clay, 340
—— _Puzosianus,_ Oxford clay, 341
Belgium, Lower Miocene of, 241
_Bellerophon costatus,_ Mountain Limestone, 436
_Belosepia sepioidea,_ Sheppey, 266
Belt, Mr., on subdivision of Lingula Flags, 484
Bembridge beds, Yarmouth, 252
Berger, Dr., on rocks altered by dikes, 515
Berlin, Miocene strata near, 242
Bernese Alps, gneiss in the, 599
Berthier on isomorphism, 502
Bertrich-Baden, columnar basalt of, 512
Beyrich on term Oligocene for Lower Miocene, 244
Billings, Mr., on trilobites, 471
Binney, Mr., on Sigillariæ in volcanic ash, 546
——, on Stigmaria, the root of Sigillaria, 426
Biotite, 499, 501
Bird in argile plastique, 276
Bischoff, Professor, on Nile and Rhine mud, 154
——, on conversion of coal into anthracite, 403
——, on hydrothermal action, 586
Blackdown beds, 301
Blacklead of Borrowdale, 65
Bog-iron-ore, 52
Bohemia, Cambrian rocks of, 487
——, silver veins in, 616
Bolderberg, in Belgium, Upper Miocene of, 224
Bone-bed of fish remains, Armagh, 437
—— of Upper Ludlow, 450
—— of the Trias, 367
Boom, Lower Miocene of, 241
Bordeaux, Upper Miocene of, 214
Borrowdale, blacklead of, 65
Bosquet, M. on chalk fossils, 283
——, on Maestricht beds, 283
Botanical nomenclature, 303
Boucher de Perthes on Abbeville alluvium, 152
Boulder-clay, whether formed by icebergs or land-ice, 166-73, 178
Boulder-clay of Canada, 182
—— fauna of, 176, 189
Boulders and pebbles in chalk, 292
Bournemouth beds (Lower Bagshot), 262
Bovey Tracey, lignites and clays of, 246
Bowerbank, Mr., on fossil fruits of London Clay, 265
——, on fossil fruits of Sheppey, 265
Bowman, Mr., on uniting of distinct coal-seams, 401
Brachiopoda, preponderance of, in older rocks, 470
——, mode of recognising shells of, 471
Bracklesham beds and Bagshot Sands, 259
Bradford encrinites, 342
Breccias of Lower Permian, 391
Brick-earth or fluviatile loam, 153
Bridlington drift, 189
Bristol, dolomitic conglomerate of, 373
Bristow, Mr., on volcanic minerals, 500
Brixham cave near Torquay, 158
Brocchi on Italian tertiary strata, 141
—— on subapennine strata, 208
Brockenhurst, corals and shells of, 257
Brodie, Rev. P. B., on Lias insects, 363
Brodie, Mr. W. R., on Purbeck mammalia, 326
Brongniart, M. Adolphe, on botanical nomenclature, 303
——, on Lias plants, 364
——, on flora of the Bunter, 380
——, on flora of the coal, 420
——, on fruit of Lepidodendron, 424
——, M. Alex., on Tertiary series, 141
_Bronteus flabellifer_, Devonian, 453
Brora, oolitic coal formation of, 350
Brown, Mr. Richard, on Stigmaria, 426
——, on carboniferous rain-prints, 416
Brown, Robert, on Eocene protaceous fruit, 264
Brown, Reverend T., on marine shells in Scotch drift, 177
Brown-coal of Germany, 540
Bryce, Mr., on Scotch till, 176
Bryozoa of Mountain Limestone, 433
—— and polyzoa, terms explained, 197
Buch, von. _See_ Von Buch.
Buckland, Dr., on Kirkdale cave, 158
——, on violent death of saurians, 362
——, on spines of fish, 359
——, on Eocene oysters, 268
——, on pot-stones in chalk, 291
Buddle, Mr., on creeps in coal-mines, 78
_Bulimus ellipticus_, Bembridge, 253
—— _lubricus_, Loess, 56
Bullock, Capt., R.N., on Atlantic mud, 287
Bunbury, Sir C., on leaf-bed of Madeira, 532
——, on ferns of the Maryland coal, 421
Bunter of Germany, 380
—— or Lower Trias of England, 372
_Buprestis? Elytron of_, Stonesfield, 346
Burmeister on trilobites, 471