_Subterranean mining_ requires two very distinct classes of workings;
the _preparatory_, and those for _extraction_.
The _preparatory_ consist in galleries, or in pits and galleries
destined to conduct the miner to the point most proper for attacking the
deposit of ore, for tracing it all round this point, for preparing
chambers of excavation, and for concerting measures with a view to the
circulation of air, the discharge of waters, and the transport of the
extracted minerals.
If the vein or bed in question be placed in a mountain, and if its
direction forms a very obtuse angle with the line of the slope, the
miner begins by opening in its side, at the lowest possible level, a
gallery of elongation, which serves at once to give issue to the waters,
to explore the deposit through a considerable extent, and then to follow
it in another direction; but to commence the real mining operations, he
pierces either shafts or galleries, according to the slope of the
deposit, across the first gallery.
For a stratum little inclined to the horizon, placed beneath a plain,
the first thing is to pierce two vertical shafts, which are usually made
to arrive at two points in the same line of slope, and a gallery is
driven to unite them. It is, in the first place, for the sake of
circulation of air that these two pits are sunk; one of them, which is
also destined for the drainage of the waters, should reach the lowest
point of the intended workings. If a vein is intersected by transverse
ones, the shafts are placed so as to follow, or, at least, to cut
through the intersections. When the mineral ores lie in nearly vertical
masses, it is right to avoid, as far as possible, sinking pits into
their interior. These should rather be perforated at one side of their
floor, even at some considerable distance, to avoid all risk of
crumbling the ores into a heap of rubbish, and overwhelming the workmen.
With a vein of less than two yards thick, as soon as the preparatory
labours have brought the miners to the point of the vein from which the
ulterior workings are to ramify, whenever a circulation of air has been
secured, and an outlet to the water and the matters mined, the first
object is to divide the mass of ore into large parallelopipeds, by means
of oblong galleries, pierced 20 or 25 yards below one another, with pits
of communication opened up, 30, 40, or 50 yards asunder, which follow
the slope of the vein. These galleries and shafts are usually of the
same breadth as the vein, unless when it is very narrow, in which case
it is requisite to cut out a portion of the roof or the floor. Such
workings serve at once the purposes of mining, by affording a portion of
ore, and the complete investigation of the nature and riches of the
vein, a certain extent of which is thus prepared before removing the
cubical masses. It is proper to advance first of all, in this manner, to
the greatest distance from the central point which can be mined with
economy, and afterwards to remove the parallelopiped blocks, in working
back to that point.
This latter operation may be carried on in two different ways; of which
one consists in attacking the ore from above; and another from below. In
either case, the excavations are disposed in steps similar to a stair
upon their upper or under side. The first is styled a _working_ in
direct or descending steps; and the second a _working_ in _reverse_, or
ascending steps.
[Illustration: 714]
1. Suppose, for example, that the post N, _fig._ 714., included between
the horizontal gallery A C, and the shaft A B, is to be excavated by
direct steps, a workman stationed upon a scaffold at the point _a_,
which forms the angle between the shaft and the elongated drift, attacks
the rock in front of him and beneath his feet. Whenever he has cut out a
parallelopiped (a rectangular mass), of from four to six yards broad,
and two yards high, a second miner is set to work upon a scaffold at
_a´_, two yards beneath the first, who, in like manner, excavates the
rock under his feet and before him. As soon as the second miner has
removed a post of four or six yards in width, by two in height, a third
begins upon a scaffold at _a´´_ to work out a third step. Thus, as many
workmen are employed as there are steps to be made between the two
oblong horizontal galleries which extend above and below the mass to be
excavated; and since they all proceed simultaneously, they continue
working in similar positions, in floors, over each other, as upon a
stair with very long wide steps. As they advance, the miners construct
before them wooden floors _c c c c_, for the purpose of supporting the
rubbish which each workman extracts from his own step. This floor, which
should be very solid, serves also for wheeling out his barrow filled
with ore. The round billets which support the planks sustain the roof or
the wall of the mineral vein or bed under operation. If the rubbish be
very considerable, as is commonly the case, the floor planks are lost.
