58 
ECONOMIC GEOLOGY 
Transvaal; the ore is at two horizons in the pre-Palazoic 
basic rocks of the Bushveld Complex. The lower horizon 
is a thick band of silicified serpentine which, according to 
P. A. Wagner and E. T. Mellor, is an altered dunite con- 
taining a variety of olivine, hortonolite. This dunite occurs 
in veins and nodules and is traversed by veins of quartz, 
chalcedonic silica, and magnesite, so that it has been subject 
to hydrothermal action. A pegmatite—with black horn- 
blende crystals 3 inches long, and diallage, phlogopite, and 
magnetite—is associated with the hortonolite-dunite. 
According to Stanley and Wagner (F. Chem. Soc. S. Afr., 
xxv, 1G25, pp. 254-9) the platinum is a primary constituent 
of the dunite; but it frequently occurs as irregular inter- 
stitial grains that solidified after the other constituents. 
The upper horizon of platinum is in a diallage-norite that 
contains pyrite and chalcopyrite, which are presumably 
secondary. 
A third platinum occurrence in the Transvaal is in a quartz- 
lode in the Waterberg Sandstone (pre-Cambrian). This lode 
has been formed by solutions circulating along a fault, 
which is post-Karroo and therefore post-Lower Jurassic. 
Some of the platinum is a replacement after pyrite. The ore 
has yielded about 24 dwt. to the ton. 
Platinum has been found in Rhodesia with sulphides in a 
long dyke-like band of diallage-norite at Makwiro. The 
rock contains up to 3 dwt. of platinum to the ton, but the 
particles are said to be so minute that they float on water 
and cannot be recovered by washing. R. S. Lightbody 
(Rep. 19, S. Rhod. G.S., 1926) remarks that the presence of 
the platinum is not easily explained, as it is not in the heaviest 
of the norite. 
Brisa CoruMsia—Platinum has been found at Tulameen 
in the Rocky Mountains of British Columbia in a Jurassic 
magnetite-olivine-diallagite (koswite), which has been hydro- 
thermally altered (Poitevin, Canada G.S., Summ. Rep. 
1013, pp. 84-101). The koswite surrounds masses of dunite, 
which also contains platinum. 
Genesis OF PraTinum OrEs—As chromite, the mineral 
most often associated with platinum, is an essential con- 
stituent of dunite, the view has been adopted that platinum 
is a primary constituent of that rock, and is normally of