To these may be added the _accumulated_ vein, or irregular mass
(_butzenwerke_), a great deposit placed without any order in the bosom
of the rocks, apparently filling up cavernous spaces.
The interlaced masses are more frequent in primitive formations, than in
the others; and tin is the ore which most commonly affects this
locality. See figure of TIN mine.
The study of the mineral substances, called _gangues_ or vein-stones,
which usually accompany the different ores, is indispensable in the
investigation and working of mines. These _gangues_, such as quartz,
calcareous spar, fluor spar, heavy spar, &c., and a great number of
other substances, although of little or no value in themselves, become
of great consequence to the miner, either by pointing out by their
presence that of certain useful minerals, or by characterising in their
several associations, different deposits of ores of which it may be
possible to follow the traces, and to discriminate the relations, often
of a complicated kind, provided we observe assiduously the accompanying
_gangues_.
Mineral veins are subject to derangements in their course, which are
called shifts or faults. Thus, when a transverse vein throws out, or
intercepts, a longitudinal one, we must commonly look for the rejected
vein on the side of the obtuse angle which the direction of the latter
makes with that of the former. When a bed of ore is deranged by a fault,
we must observe whether the slip of the strata be upwards or downwards;
for in either circumstance, it is only by pursuing the direction of the
fault that we can recover the ore; in the former case by mounting, in
the latter by descending beyond the dislocation.
When two veins intersect each other, the direction of the _offcast_ is a
subject of interest, both to the miner and the geologist. In Saxony it
is considered as a general fact that the portion thrown out is always
upon the side of the obtuse angle, a circumstance which holds also in
Cornwall; and the more obtuse the angle, the out-throw is the more
considerable. A vein may be thrown out on meeting another vein, in a
line which approaches either towards its inclination or its direction.
The Cornish miners use two different terms to denote these two modes of
rejection; for the first case, they say the vein is _heaved_; for the
second, it is _started_.
[Illustration: 699]
The great copper lode of Carharack, _d_, _fig._ 699. in the parish of
Gwenap, is one of the most instructive examples of intersection. The
power or thickness of this vein is 8 feet; its direction is nearly due
east and west, and it dips towards the north at an inclination of two
feet per fathom; its upper part being in the _killas_ (a greenish
clay-slate); its lower part in the granite. The lode has suffered two
intersections; the first produced by meeting the vein _h_, called
_Steven’s fluckan_, which runs from north-east to south-west, and which
throws the lode several fathoms out; the second is produced by another
vein _i_, almost at right angles with the first, and which occasions
another out-throw of 20 fathoms to the right side. The fall of the vein
occurs therefore in the one case to the right, and in the other to the
left; but in both it is towards the side of the obtuse angle. This
distribution is very singular; for one part of the vein appears to have
mounted while the other has descended. N, S denotes North and South. _d_
is the copper lode running east and west. _h_, _i_, are systems of
clay-slate veins called fluckans; the line over S, represents the
down-shift, and _d´_ the up-shift.
_General observations on the localities of ores, and on the indications
of metallic mines._
1. _Tin_, exists principally in primitive rocks, appearing either in
interlaced masses, in beds, or as a constituent part of the rock itself,
and more rarely in distinct veins. Tin ore is found indeed sometimes in
alluvial land, filling up low situations between lofty mountains.
2. _Gold_, occurs either in beds, or in veins, frequently in primitive
rocks; though in other formations, and particularly in alluvial earth,
it is also found. When this metal exists in the bosom of primitive
rocks, it is particularly in schists; it is not found in serpentine, but
it is met with in greywacke in Transylvania. The gold of alluvial
districts, called gold of washing or transport, occurs, as well as
alluvial tin, among the debris of the more ancient rocks.
3. _Silver_, is found particularly in veins and beds, in primitive and
transition formations; though some veins of this metal occur in
secondary strata. The rocks richest in it are, gneiss, mica-slate,
clay-slate, greywacke, and old alpine limestone. Localities of
silver-ore itself are not numerous, at least in Europe, among secondary
formations; but it occurs in combination with the ores of copper or of
lead.
4. _Copper_, exists in the three mineral epochas; 1. in primitive rocks,
principally in the state of pyritous copper, in beds, in masses, or in
veins; 2. in transition districts, sometimes in masses, sometimes in
veins of copper pyrites; 3. in secondary strata, especially in beds of
cupreous schist.
5. _Lead_, occurs also in each of the three mineral epochas; abounding
particularly in primitive and transition grounds, where it usually
constitutes veins, and occasionally beds of sulphuretted lead (galena).
The same ore is found in strata or in veins among secondary rocks,
associated now and then with ochreous iron-oxide and calamine (carbonate
of zinc); and it is sometimes disseminated in grains through more recent
strata.
6. _Iron_, is met with in four different mineral eras, but in different
ores. Among primitive rocks, magnetic iron ore and specular iron ore
occur chiefly in beds, sometimes of enormous size; the ores of red or
brown oxide of iron (hæmatite) are found generally in veins, or
occasionally in masses with sparry iron, both in primitive and
transition rocks; as also sometimes in secondary strata; but more
frequently in the coal-measure strata, as beds of clay-ironstone, of
globular iron oxide, and carbonate of iron. In alluvial districts we
find ores of clay-ironstone, granular iron-ore, bog-ore, swamp-ore, and
meadow-ore. The iron ores which belong to the primitive period have
almost always the metallic aspect, with a richness amounting even to 80
per cent. of iron, while the ores in the posterior formations become in
general more and more earthy, down to those in alluvial soils, some of
which present the appearance of a common stone, and afford not more than
20 per cent. of metal, though its quality is often excellent.
7. _Mercury_, occurs principally among secondary strata, in disseminated
masses, along with combustible substances; though the metal is met with
occasionally in primitive countries.
