ORES OF FIVE MINOR METALS 119
dyke, and penetrate and replace the pyroxene; hence, there
also, the nickel ores are later than the consolidation of the
igneous rock.
Sout Arrica—Nickel ores of no economic importance
occur at Insizwa in the N.E. of Cape Colony (at 29° 20’ E,,
20° 45’ S.) in an intrusive sill which W. H. Goodchild has ex-
Plained as due to the separation of the sulphides from a molten
norite, T. C. Phemister (Tr. IL. M.M., xxxiii, 1924, pp. 519-
20) has shown that the rock is an olivine-gabbro with some
Picrite on the lower margin; and that the sulphides were
not the first formed constituents in the gabbro, as they
often replace biotite, occur in secondary quartz, and were
deposited along fractures.
The Bushveld laccolite in the Transvaal includes dissemin-
ated and massive nickel-bearing sulphides which are regarded
by P. A. Wagner as primary and magmatic ; but he describes
the sulphides in the ore as unquestionably later than the
silicates, and as in part replacing the silicates; and the ore
bodies cut across the ** pseudo-stratification * of the rocks
and their mineralization has been guided by older fractures
(G.S.S. Afr., Mem. 21, 1924, p- 147).
A small occurrence of nickel-cobalt ore in a diorite at
Talnotry, near Newton-Stewart in S.W. Scotland, has been
attributed to igneous segregation. The sulphides were de-
posited along a fault after the igneous rock had undergone
great hydrothermal changes.
MERCURY
MercuRY—Uses AnD Price—Mercury or Quicksilver
(Hg; at. wt, 200; sp. gr. liquid, 13-6; melting-point,
— 38°F. ; boiling-point, 357° F. with slow volatilization at
ordinary temperatures) is the only metal which is liquid at
ordinary climatic temperatures. It is used in barometers
and thermometers owing to its heaviness and its regular
®Xpansion and contraction with changes of temperature.
Its most extensive use is for chemical purposes, and as a
drug; its other chief uses are as a pigment, a detonator—
its use in percussion caps developed the fire-arm from the
ancient flintlock—as amalgam in dentistry, and in mirrors.
It 1s of great service in gold mining, as it enables minute
particles of gold to be recovered by amalgamation.
