ORES OF LEAD, ZINC, AND SILVER 109
Stone, or limestone. The ores at Ceeur d'Alene in Idaho and
Leadville in Colorado, for example, are remarkably alike, in
SPite of the difference in structure between, the two fields.
This similarity indicates that the ores are not derived from
ither the adjacent sedimentary or intrusive rocks, but
‘Ome from an ore-zone beneath the igneous rocks of the
‘rust. Like the plutonic waters they work their way up-
Vard through igneous and sedimentary rocks alike.
ZINC
Zivc (Zn ; at. wt, 65; sp. gr, 71; melting-point, 790° F. ;
volatilizing-point, 1520° F.) is a comparatively modern metal
‘0 the western world, though it has been long used by the
Chinese. The origin of the name is uncertain, but is probably
from #inn, the Greek for tin. Spelter, the trade name for
the metal, was used in Germany and Spain for the alloy
Pewter. Zinc is bluish-white, brittle when cold, but malle-
able when hot. Most of it is used for galvanizing iron as a
Protection against rust; its other chief uses are as white
Paint, a drug, and in many alloys, such as German silver,
which is composed of nickel, zinc, and copper. The price of
nc before the War was generally about £22 a ton; it rose
to £100 a ton in 1015, after which it fell to £52 and early in
1927 was about £30. The bulk of the zinc ores comes, in
order of quantity, from Germany, Australia, the United States,
Italy, ang Spain. The zinc is seldom extracted at the mines,
and the largest production of spelter is in the United States,
Belgium, ang Germany. Belgium was the home of this
‘ndustry which is now threatened by the electrical pro-
cesses of zine extraction. The common ore is blende (or
sphalerite, ZnS), which is constantly associated with galena,
and most zinc is obtained from lead ores.
FrankLin OrEs—1In the secondary ores lead and zinc are
often Separated, and the zinc is frequently deposited in con-
Centric layers known as schalenblende. Some secondary
Ores are of special value as free from lead. Thus at Franklin,
New Jersey, a trough-shaped sheet of ore in a pre-Cambrian
limestone includes willemite (ZnSi0,), zincite (ZnO), and
franklinite (oxide of zinc, iron, and manganese). The origin of
this deposit js a vexed question. The absence of lead suggests
that the ore is secondary. It has probably been formed by
