The Student's Elements of Geology
by
Sir Charles Lyell
Part 12 out of 14
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 Senarmont,
Daubree, 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 Plombieres
springs. The quantity of water required, according to Daubree, 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. (See Principles
Index Carbonated Springs etc.)
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 (Chapter 31), 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 degrees to
167 degrees 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.
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.
(FIGURE 624. Parallel planes of cleavage intersecting curved strata.
(Sedgwick.))
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 Figure 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 degrees. Sometimes the cleavage
planes dip towards the same point of the compass as those of stratification, but
often to opposite points. (Geological Transactions second series volume 3 page
461.) The cleavage, as represented in Figure 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.
(FIGURE 625. Section in Lower Silurian slates of Cardiganshire, showing the
cleavage planes bent along the junction of the beds. (T. McK. Hughes.))
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 degrees 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 Figure
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. (Silurian
System page 246.)
(FIGURE 626. Stratification, joints, and cleavage (From Murchison's Silurian
System page 245.))
In Figure 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.
(Introduction to Geology chapter 4.)
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 exist (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.),
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." (Letter to
the author dated Cape of Good Hope February 20, 1836.)
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. (Report British Association Cork 1843
Section page 60.) 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. (Quarterly Geological Journal volume 3 page 87 1847.)
(FIGURE 627. Vertical section of slate rock in the cliffs near Ilfracombe, North
Devon. Scale one inch to one foot. (Drawn by H.C. Sorby.)
a, b, c, e. Fine-grained slates, the stratification being shown partly by
lighter or darker colours, and partly by different degrees of fineness in the
grain.
d, f. A coarser grained light-coloured sandy slate with less perfect cleavage.)
Subsequently (1853) Mr. Sorby demonstrated the great extent to which this
mechanical theory is applicable to the slate rocks of North Wales and Devonshire
(On the Origin of Slaty Cleavage by H.C. Sorby Edinburgh New Philosophical
Journal 1853 volume 55 page 137.), 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 Figure 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.
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. (Sorby as cited above page 741 note.)
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
laminae of surpassing tenuity." (Tyndall View of the Cleavage of Crystals and
Slate rocks.) 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 laminae 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 laminae.
The zones consist sometimes of layers of air-cells drawn out and lengthened in
the supposed direction of the moving mass. (Darwin Volcanic Islands pages 69,
70.)
FOLIATION OF CRYSTALLINE SCHISTS.
After studying, in 1835, the crystalline rocks of South America, Mr. Darwin
proposed the term FOLIATION for the laminae 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. (Norske Mag. Naturvidsk. volume 1 page 71.) 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 degrees N.E. (Memoir read before the Geological Society
London January 31, 1855.) 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 Chapter 4). 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 (Chapter 2)-- 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. (H.C. Sorby
Quarterly Geological Journal volume 19 page 401.)
(FIGURE 628. Lamination of clay-stone. Montagne de Seguinat, near Gavarnie, in
the Pyrenees.)
In Figure 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.
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 Chapters 33 and 34.) 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 33, 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 Palaeozoic 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 Palaeozoic 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 Chapter 16), 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 can not 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,
Boue, 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. (See notices of
Savi, Hoffman, and others, referred to by Boue, Bull. de la Soc. Geol. de France
tome 5 page 317 and tome 3 page 44; also Pilla, cited by Murchison Quarterly
Geological Journal volume 5 page 266.) 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 (Figure 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 (Chapter 27), 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 (Chapter 27).
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 Figure 82. It is the same as that intersected by
numerous granite veins which forms the cliffs of Cape Wrath, in Sutherlandshire
(see Figure 613), and is conjectured to be of Laurentian age. Above it, as shown
in the section (Figure 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 (Chapter 7), 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; secondly, talcose-schist;
and thirdly, of quartz-rock and gneiss: where unaltered, of, first,
fossiliferous limestone; secondly, shale; and thirdly, 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 (Chapter 28 and Table 28.1).
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 can not 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.
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.
(FIGURES 629, 630 and 631. Vertical sections of the mine of Huel Peever,
Redruth, Cornwall.
(Figure 629. Vertical section of the mine of Huel Peever, Redruth, Cornwall.
Tin.)
(FIGURE 630. Vertical section of the mine of Huel Peever, Redruth, Cornwall.
Copper.)
(FIGURE 631. Vertical section of the mine of Huel Peever, Redruth, Cornwall.
Clay and copper.))
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
(Chapter 5). 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, Figure 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, Figure 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.
