THE GENESIS OF ORE DEPOSITS 491 
associations, namely, in Tavoy, dominance of tungsten; in the 
Malay peninsula, Banka, and Billiton, dominance of tin alone; in 
Queensland, tin, tungsten, molybdenum, bismuth; in New South 
Wales, tin, tungsten, and molybdenum; in Tasmania, mainly tin. 
The second most important tin-producing country of the world 
is Bolivia. Here along the Cordilleran chains, in association with 
the great Tertiary volcanic activity, we find extraordinarily rich 
veins carrying tin, tungsten, bismuth, and silver; a remarkable 
and apparently unique type.t. These veins present features of 
great interest from the theoretical point of view. Three chief 
types of veins can be recognized, as follows: 
a) Tin-bismuth veins with tourmaline and other pneumatolytic 
minerals, associated with deep-seated granites. 
6) Tin-bismuth-silver veins, carrying most of the tin as com- 
plex sulphides, stannite, etc., associated with hypabyssal porphyry 
intrusions. 
c) Silver veins without tin, associated with extrusive volcanic 
rocks. 
A noteworthy feature is the occurrence of the tin in type (0) 
mainly as complex sulphides associated with argentiferous tetrahe- 
driteand sulphosalts of copper, lead, bismuth, arsenic, and anti- 
mony. There is thus a distinct gradation in the metal contents of the 
veins, minerals with boron and fluorine occurring in quantity only 
in the high-temperature veins associated with granites. Thus the 
temperature-depth relations of tin and silver are clearly shown. 
In all these cases there can be no possible doubt of the associa- 
tion of the tin and other metals with igneous rocks. When we 
turn to other metals the facts are equally striking. It is impossible 
to enumerate all of them, but a few examples must suffice, and 
these may be taken as typical of the rest. It has long been known 
that platinum has its original home for the most part in peridotites 
and their alteration product, serpentine, and to a less extent in 
certain nickel-bearing and other eruptives to be mentioned pres- 
ently; in short, in ultrabasic and basic rocks. The peridotites 
also commonly contain large quantities of chromite and also 
chrome-bearing spinels: hence the presence of chromite is a useful 
t Davy, Econ. Geol., Vol. XV (1920), pp. 463-06. 
