Having thus enumerated the several species of lead ore, we may remark,
that galena is the only one which occurs in sufficiently great masses to
become the object of mining and metallurgy. This mineral is found in
small quantity among the crystalline primitive rocks, as granite. It is
however among the oldest talc-schists and clay slates, that it usually
occurs. But galena is much more abundant among the transition rocks,
being its principal locality, where it exists in interrupted beds,
masses, and more rarely in veins. The blackish transition limestone is
of all rocks that which contains most galena; as at Pierreville in
Normandy; at Clausthal, Zellerfeldt, and most mines of the Harz; at
Fahlun, in Sweden; in Derbyshire and Northumberland, &c. In the
transition graywacke of the south of Scotland, the galena mines of
Leadhills occur. The galena of the primitive formations contains more
silver than that of the calcareous.
The principal lead mines at present worked in the world, are the
following: 1. Poullaouen and Huelgoët near Carhaix in France, department
of Finisterre, being veins of galena, which traverse a clay slate
resting upon granite. They have been known for upwards of three
centuries; the workings penetrate to a depth of upwards of 300 yards,
and in 1816 furnished 500 tons of lead per annum, out of which 1034
pounds avoirdupois of silver were extracted. 2. At Villeforte and
Viallaz, department of the Lozère, are galena mines said to produce 100
tons of lead _per annum_, with 400 kilogrammes of silver (880 libs.
avoird.). 3. At Pezey and Macot, to the east of Moutiers in Savoy, a
galena mine exists in talc-schist, which has produced annually 200 tons
of lead, and about 600 kilogrammes of silver (1320 libs avoird.). 4. The
mine of Vedrin, near Namur in the Low Countries, is opened upon a vein
of galena, traversing compact limestone of a transition district; it has
furnished 200 tons of lead, from which 385 pounds avoird. of silver were
extracted. 5. In Saxony the galena mines are so rich in silver as to
make the lead be almost overlooked. They are enumerated under silver
ores. 6. The lead mines of the Harz, have been likewise considered as
silver ores. 7. Those of Bleyberg in the Eifel are in the same
predicament. 8. The galena mines of Bleyberg and Villach in Carinthia,
in compact limestone. 9. In Bohemia, to the south-west of Prague. 10.
The mines of Joachimsthal, and Bleystadt, on the southern slope of the
Erzgebirge, produce argentiferous galena. 11. There are numerous lead
mines in Spain, the most important being in the granite hills of
Linarès, upon the southern slope of the Sierra Morena, and in the
district of the small town of Canjagar. Sometimes enormous masses of
galena are extracted from the mines of Linarès. There are also mines of
galena in Catalonia, Grenada, Murcia, and Almeria, the ore of the last
locality being generally poor in silver. 12. The lead mines of Sweden
are very argentiferous, and worked chiefly with a view to the silver.
13. The lead mines of Daouria are numerous and rich, lying in a
transition limestone, which rests on primitive rocks; their lead is
neglected on account of the silver.
14. Of all the countries in the world, Great Britain is that which
annually produces the greatest quantity of lead. According to M.
Villefosse, in his _Richesse Minerale_, published in 1810, we had
furnished every year 12,500 tons of lead, whilst all the rest of Europe
taken together, did not produce so much; but from more recent documents,
that estimate seems to have been too low. Mr. Taylor has rated the total
product of the United Kingdom _per annum_ at 31,900 tons, a quantity
fully 2-1/2 times greater than the estimate of Villefosse (see Conybeare
and Phillips’ Geology, p. 354). Mr. Taylor distributes this product
among the different districts as follows:--
Tons.
Wales, (Flintshire and Denbighshire) 7,500
Scotland, (in transition graywacke) 2,800
Durham, Cumberland, and Yorkshire, (in carboniferous lime) 19,000
Derbyshire, (probably in carboniferous lime) 1,000
Shropshire 800
Devon and Cornwall, (transition and primitive rocks) 800
------
Total 31,900
We thus see that Cumberland, and the adjacent parts of the counties of
Durham and York, furnish of themselves nearly three-fifths of the total
product. Derbyshire was formerly much more productive. In Cornwall and
Devonshire, the lead ore is found in veins in _killas_, a clay-slate
passing into greywacke. In North Wales and the adjacent counties, as
well as in Cumberland and Derbyshire, the lead occurs in the
carboniferous limestone.
The English lead-miners distinguish three different kinds of deposits of
lead ore; _rake-veins_, _pipe-veins_, and _flat-veins_. The English word
vein corresponds to the French term _filon_; but miners make use of it
indifferently in England and France, to indicate all the deposits of
this ore, adding an epithet to distinguish the different forms; thus,
_rake veins_ are true veins in the geological acceptation of the word
vein; _pipe-veins_ are masses usually very narrow, and of oblong shape,
most frequently parallel to the plane of the rocky strata; and
_flat-veins_ are small beds of ores interposed in the middle of these
strata.