Again, in Figure 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. (Geological Transactions volume 4 page 139;
Transactions of the Royal Geological Society Cornwall volume 2 page 90.) The
principal movement here referred to, or that of c-c, Figure 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 Chapter 5.) 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
Figure 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 striae 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. (Conybeare and Phil. Geol. page 401 and Farey's
Derbyshire page 243.)
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 gryphaea 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. (Fournet Etudes sur les
Depots Metalliferes.) 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. (carne
Transactions of the Geological Society Cornwall volume 3 page 238.) 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.
(FIGURE 632. Copper lode, near Redruth, enlarged at six successive periods.)
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
(Figure 632), from a copper-mine in granite, near Redruth. (Geological Report on
Cornwall page 340.) 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.
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. (Principles chapter 27 8th
edition page 422.) 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.
(FIGURES 633 to 635. Irregular fissures.
(FIGURE 633.)
(FIGURE 634.)
(FIGURE 635.))
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 Figures 633 to 635.
Let a-b, Figure 633, be a line of fracture traversing a rock, and let a-b,
Figure 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, Figure 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.
(FIGURE 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, Figure 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. (See Dr. Daubeny's Volcanoes.) 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. (Bulletin 4 page
1278.)
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 can not 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 can not 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. (Sir H. De la Beche MS. Notes on Irish Survey.)
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 Figure 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. 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. (Report on the Geology of Cornwall page 310.) 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. (Darwin's South America page 209 etc.) 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
degrees and 39 degrees 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, 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. (Proceedings of the
Royal Society 1868 page 294.) 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, 32d, and 35th chapters on the chronology of the
volcanic and hypogene formations.
INDEX.
Abbeville, flint tools of.
Aberdeenshire, granite of.
Abich, M., on trachytic rocks.
Acer trilobatum, Miocene.
Acrodus nobilis, Lias.
Acrogens, term explained.
Acrolepis Sedgwickii, Permian.
Actaeon acutus, Great Oolite.
Actinocyclas, in Atlantic mud.
Actinolite.
-- schist.
Aechmodus Leachii, Lias.
Adiantites Hibernica, Old Red.
Agassiz on fish of Sheppey.
-- on fish of the Brown-Coal.
-- on fish of Monte Bolca.
-- on Old Red fossil fish.
-- on Silurian fish.
Age of metamorphic rocks.
-- of Plutonic rocks.
-- of strata, tests of.
-- of volcanic rocks.
Agglomerate described.
Agnostus integer. A. Rex.
Air-breathers of the Coal.
Aix-la-Chapelle, Cretaceous flora of.
Alabaster defined.
Alberti on Keuper.
Albite.
Aldeby and Chillesford beds.
Alkali, present in the Palaeozoic strata.
Alpine blocks on the Jura.
Alps, age of metamorphic rocks in.
--, nummulitic limestone and flysch of.
Alum schists of Norway and Sweden.
Alluvial deposits, Recent and Post-pliocene.
Alluvium, term explained.
-- in Auvergne.
Alternations of marine and fresh-water strata.
Alum Bay beds, plants of the.
Amblyrhynchus cristatus, a living marine saurian.
America. See United States, Canada, Nova Scotia.
--, North, Glacial formations of.
--, South, gradual rise of land in.
--, Silurian strata of.
American character of Lower Miocene flora.
-- forms in Swiss Miocene flora.
Amiens, flint tools of.
Ammonites bifrons, Lias.
-- Braikenridgii, Oolite.
-- Bucklandi, Lias.
-- Deshayesii, Neocomian.
-- Humphresianus, Inferior Oolite.
-- Jason, Oxford Clay.
-- Noricus, Speeton.
-- macrocephalus, Oolite.
-- margaritatus, Lias.
-- planorbis, Lias.
-- rhotomagensis, Chalk marl.
Amphibole group of minerals.
Amphistegina Hauerina, Vienna basin.
Amphitherium Broderipii, in Stonesfield.
-- Prevostii, Stonesfield slate.
Ampullaria glauca.
Amygdaloid.
Analcime.
Anamesite, a variety of basalt.
Ananchytes ovatus, White chalk.
--, with crania attached.
Ancillaria subulata, Eocene.
Ancyloceras gigas.
-- spinigerum, Gault.
-- Duvallei, Neocomian.
Ancylus velletia (A. elegans).
Andalusite.
Andes, Plutonic rocks of the.
Andreasburg, metalliferous vein of.
Angelin, on Cambrian of Sweden.
Angiosperms.
-- of the Coal.
Anglesea, dike cutting through shale in.
Anodonta Cordierii.
-- Jukesii, Upper Old Red.
-- latimarginata.