However strongly they may be made, as they cannot be repaired, they
sooner or later give way under the enormous pressure of the rubbish; and
as all the weight is borne by the roof of the oblong gallery underneath,
this must be sufficiently timbered. By this ingenious plan, a great many
miners may go to work together upon a vein without mutual interference;
as the portions which they detach have always two faces at least free,
they are consequently more easily separable, either with gunpowder or
with the pick. Should the vein be more than a yard thick, or if its
substance be very refractory, two miners are set upon each step. _b b b
b_ indicate the quadrangular masses that are cut out successively
downwards; and 1 1, 2 2, 3 3, forwards; the lines of small circles are
the sections of the ends of the billets which support the floors.
[Illustration: 715]
2. To attack a mass Y, _fig._ 715., a scaffold _m_, is erected in one of
its terminal pits P P, at the level of the ceiling of the gallery R R´,
where it terminates below. A miner placed on this scaffold, cuts off at
the angle of this mass a parallelopiped 1, from one to two yards high,
by six or eight long. When he has advanced thus far, there is placed in
the same pit, upon another scaffold _m´_, a second miner, who attacks
the vein above the roof of the first cutting, and hews down, above the
parallelopiped 1, a parallelopiped of the same dimensions 1´, while the
first is taking out another 2, in advance of 1. When the second miner
has gone forward 6 or 8 yards, a third is placed also in the same pit.
He commences the third step, while the first two miners are pushing
forwards theirs, and so in succession.
In this mode of working, as well as in the preceding, it is requisite to
support the rubbish and the walls of the vein. For the first object, a
single floor _n n n_, may be sufficient, constructed above the lower
gallery, substantial enough to bear all the rubbish, as well as the
miners. In certain cases, an arched roof may be substituted; and in
others, several floors are laid at different heights. The sides of the
vein are supported by means of pieces of wood fixed between them
perpendicularly to their planes. Sometimes, in the middle of the
rubbish, small pits are left at regular distances apart, through which
the workmen throw the ore coarsely picked, down into the lower gallery.
The rubbish occasionally forms a slope _f f f_, so high that miners
placed upon it can work conveniently. When the rich portions are so
abundant as to leave too little rubbish to make such a sloping platform,
the miners plant themselves upon movable floors, which they carry
forward along with the excavations.
These two modes of working in the _step-form_, have peculiar advantages
and disadvantages; and each is preferred to the other according to
circumstances.
In the _descending workings_ or in _direct steps_, _fig._ 714., the
miner is placed on the very mass or substance of the vein; he works
commodiously before him; he is not exposed to the splinters which may
fly off from the roof; but by this plan he is obliged to employ a great
deal of timber to sustain the rubbish; and the wood is fixed for ever.
In the _ascending workings_, or in _reversed steps_, _fig._ 715., the
miner is compelled to work in the re-entering angle formed between the
roof and the front wall of his excavation, a posture sometimes
oppressive; but the weight of the ore conspires with his efforts to make
it fall. He employs less timber than in the _workings_ with _direct
steps_. The _sorting_ of the ore is more difficult than in the
_descending working_, because the rich ore is sometimes confounded with
the heap of rubbish on which it falls.
When seams of diluvium or gravel-mud, occur on one of the sides of the
vein, or on both, they render the quarrying of the ore more easy, by
affording the means of uncovering the mass to be cut down, upon an
additional face.
Should the vein be very narrow, it is necessary to remove a portion of
the sterile rock which encloses it, in order to give the work a
sufficient width to enable the miner to advance. If, in this case, the
vein be quite distinct from the rock, the labour may be facilitated, as
well as the separation of the ore, by disengaging the vein, on one of
its faces through a certain extent, the rock being attacked separately.
This operation is called _stripping the vein_. When it is thus
uncovered, a shot of gunpowder is sufficient to detach a great mass of
it, unmixed with sterile stones.
By the methods now described, only those parallelopipeds are cut out,
either in whole or in part, which present indications of richness
adequate to yield a prospect of benefit. In other cases, it is enough to
follow out the threads of ore which occur, by workings made in their
direction.
The miner, in searching within the crust of the earth for the riches
which it conceals, is exposed to many dangers. The rocks amidst which he
digs are seldom or never entire, but are almost always traversed by
clefts in various directions, so that impending fragments threaten to
fall and crush him at every instant. He is even obliged at times to cut
through rotten friable rocks or alluvial loams. Fresh atmospheric air
follows him with difficulty in the narrow channels which he lays open
before him; and the waters which circulate in the subterranean seams and
fissures filter incessantly into his excavation, and tend to fill it.
Let us now take a view of the means he employs to escape from these
three classes of dangers.