8. _Cobalt_, belongs to the three mineral epochas; its most abundant
deposits are veins in primitive rocks; small veins containing this metal
are found, however, in secondary strata.
9. _Antimony_, occurs in veins or beds among primitive and transition
rocks.
10, 11. Bismuth and nickel do not appear to constitute the predominating
substance of any mineral deposits; but they often accompany cobalt.
12. _Zinc_, occurs in the three several formations: namely, as sulphuret
or blende, particularly in primitive and transition rocks; as calamine,
in secondary strata, usually along with oxide of iron, and sometimes
with sulphuret of lead.
An acquaintance with the general results collected and classified by
geology must be our first guide in the investigation of mines. This
enables the observer to judge whether any particular district, should
from the nature and arrangement of its rocks, be susceptible of
including within its bosom, beds of workable ores; it indicates also, to
a certain degree, what substances may probably be met with in a given
series of rocks, and what locality these substances will preferably
affect. For want of a knowledge of these facts, many persons have gone
blindly into researches equally absurd and ruinous.
Formerly indications of mines were taken from very unimportant
circumstances; from thermal waters, the heat of which was gratuitously
referred to the decomposition of pyrites; from mineral waters, whose
course is however often from a far distant source; from vapours
incumbent over particular mountain groups; from the snows melting faster
in one mineral district than another; from the different species of
forest trees, and from the greater or less vigour of vegetation, &c. In
general, all such indications are equally fallacious with the divining
rod, and the compass made of a lump of pyrites suspended by a thread.
Geognostic observation has substituted more rational characters of
metallic deposits, some of which may be called _negative_ and others
_positive_.
The _negative_ indications are derived from that peculiar geological
constitution, which from experience or general principles excludes
certain metallic matters; for example, granite, and in general every
primitive formation, forbids the hope of finding within them combustible
fossils (pit-coal), unless it be beds of anthracite; there also it would
be vain to seek for sal gem. It is very seldom that granite rocks
include silver; or limestones, ores of tin. Volcanic territories never
afford any metallic ores worth the working; nor do extensive veins
usually run into secondary and alluvial formations. The richer ores of
iron do not occur in secondary strata; and the ores of this metal
peculiar to these localities, do not exist among primary rocks.
Among _positive_ indications, some are proximate and others remote. The
proximate are, an efflorescence, so to speak, of the subjacent metallic
masses; magnetic attraction for iron ores; bituminous stone, or
inflammable gas for pit-coal; the frequent occurrence of fragments of
particular ores, &c. The remote indications consist in the geological
epocha, and nature of the rocks. From the examples previously adduced,
marks of this kind acquire new importance when in a district susceptible
of including deposits of workable ores, the _gangues_ or vein-stones are
met with which usually accompany any particular metal. The general
aspect of mountains whose flanks present gentle and continuous slopes,
the frequency of sterile veins, the presence of metalliferous sands, the
neighbourhood of some known locality of an ore, for instance that of
iron-stone in reference to coal, lastly the existence of salt springs
and mineral waters, may furnish some indications; but when ferruginous
or cupreous waters issue from sands or clays, such characters merit in
general little attention, because the waters may flow from a great
distance. No greater importance can be attached to metalliferous sands
and saline springs.
In speaking of remote indications, we may remark that in several places,
and particularly near Clausthal in the Hartz, a certain ore of red oxide
of iron occurs above the most abundant deposits of the ores of lead and
silver; whence it has been named by the Germans the _iron-hat_. It
appears that the iron ore rich in silver, which is worked in America
under the name of _pacos_, has some analogy with this substance; but
iron ore is in general so plentifully diffused on the surface of the
soil, that its presence can be regarded as only a remote indication,
relative to other mineral substances, except in the case of clay
ironstone with coal.
_Of the instruments and operations of subterranean operations._--It is
by the aid of geometry in the first place that the miner studies the
situation of the mineral deposits, on the surface and in the interior of
the ground; determines the several relations of the veins and the rocks;
and becomes capable of directing the perforations towards a suitable
end.
The instruments are, 1. the magnetic compass, which is employed to
measure the direction of a metallic ore, wherever the neighbourhood of
iron does not interfere with its functions; 2. the graduated semicircle
which serves to measure the inclination, which is also called the
clinometer.
3. The chain or cord for measuring the distance of one point from
another.
4. When the neighbourhood of iron renders the use of the magnet
uncertain, a plate or plane table is employed.
The dials of the compasses generally used in the most celebrated mines,
are graduated into hours; most commonly into twice 12 hours. Thus the
whole limb is divided into 24 spaces, each of which contains 15° = 1
hour. Each hour is subdivided into 8 parts.
_Means of penetrating into the interior of the earth._--In order to
penetrate into the interior of the earth, and to extract from it the
objects of his toils, the miner has at his disposal several means, which
may be divided into three classes: 1. _manual tools_, 2. _gunpowder_,
and 3. _fire_.
The tools used by the miners of Cornwall and Devonshire are the
following:
[Illustration: 700 701 702 703 704 705 706 708 710]
_Fig._ 700. The _pick_. It is a light tool, and somewhat varied in shape
according to circumstances. One side used as a hammer is called the
_poll_, and is employed to drive in the _gads_, or to loosen and detach
prominences. The _point_ is of steel, carefully tempered, and drawn
under the hammer to the proper form. The French call it _pointerolle_.
_Fig._ 701. The _gad_. It is a wedge of steel, driven into crevices of
rocks, or into small openings made with the point of the pick.
_Fig._ 702. The _miner’s shovel_. It has a pointed form, to enable it to
penetrate among the coarse and hard fragments of the mine rubbish. Its
handle being somewhat bent, a man’s power may be conveniently applied
without bending his body.