Anoplotherium commune, Binstead.
-- gracile, Paris basin.
Anorthite.
Annularia sphenophylloides, Coal.
Antholithes, coal-measures.
Anthracite, conversion of coal into.
Anticlinal and synclinal curves.
Antrim, Chalk altered by a dike in.
--, Lower Miocene, volcanic rocks of.
Antwerp Crag.
Apateon pedestris, a carboniferous reptile.
Apatite.
Apennines, Northern, metamorphic rocks of.
Apes, fossil of the Upper Miocene.
Apiocrinites rotundus, Bradford.
Appalachians, long lines of flexures in.
--, vast thickness of successive strata in.
Aptychus, part of ammonite.
Aqueous rocks defined.
Araucaria sphaerocarpa, Inferior Oolite.
Arbroath, section of Old Red at.
Archaeopteryx macrura, Solenhofen.
Archegosaurus minor and A. medius, coal measures.
Archiac, M. de, on nummulites.
--, on chalk of France.
Arctic Miocene Flora.
Area of the Wealden.
Areas, permanence of continental.
Arenaceous rocks described.
Arenicolites linearis, Arenig beds.
Arenig or Stiper-Stones group.
--, volcanic formations of.
Argile plastique.
Argillaceous rocks described.
Argillite, Argillaceous schist.
Argyll, Duke of, on Isle of Mull leaf-beds.
Armagh, bone-beds in Mountain Limestone at.
Arran, amygdaloid filled with spar near.
--, erect trees in volcanic ash of.
--, Greenstone dike in.
Arthur's seat, trap rocks of.
Arvicola, tooth of.
Asaphus caudatus, Silurian.
-- tyrannus, A. Buchii.
Ascension, lamination of volcanic rocks in.
Ash, Mr., on fossils of Tremadoc beds.
Ashby-de-la-Zouch, fault in coal field of.
Aspidura loricata, Muschelkalk.
Astarte borealis (=A. arctica = A. compressa).
-- Omalii, Crag.
Asterophyllites foliosus, Coal.
Astrangia lineata (Anthophyllum lineatum).
Astraea basaltiforme, Carboniferous.
Astropecten crispatus, London clay.
Atherfield clay.
Atlantic mud, composition of.
Atrypa reticularis, Aymestry.
Aturia ziczac (Nautilus ziczac).
Augite.
Auricula, recent.
Austen, Mr. Godwin, on marine deposit of Selsea Bill.
--, on boulders in chalk.
Australian cave breccias.
Australia, auriferous gravel of.
Auvergne, alluvium in.
--, chain of extinct volcanoes in.
--, granite veins in.
--, Lower Miocene of.
--, Miocene volcanic rocks of.
--, Post-pliocene volcanic eruptions in.
--, springs from spent volcanoes in.
Aveline Mr., on Tarannon shales.
Avicula contorta, Rhaetic beds.
-- cygnipes, Lias.
-- inaequivalvis, Lias.
-- socialis, Muschelkalk.
Aviculopecten papyraceus, coal measures.
-- sublobatus, mountain limestone.
Aymestry Limestone.
Azoic period, supposed.
Azores, Miocene lavas with shells.
Bacillaria paradoxa.
Baculites anceps, Lower Chalk.
-- Fauiasii, chalk.
Baffin's Bay, formation of drift in.
Bagshot sands.
Baiae, Bay of, subterranean igneous action in.
Bakewell, Mr., on cleavage in Swiss Alps.
Bala and Caradoc beds.
Balistidae, defensive spine of.
Bangor, or Longmynd group.
Banksia, seed and fruit of, Lower Miocene.
Barmouth sandstones.
Barnes, Mr. J., on insects in American coal.
Barnstaple, Upper Devonian of.
Barrande, M. Joachim, his "Primordial Zone."
--, on metamorphosis of trilobites.
Barrett, Mr., on bird in Blackdown beds.
Barton series sands and clays.
-- shells, percentage of, common to London clay.
Basalt, columnar.
--, composition of.
Basaltic rocks, poor in silica.
--, specific gravity of minerals in.
Basilosaurus, Eocene, United States.
Basset, term explained.
Basterot, M. de, on Bordeaux tertiary strata.
Bath Oolite.
Batrachian reptiles in coal.
Bay of Fundy, denudation in coalfield in.
Bean, Mr., on Yorkshire Oolite.
Bear Island carboniferous flora.
Beaumont, M. E. de, on island in Cretaceous sea.
--, on mineral veins.
--, on Jurassic plutonic rocks.
--, on formation of granite.
Beckles, Mr. S.H., on footprints in Hastings sands.