1. _Of the timbering of excavations._--The excavations of mines, are
divisible into three principal species; _shafts_, _galleries_, and
_chambers_. When the width of these excavations is inconsiderable, as is
commonly the case with shafts and galleries, their sides can sometimes
stand upright of themselves; but more frequently they require to be
propped or stayed by billets of wood, or by walls built with bricks or
stones; or even by stuffing the space with rubbish. These three kinds of
_support_ are called _timbering_, _walling_, and _filling up_.
Timbering is most used. It varies in form for the three species of
excavations, according to the solidity of the walls which it is destined
to sustain.
[Illustration: 716]
In a gallery, for example, it may be sufficient to support merely the
roof, by means of joists placed across, bearing at their two ends in the
rock; or the roof and the two walls by means of an upper joist S, _fig._
716., which is then called a _cap_ or _cornice beam_, resting on two
lateral upright posts or _stanchions_, _a_, _b_, to which a slight
inclination towards each other is given, so that they approach a little
at the top, and rest entirely upon the floor. At times, only one of the
walls and the roof need support. This case is of frequent occurrence in
pipe veins. Pillars are then set up only on one side, and on the other
the joists rest in holes of the rock. It may happen that the floor of
the gallery shall not be sufficiently firm to afford a sure foundation
to the standards; and it may be necessary to make them rest on a
horizontal piece called the _sole_. This is timbering with _complete
frames_. The upright posts are usually set directly on the sole; but the
extremities of the _cap_ or ceiling, and the upper ends of the
_standards_, are mortised in such a manner that these cannot come
nearer, whereby the cap shall possess its whole force of resistance. In
friable and shivery rocks there is put behind these beams, both upon the
ceiling and the sides, _facing boards_, which are planks placed
horizontally, or spars of cleft wood, set so close together as to leave
no interval. They are called _fascines_ in French. In ordinary ground,
the miner puts up these _planks_ in proportion as he goes forwards; but
in a loose soil, such as sand or gravel, he must mount them a little in
advance. He then drives into the mass behind the wooden frame-work,
thick but sharp-pointed planks or stakes, and which, in fact, form the
sides of the cavity, which he proceeds to excavate. Their one extremity
is thus supported by the earth in which it is thrust, and their other
end by the last framing. Whenever the miner gets sufficiently on, he
sustains the walls by a new frame. The size of the timber, as well as
the distance between the frames or _stanchions_, depends on the degree
of pressure to be resisted.
When a gallery is to serve at once for several distinct purposes, a
greater height is given to it; and a flooring is laid on it at a certain
level. If, for example, a gallery is to be employed, both for the
transport of the ores and the discharge of the waters, a floor _e e_,
_fig._ 715., is constructed above the bottom, over which the carriages
are wheeled, and under which the waters are discharged.
The timbering of shafts varies in form, as well as that of galleries,
according to the nature and the locality of the ground which they
traverse, and the purposes which they are meant to serve. The shafts
intended to be stayed with timber are usually square or rectangular,
because this form, in itself more convenient for the miner, renders the
execution of the timbering more easy. The wood-work consists generally
of rectangular frames, the spars of which are about eight inches in
diameter, and placed at a distance asunder of from a yard to a yard and
a half. The spars are never placed in contact, except when the pressure
of the earth and the waters is very great. The pieces composing the
frames are commonly united by a half-check, and the longer of the two
pieces extends often beyond the angles, to be rested in the rock.
Whether the shaft is vertical or inclined, the frame-work is always
placed so that its plane may be perpendicular to the axis of the pit. It
happens sometimes in inclined shafts that there are only two sides, or
even a single one, which needs to be propped. These are stayed by means
of cross beams, which rest at their two ends in the rock. When the
frames do not touch one another, strong planks or stakes are fastened
behind them to sustain the ground. To these planks the frames are firmly
connected, so that they cannot slide. In this case the whole timbering
will be supported, when the lower frame is solidly fixed, or when the
pieces from above pass by its angles to be abutted upon the ground.
In the large rectangular shafts, which serve at once for extracting the
ores, for the discharge of the waters, and the descent of the workmen,
the spaces destined for these several purposes are in general separated
by partitions, which also serve to increase the strength of the
timberings, by acting as buttresses to the planks in the long sides of
the frame-work. Occasionally a partition separates the ascending from
the descending basket, to prevent their jostling.--Lastly, particular
passages are left for ventilation.