_Rake-veins_ are the most common form in which lead ore occurs in
Cumberland. They are in general narrower in the sandstone which covers
the limestone, than in the calcareous beds. A thickness of less than a
foot in the former, becomes suddenly 3 or 4 feet in the latter; in the
rich vein of Hudgillburn, the thickness is 17 feet in the _Great
limestone_, while it does not exceed 3 feet in the overlying _Watersill_
or sandstone. This influence exercised on the veins by the nature of the
enclosing rock, is instructive; it determines at the same time almost
uniformly their richness in lead ore, an observation similar to what has
been made in other countries, especially in the veins of Kongsberg in
Norway. The Cumberland veins are constantly richer, the more powerful
they are, in the portions which traverse the calcareous rocks, than in
the beds of sandstone, and more particularly the schistose rocks. It is
rare in the rock called _plate_ (a solid slaty clay) for the vein to
include any ore; it is commonly filled with a species of potter’s earth.
The upper calcareous beds are also in general more productive than the
lower ones. In most of these mines, the veins were not worked till
lately below the fifth calcareous bed (the four-fathom limestone), which
is 307 yards beneath the millstone-grit; and as the first limestone
stratum is 108 yards beneath it, it follows that the thickness of the
part of the ground where the veins are rich in lead does not in general
exceed 200 yards. It appears however that veins have been mined in the
neighbourhood of Alston Moor, downwards to the eleventh calcareous
stratum, or Tyne bottom limestone, which is 418 yards under the
millstone-grit of the coal formation, immediately above the whin-sill;
and that they have been followed above the first limestone stratum, as
high as the grindstone sill, which is only 83 yards below the same
stratum of millstone-grit; so that in the total thickness of the
plumbiferous formation there is more than 336 yards. It has been
asserted that lead veins have been traced even further down, into the
_Memerby_ scar limestone; but they have not been mined.
The greatest enrichment of a vein takes place commonly in the points
where its two sides, being not far asunder, belong to the same rock; and
its impoverishment occurs when one side is calcareous and the other a
schistose clay. The minerals which most frequently accompany the galena,
are carbonate of lime, fluate of lime, sulphate of baryta, quartz, and
pyrites.
The pipe-veins (_amas_ in French), are seldom of great length; but some
have a considerable width; their composition being somewhat similar to
that of the _rake-veins_. They meet commonly in the neighbourhood of the
two systems, sometimes being in evident communication together; they are
occasionally barren; but when a wide pipe-vein is metalliferous, it is
said to be very productive.
The _flat veins_, or _strata veins_, seem to be nothing else than
expansions of the matter of the vein between the planes of the strata;
and contain the same ores as the veins in their vicinity. When they are
metalliferous, they are worked along with the adjacent rake vein; and
are productive to only a certain distance from that vein, unless they
get enriched by crossing a rake vein. Some examples have been adduced of
advantageous workings in _flat veins_ in the _great limestone_ of
Cumberland, particularly in the mines of Coalcleugh and Nenthead. The
_rake veins_, however, furnish the greater part of the lead which
Cumberland and the adjacent counties send every year into the market.
Mr. Forster gives a list of 165 lead mines, which have been formerly, or
are now, worked in that district of the kingdom.
The metalliferous limestone occupies, in Derbyshire, a length of about
25 miles from north-west to south-east, under a very variable breadth,
which towards the south, amounts to 25 miles. Castleton to the north,
Buxton to the north-west, and Matlock to the south-east, lie nearly upon
its limits. It is surrounded on almost all sides by the millstone grit
which covers it, and which is, in its turn, covered by the coal strata.
The nature of the rocks beneath the limestone is not known. In
Cumberland the metalliferous limestone includes a bed of trap,
designated under the name of _whinsill_. In Derbyshire the trap is much
more abundant, and it is thrice interposed between the limestone. These
two rocks constitute of themselves the whole mineral mass, through a
thickness of about 550 yards, measuring from the millstone grit; only in
the upper portion, that is near the millstone grit, there is a pretty
considerable thickness of argillo-calcareous schists.
Four great bodies or beds of limestone are distinguishable, which
alternate with three masses of trap, called toadstone. The lead veins
exist in the calcareous strata, but disappear at the limits of the
toadstone. It has now been ascertained however that they recur in the
limestone underneath.
_Treatment of the Ores of Lead._
The mechanical operations performed upon the lead ores in Great Britain,
to bring them to the degree of purity necessary for their metallurgic
treatment, may be divided into three classes, whose objects are,--