-- on Mammalia of Purbeck.
Belemnitella mucronata, Chalk.
Belemnites hastatus, Oxford clay.
-- Puzosianus, Oxford clay.
Belgium, Lower Miocene of.
Bellerophon costatus, Mountain Limestone.
Belosepia sepioidea, Sheppey.
Belt, Mr., on subdivision of Lingula Flags.
Bembridge beds, Yarmouth.
Berger, Dr., on rocks altered by dikes.
Berlin, Miocene strata near.
Bernese Alps, gneiss in the.
Berthier on isomorphism.
Bertrich-Baden, columnar basalt of.
Beyrich on term Oligocene for Lower Miocene.
Billings, Mr., on trilobites.
Binney, Mr., on Sigillariae in volcanic ash.
--, on Stigmaria, the root of Sigillaria.
Biotite.
Bird in argile plastique.
Bischoff, Professor, on Nile and Rhine mud.
--, on conversion of coal into anthracite.
--, on hydrothermal action.
Blackdown beds.
Blacklead of Borrowdale.
Bog-iron-ore.
Bohemia, Cambrian rocks of.
--, silver veins in.
Bolderberg, in Belgium, Upper Miocene of.
Bone-bed of fish remains, Armagh.
-- of Upper Ludlow.
-- of the Trias.
Boom, Lower Miocene of.
Bordeaux, Upper Miocene of.
Borrowdale, blacklead of.
Bosquet, M. on chalk fossils.
--, on Maestricht beds.
Botanical nomenclature.
Boucher de Perthes on Abbeville alluvium.
Boulder-clay, whether formed by icebergs or land-ice.
Boulder-clay of Canada.
-- fauna of.
Boulders and pebbles in chalk.
Bournemouth beds (Lower Bagshot).
Bovey Tracey, lignites and clays of.
Bowerbank, Mr., on fossil fruits of London Clay.
--, on fossil fruits of Sheppey.
Bowman, Mr., on uniting of distinct coal-seams.
Brachiopoda, preponderance of, in older rocks.
--, mode of recognising shells of.
Bracklesham beds and Bagshot Sands.
Bradford encrinites.
Breccias of Lower Permian.
Brick-earth or fluviatile loam.
Bridlington drift.
Bristol, dolomitic conglomerate of.
Bristow, Mr., on volcanic minerals.
Brixham cave near Torquay.
Brocchi on Italian tertiary strata.
-- on subapennine strata.
Brockenhurst, corals and shells of.
Brodie, Reverend P.B., on Lias insects.
Brodie, Mr. W.R., on Purbeck mammalia.
Brongniart, M. Adolphe, on botanical nomenclature.
--, on Lias plants.
--, on flora of the Bunter.
--, on flora of the coal.
--, on fruit of Lepidodendron.
--, M. Alex., on Tertiary series.
Bronteus flabellifer, Devonian.
Brora, oolitic coal formation of.
Brown, Mr. Richard, on Stigmaria.
--, on carboniferous rain-prints.
Brown, Robert, on Eocene protaceous fruit.
Brown, Reverend T., on marine shells in Scotch drift.
Brown-coal of Germany.
Bryce, Mr., on Scotch till.
Bryozoa of Mountain Limestone.
-- and polyzoa, terms explained.
Buch, von. See Von Buch.
Buckland, Dr., on Kirkdale cave.
--, on violent death of saurians.
--, on spines of fish.
--, on Eocene oysters.
--, on pot-stones in chalk.
Buddle, Mr., on creeps in coal-mines.
Bulimus ellipticus, Bembridge.
-- lubricus, Loess.
Bullock, Captain, R.N., on Atlantic mud.
Bunbury, Sir C., on leaf-bed of Madeira.
--, on ferns of the Maryland coal
Bunter of Germany.
-- or Lower Trias of England.
Buprestis? Elytron of, Stonesfield.
Burmeister on trilobites.
Cainozoic, term defined.
Caithness, fish beds of.
Calamite, root of.
Calamites Sucowii, coal, and restored stem.
Calamophyllia radiata, Bath Oolite.
Calcaire de la Beauce, age of the.
-- grossier, fossils of the.
-- siliceux of France.
Calcareous matter poured out by springs.
-- rocks described.
-- nodules in Lias.
Calcarina rarispina, Eocene.
Calceola sandalina, Devonian.
--, schiefer of Germany.
California, aurifrous gravel of.
--, gold in petrified wood of age of alluvium.
Calymene Blumenbachii, Silurian.
Cambrian Group, classification of the.
Cambrian, Upper.