As it is desirable that the wood shall retain its whole force, only
those pieces are squared which absolutely require it. The spars of the
frames in shafts and galleries are deprived merely of their bark, which
by holding moisture, would accelerate the decomposition of the wood. The
alburnum of oak is also removed.
Resinous woods, like the pine, last much shorter than the oak, the
beech, and the cherry-tree; though the larch is used with advantage. The
oak has been known to last upwards of 40 years; while the resinous woods
decay frequently in 10. The fresher the air in mines, the more durable
is the timbering.
[Illustration: 717 718]
The marginal _figs._ 717, 718. represent two vertical sections of a
shaft, the one at right angles to the other, with the view of showing
the mode of sustaining the walls of the excavation by timbering. It is
copied from an actual mine in the Hartz. There we may observe the spaces
allotted to the descent of the miners by ladders, to the drainage of the
waters by pumps P, and rods _t_, and to the extraction of the mineral
substances by the baskets B. _a_, _b_, _c_, _f_, _h_, _k_, various cross
timbers; A, C, E, upright do.; R, pump cistern; V, W, corve-ways. The
shafts here shown, are excavated in the line of the vein itself,--the
rock enclosing it being seen in the second figure.
In a great many mines it is found advantageous to support the
excavations by brick or stone buildings, constructed either with or
without mortar. These constructions are often more costly than wooden
ones, but they last much longer, and need fewer repairs. They are
employed instead of timberings, to support the walls and roof of
galleries, to line the sides of shafts, and to bear up the roofs of
excavations.
Sometimes the two sides of a gallery are lined with vertical walls, and
its roof is supported by an ogee vault, or an arch. If the sides of the
mine are solid, a simple arch is sufficient to sustain the roof and at
other times the whole surface of a gallery is formed of a single
elliptic vault, the great axis of which is vertical; and the bottom is
surmounted by a wooden plank, under which the waters run off; see _fig._
719.
Walled shafts also are sometimes constructed in a circular or elliptic
form, which is better adapted to resist the pressure of the earth and
waters. Rectangular shafts of all dimensions, however, are frequently
walled.
The sides of an excavation may also be supported by filling it
completely with rubbish. Wherever the sides need to be supported for
some time without the necessity of passing along them, it is often more
economical to stuff them up with rubbish, than to keep up their
supports. In the territory of Liege, for example, there have been shafts
thus filled up for several centuries; and which are found to be quite
entire when they are emptied. The rubbish is also useful for forming
roads among steep strata, for closing air-holes, and forming canals of
ventilation.
[Illustration: 719 720 721 722]
_Figs._ 719. 720. 721. represent the principal kinds of mason-work
employed in the galleries and shafts of mines. _Fig._ 722. exhibits the
walling in of the cage of an overshot water-wheel, as mounted within a
mine. Before beginning to build, an excavation large enough must be
made in the gallery to leave a space three feet and a half high for the
workmen to stand in, after the brick-work is completed. Between the two
opposite sides, cross beams of wood must be fixed at certain distances,
as chords of the vault, over which the rock must be hollowed out to
receive the arch-stones, and the centring must then be placed, covered
with deals to receive the _voussoirs_, beginning at the flanks and
ending with the key-stone. When the vault is finished through a certain
extent, the interval between the arch and the rock must be rammed full
of rubbish, leaving passages if necessary through it and the arch, for
currents of water.
In walling galleries, attention must be paid to the direction of the
pressure, and to build vertically or with a slope accordingly. Should
the pressure be equal in all directions, a closed vault, like _fig._
719., should be formed. For walls not far from the vertical, salient or
buttressed arches are employed, as shown in _fig._ 720., called in
German _überspringende bogen_; for other cases, twin-arches are
preferred, with an upright wall between.
_Fig._ 721. is a transverse section of a walled drain-gallery, from the
grand gallery of the Hartz; see also _fig._ 722. _a_ is the rock which
needs to be supported only at the sides and top; _b_, the masonwork, a
curve formed of the three circular arcs upon one level; _c_, the floor
for the watercourse. _Fig._ 719. is a cross section of a walled gallery,
as at Schneeberg, Rothenburg, Idria, &c.; _d_, is the rock, which is not
solid either at the flanks, roof, or floor; _e_, the elliptic masonwork;
_f_, the wooden floor for the waggons, which is sometimes, however,
arched in brick to allow of a watercourse beneath it.
