CHAPTER VII

ORES OF LEAD, ZINC, AND SILVER

Leap—Irs Uses, etc.—Lead (Pb; at. wt, common lead,
207, uranium lead, 206 ; sp. gr., common lead, 11-33, uranium
lead, 11-27; melting-point, 620° F.) is one of the six metals
that was used by prehistoric man ; it is easy to work, being
soft, ductile, and malleable. Among its valuable properties
are its flow under pressure while’ cold, melting at a low temperature,
 and insolubility in concentrated sulphuric and
hydrochloric acids. It is a constituent of some alloys including
 pewter, and being the heaviest of common metals
is used for bullets. In 1025 40 per cent. of the supply was
used by the electrical industry and 25 per cent. for paints.
British lead mining was at its zenith in 1856 with 353 mines
at work, and an output of 73,129 tons of lead and 61,400
oz. of silver. The price of lead was then £32 a ton. The
price fell, after the opening of lead mines in the United States,
Mexico, and Australia, to as low as £13 16s. per ton in 1896.
The United States has in recent years supplied 40 per cent.,
Mexico 11 per .cent., Spain 10 per cent., and Australia 10
per cent. of the world’s output. The British Isles in 1924
produced only 10,863 tons, mainly from eight mines. The
declining yield of the six chief mining fields has led to the
price rising to £40 per ton in 1924, but it has fallen again to
£28 in 1927. Native lead is rare, as it is slowly oxidized,
but it has been often recorded from torn shot found in gravels.
The chief lead mineral is galena (PbS), which is usually
associated with blende (ZnS), and nearlv alwavs contains
silver as argentite (Ag,S).
CrassiFicatioNn oF OrRes—The primary lodes occur along
faults and fractures. Those due to the filling of fissures are
often well crustified. Many lodes that are productive in
ab