--, Lower.
--, of Sweden and Norway.
--, strata of Bohemia.
--, of North America.
--, volcanic rocks.
Campophyllum flexuosum.
Canada, Cambrian of.
--, Devonian of.
--, trap-rocks of.
Canadian drift.
Canary, Grand, shelly tuffs of.
Cantal, Lower Miocene of the.
Cape Breton, rain-prints in coal-measures of.
Cape Wrath, granite veins in gneiss at.
Caradoc and Bala beds.
Carbonate of lime in rocks, how tested.
Carboniferous Group, subdivisions of the.
-- flora.
-- limestone, thickness of.
--, marine fauna of the.
-- Period, trap-rocks of.
-- plutonic rocks.
-- reptiles.
-- insects.
Carcharodon angustidens, Bracklesham.
Cardiganshire, section of slaty cleavage in.
Cardiocarpon Ottonis, Permian.
Cardita (Venericardia) planicosta.
-- sulcata, Barton.
Cardium dissimile, Portland Stone.
-- rhaeticum, Rhaetic Beds.
-- striatulum, Kimmeridge clay.
Carne, Mr. N., on Cornish lodes.
Carpenter, Dr., on Atlantic mud.
--, on Eozoon Canadense.
Carrara, marble of.
Carruthers, Mr., on Eocene proteaceous fruit.
--, on cycads of the Purbeck.
--, on leaves of calamite.
--, on spores of carboniferous Lycopodiaceae.
--, on structure of sigillaria.
--, on trees in volcanic ash.
Cashmere, recent formations in.
Cassian, St., Triassic strata of.
Castrogiovanni, curved strata near.
Catania, laterite formed in.
--, Tertiary beds in.
Catillus Lamarckii, White Chalk.
Caucasus, absence of lakes in the.
Caulopteris primaeva, Coal.
Cave-breccias of Australia.
Cavern deposits with human and animal remains.
Caves of Kirkdale and Brixham.
Celts described.
Cementing of strata.
Cephalaspis Lyelli, Old Red.
Ceratites nodosus, Muschelkalk.
Cerithium concavum, Headon.
-- elegans, Hempstead beds.
-- (Terebra) Portlandicum.
-- plicatum, Hempstead beds.
-- melanoides.
Cervus alces, tooth of.
Cestracion Phillippi, Recent.
Chabasite.
Chalk, composition, extent, and origin of.
-- of Faxoe.
-- flints, origin of.
-- fossils of the White.
--, iceborne boulders in the.
-- of North and South Europe.
--, Lower White, without flints.
-- marl, fossils of the.
-- Period, popular error concerning.
Chalk-pit with pot-stones, view of.
Chama squamosa, Barton.
Champoleon, junction of granite with Jurassic strata near.
Chara elastica, C. medicaginula.
-- tuberculata, Bembridge.
Charpentier, M., on Alpine glaciers.
--, on depression of Alps in Glacial Period.
Chatham coal-field.
Cheirotherium, footprints of.
Chemical deposits in veins.
-- and mechanical deposits.
Chiapa, fall of volcanic dust at.
Chichester, erratics near.
Chili, copper pyrites with gold in.
--, walls cracked by earthquake in.
Chillesford and Aldeby beds.
Chimaera monstrosa, Lias.
Chlorite-schist.
Chloritic series, or Upper Greensand.
Christiania, Euritic porphyry at.
--, granite veins in Silurian strata of.
--, quartz vein in gneiss at.
Chronological groups of formations.
Chronology, test of, in rocks.
Cinder-bed of the Purbeck.
Cinnamomum polymorphum, Miocene.
-- Rossmassleri, Miocene.
Claiborne beds, Eocene fossils of.
Clarke County, United States, Zeuglodon of.
Classification of Tertiary formations.
--, value of shells in.
Clausilia bidens, Loess.
Clay defined.
-- iron-stone defined.
--, plastic.
-- slate.
--, Weald.
Cleavage explained.
--, crystalline theory of.
--, mechanical theory of.
-- of metamorphic rocks.
Cleidotheca operculata.
Clermont, metalliferous gneiss near.
Climate of the Crags.
-- of the Coal.
-- of the Miocene in the Arctic regions.
-- of the Post-pliocene period.
Clinkstone.
Clinton group, fossils of the.
Clyde, buried canoes in estuary of.
--, arctic marine shells in drifts of.
Clymenia linearis, Devonian.
Clymenien-Kalk of Germany.
Coal, conversion into anthracite of.
-- a land and swamp formation.
--, cause of the purity of.
--, conversion of lignite into.
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