_Fig._ 720. shows two vertical projections of a portion of a walled
shaft with buttresses, as built at the mine _Vater Abraham_, near
Marienberg. J is a section in the direction of the vein _g h_, to show
the roof of the shaft. I, a section exhibiting the slope of the vein _g
h_, into which the shaft is sunk; _m_ is the wall of the vein; _k_ is
the roof of the same vein; _n_, buttresses resting upon the flanks of
the shaft; _g_, great arcs on which the buttresses bear; _y_, vertical
masonwork; _z_, a wall which divides the shaft into two compartments, of
which the larger _p_ is that for extracting the ore, and the smaller for
the draining and descent of the miners.
_Fig._ 722. C D is the shaft in which the vertical crank-rods _c g_, _e
d_, move up and down. F, is a double hydraulic wheel, which can be
stopped at pleasure by a brake mounted upon the machine of extraction.
G, is the drum of the gig or whim for raising the _corves_ or tubs
(_tonnes_); H, is the level of the ground, with the carpentry which
supports the whim and its roof. _k_, is the key-stone of the _ogee_ arch
which covers the water-wheel; _a_, is the opening or window, traversed
by the extremity of the driving shaft, upon each side of the
water-wheel, through which a workman may enter to adjust or repair it;
_c b_, line of conduits for the streams of water which fall upon the
hydraulic wheel; _c_, _g_, double crank with rods, whose motion is
taken off the left side of the wheel; _e_, _d_, the same upon the right
side. The distance from H to F is about 22 yards.
_Figs._ 723. 724. present two vertical sections of the shaft of a mine
walled, like the roof of a cavern, communicating with the galleries of
the roof and the wall of the vein, and well arranged for both the
extraction of the ore, and the descent of the miners. The vertical
partition of the shaft for separating the passage for the corves or tubs
from the ladders is omitted in the figure, for the sake of clearness.
[Illustration: 723 724]
In _fig._ 723., A, B are the side walls supported upon the buttresses C
and D; in _fig._ 724., E is the masonry of the wall, borne upon the arch
F at the entrance to a gallery; the continuation being at G, which is
sustained by a similar arch built lower.
L, is the vault arch of the roof, supported upon another vault M, which
presents a double curvature, at the entrance of a gallery; at H is the
continuation of the arch or vault L, which underneath is supported in
like manner at the entrance of a lower gallery.
_a b_, _c d_, _fig._ 723., are small upright guide-bars or rods for one
of the corves, or kibbles.
_e f_, _g h_, are similar guide-bars for the other corf.
_i i_, are cross-bars of wood, which support the stays of the ladders of
descent.
_k k_, are also cross-bars by which the guide-rods are secured.
_t_, a _corf_, or extraction kibble, furnished with friction rollers;
the other corf is supposed to be drawn up to a higher level, in the
other vertical passage.
[Illustration: 725 726]
_Figs._ 725. 726. represent in a vertical section the mode of timbering
the galleries of the silver and lead mines at Andreasberg in the Hartz.
_Fig._ 725. shows the plan viewed from above. Upon the roof of the
timbering, the workman throws the waste rubbish, and in the empty space
below, which is shaded black, he transports in his waggons or
wheelbarrows the ores towards the mouth of the mine. _Fig._ 726. is the
cross section of the gallery. In the two figures, _a_ represents the
rock, and _b_ the timbering; round which there is a garniture of small
spars or lathes for the purpose of drainage and ventilation, with the
view of promoting the durability of the wood-work.
The working of minerals by the _mass_ is well exemplified a few leagues
to the north of Siegen, near the village of Müsen, in a mine of iron and
other metals, called _Stahlberg_, which forms the main wealth of the
country. The plan of working is termed _the excavation of a direct or
transverse mass_. It shows in its upper part the danger of bad mining,
and in its inferior portion, the regular workings, by whose means art
has eventually prevented the destruction of a precious mineral deposit.
[Illustration: 727]
_Fig._ 727. is a vertical section of the bed of ore, which is a _direct_
mass of spathose iron, contained in transition rock (greywacke). _a_,
_a_, _a_, are pillars of the sparry ore, reserved to support the
successive stages or floors, which are numbered 1. 2. 3. &c.; _b_, _b_,
_b_, are excavations worked in the ore; which exhibit at the present day
several floors of arches, of greater or less magnitude, according to the
localities. It may be remarked, that where the metallic deposit forms
one entire mass, rich in spathose iron ore of good quality, there is
generally given to the vaults a height of three fathoms; leaving a
thickness over the roof of two fathoms, on account of the numerous
fissures which pervade the mass. But where this mass is divided into
three principal branches, the roof of the vaults has only a fathom and a
half of thickness, while the excavation is three fathoms and a half
high. In the actual state of the workings, it may be estimated that from
all this direct mass, there is obtained no more out of every floor than
one-third of the mineral. Two-thirds remain as labours of reserve, which
may be resumed at some future day, in consequence of the regularity and
the continuation of the subterranean workings. _e_ is a shaft for
extraction, communicating below with the gallery of efflux _k_; _h_ is
an upper gallery of drainage, which runs in different directions (one
only being visible in this section) over a length of 400 fathoms. The
lower gallery _k_ runs 646 fathoms in a straight line. The mine of
Stahlberg has furnished annually on an average since 1760 about 25,000
cubic feet (French) of an excellent spathose ore of iron. _m m_,
represents the mass of sparry iron.
_Figs._ 728, 729, 730. represent the cross system of mining, which
consists in forming galleries through a mineral deposit, from its wall
or floor towards its roof, and not, as usual, in the direction of its
length. This mode was contrived towards the middle of the 18th century,
for working the very thick veins of the Schemnitz mine in Hungary, and
it is now employed with advantage in many places, particularly at Idria
in Carniola. In the two sections _figs._ 728., 730., as well as in the
ground plan _fig._ 729., the wall is denoted by _m m_, and the roof by
_t t_. A first gallery of prolongation E F, _fig._ 730., being formed to
the wall, transverse cuts, _a a_, are next established at right angles
to this gallery, so that between every two there may be room enough to
place three others, _b_, _c_, _b_, _fig._ 729. From each of the cuts
_a_, ore is procured by advancing with the help of timbering, till the
roof _t_ be reached. When this is done, these first cuts _a_, are filled
up with rubbish, laid upon pieces of timber with which the ground is
covered, so that if eventually, it should be wished to mine underneath,
no downfall of detritus is to be feared. These heaps of rubbish rise
only to within a few inches of the top of the cuts _a_, in order that
the working of the upper story may be easier, the bed of ore being there
already laid open upon its lower face.
[Illustration: 728 729 730]
In proportion as the cuts _a_, of the first story E F, are thus filled
up, the greater part of the timbering is withdrawn, and made use of
elsewhere. The intermediate cuts _b_, _c_, _b_, are next mined in like
manner, either beginning with the cuts _c_, or the cuts _b_, according
to the localities. From _fig._ 729. it appears that the working may be
so arranged, that in case of necessity, there may be always between two
cuts in activity the distance of three cuts, either not made, or filled
up with rubbish. Hence, all the portion of the bed of ore may be
removed, which corresponds to a first story E F _fig._ 730., and this
portion is replaced by rubbish.
The exploration of the upper stories E´ F´, E² F², E³ F³, is now
prepared in a similar manner; with which view shafts _h h_³, _k k_³, are
formed from below upwards in the wall _m_ of the deposit, and from these
shafts oblong galleries proceed, established successively on a level
with the stories thus raised over one another. See _fig._ 730. The
following objects may be specified in the figures:--
_a a_, the first cuts filled up with rubbish, upon the first story E F,
_fig._ 729.
_b b_, other cuts subsequently filled up, upon the same story.
_c_, the cut actually working.
_d_, the front of the cut, or place of actual excavation of the mineral
deposit.
_e_, masses of the barren rock, reserved in the cutting, as pillars of
safety.
_f_, galleries, by means of which the workmen may turn round the mass
_e_, in order to form, in the roof _t_, an excavation in the direction
of the deposit.
_g_, rubbish behind the mass _e_.
_k k_, two shafts leading from the first story E F, to the upper stories
of the workings, as already stated.
_m_, the wall, and _t_ the roof of the mineral bed.
In the second story E´ F´, the gallery of prolongation F´, _figs._ 728.
and 730., is not entirely perforated; but it is further advanced than
that of the third story, which, in its turn, is more than the gallery of
the fourth.
From this arrangement there is produced upon _fig._ 730. the general
aspect of a working by reversed steps.
Whenever the workings of the cuts _c_ in the first story are finished,
those of the second, _a´ a´_, may be begun in the second; and thus by
mounting from story to story, the whole deposit of ore may be taken out
and replaced with rubbish. One great advantage of this method is, that
nothing is lost; but it is not the only one. The facilities offered by
the system of _cross workings_ for disposing of the rubbish, most
frequently a nuisance to the miner, and expensive to get rid of, the
solidity which it procures by the banking up, the consequent economy of
timbering, and saving of expense in the excavation of the rock,
reckoning from the second story, are so many important circumstances
which recommend this mode of mining. Sometimes, indeed, rubbish may be
wanted to fill up, but this may always be procured by a few accessory
perforations; it being easy to establish in the vicinity of the workings
a vast excavation in the form of a vault, or kind of subterraneous
quarry, which may be allowed to fall in with proper precautions, and
where rubbish will thus accumulate in a short time, at little cost.
[Illustration: 731]
_Fig._ 731. represents a section of the celebrated lead mines of
Bleyberg in Carinthia, not far from Villach.
_b_, _c_, is the ridge of the mountains of compact limestone, in whose
bosom the workings are carried on.
_e_, is the metalliferous valley, running from east to west, between the
two parallel valleys of the Gail and the Drave, but at a level
considerably above the waters of these rivers.
_f g_, is the direction of a great many vertical beds of metalliferous
limestone.
On considering the direction and dip of the marly schist, and
metalliferous limestone, in the space _w_, _w_, to the west of the line
1, _s_, it would appear that a great portion of this system of mountains
has suffered a slip between 1, _s_, and a parallel one towards the east;
whereby, probably, that vertical position of the strata has been
produced, which exists through a considerable extent. The metalliferous
limestone is covered to a certain thickness with a marly schist, and
other more recent rocks. It is in this schist that the fine marble known
under the name of the _lumachella of Bleyberg_ is quarried.
The galena occurs in the bosom of this rock in flattened masses, or
blocks of a considerable volume, which are not separated from the rest
of the calcareous beds by any seam. It is accompanied by zinc ore
(_calamine_), especially in the upper parts of the mountain.
Several of the workable masses are indicated by _r_, _r_³; each presents
itself as a solid analogous to a very elongated ellipse, whose axis
dips, not according to the inclination of the surrounding rock, but to
an oblique or intermediate line between this inclination, and the
direction of the beds of limestone; as shown by _r w_, _r´ u_. Every
thing indicates the contemporaneous formation of the limestone, and the
lying beds of the lead ore.
The accidents or faults called _kluft_ (_rent_) at Bleyberg are visible
on the surface of the ground. Experienced miners have remarked that the
rich masses occur more frequently in the direction of these accidents
than elsewhere.
It is in general by galleries cut horizontally in the body of the
mountain, and at different levels, _s_, _g_, _s f_, that the miner
advances towards the masses of ore _r_, _r_³. Many of these galleries
are 500 fathoms long before they reach a workable mass. The several
galleries are placed in communication by a few shafts, such as _t_; but
few of these are sunk deeper than the level of the valley _e_.
The total length of the mines of Bleyberg is about 10,000 yards,
parallel to the valley _e_; in which space there are 500 concessions
granted by the government to various individuals or joint stock
societies, either by themselves or associated with the government.
The metalliferous valley contains 5000 inhabitants, all deriving
subsistence from the mines; 300 of whom are occupied in the government
works.
Each concession has a number and a name; as Antoni, Christoph, Matthæus,
Oswaldi, 2, 8, 36, &c.
[Illustration: 732]
_Fig._ 732 is a section in the quicksilver mine of Idria. 1. is the
gray-limestone; 2. is a blackish slate; 5. is a grayish slate.
Immediately above these transition rocks lies the bed containing the
ores called _corallenerz_, which consist of an intimate mixture of
sulphuret of mercury and argillaceous limestone; in which four men can
cut out, in a month, 2-1/2 toises cube of rock.
[Illustration: 733]
_Fig._ 733. represents a section of part of the copper mine of
Mansfeldt; containing the cellular limestone, called _rauchwacke_,
always with the compact marl-limestone called zechstein; the cupreous
schist, or _kupferschiefer_; the wall of grayish-white sandstone, called
the _weisse liegende_; and the wall of red sandstone, or the _rothe
liegende_. The thin dotted stratum at top is vegetable mould; the large
dotted portion to the right of the figure is oolite; the vein at its
side is sand; next is _rauchwacke_; and lastly, the main body of fetid
limestone, or _stinkstein_.
[Illustration: 734]
_Fig._ 734. represents one of the Mansfeldt copper schist mines in the
district called Burgoerner, or Preusshoheit.