HESS    AND 
GEATON. 
OCCURRENCE    AND    DISTRIBUTION    OF    TIN.  173 
occurs  some  distance  from  the  mines.  The  porphyries  have  weathered 
deeply,  and  the  residual  material  has  been  cemented  with  iron  oxides, 
so  that  much  of  it  has  to  be  blasted.  The  Mount  Bischoff  mine  is  one  of 
the  largest  tin  mines  in  the  world,  its  product  for  the  year  ending 
June  30,  1903,  having  been  between  1,350  and  1,450  tons  of  tin.  The 
mine  has  been  worked  both  by  greatxopen  cuts  and  by  tunnels.  One 
cut  1,000  feet  wide  and  100  feet  deep  gave  ore  carrying  3  per  cent  of 
tin  oxide,  but  the  average  percentage  is  probably  less.  From  one 
excavation,  66  by  66  feet,  240  tons  of  ore  were  taken,  while  20  yards 
away  there  were  barely  traces  of  tin.  Masses  of  very  pure  cassiterite, 
the  largest  weighing  over  600  pounds,  were  taken  out. 
In  the  Blue  Tier,  a  range  of  mountains  near  the  east  coast,  between 
the  Ringarooma  River  and  Georges  Bay,  tin  ore  occurs  in  impregna- 
tions and  stockworks  in  the  granite.  The  deposits  are  of  low  grade, 
from  under  three-eighths  to  about  five-eighths  per  cent  of  black  tin, 
but  occur  in  comparatively  large  masses.  Topaz,  fluorite,  apatite, 
wolframite,  scheelite,  molybdenite,  galena,  copper  and  iron  pyrites, 
and  sapphire  occur  with  the  tin  ore.     Tourmaline  is  rare. 
There  are  promising  veins  of  tin  ore  in  the  Mount  Heemskirk  dis- 
trict of  western  Tasmania,  and  some  veins  are  found  also  on  the 
Stanley  River  farther  north,  where  monazite  occurs  in  considerable 
quantity  with  the  ore. 
The  principal  alluvial  workings  are  on  the  Ringarooma  and  Georges 
rivers  and  their  branches  in  the  northeastern  portion  of  the  island. 
The  deposits  are  of  different  ages,  from  Miocene  to  recent.  Some  of 
the  older  gravels  are  covered  with  basalt  and  are  worked  by  under- 
ground mining,  but  most  of  the  ore  is  obtained  from  shallow  work- 
ings by  ground  sluicing.  Most  of  the  easily  mined  ore  is  worked  out, 
and  the  larger  and  poorer  deposits  are  now  being  hydraulicked. 
These  placers  give  promise  of  long  production.  Tin  was  discovered 
in  Tasmania  in  1871,  and  the  Mount  Bischoff  deposit  was  discovered 
the  next  year.  The  production  reached  its  maximum  of  6,013  tons 
of  tin  in  1878.     For  the  year  1902  the  output  was  2,193  tons. 
NEW    ZEALAND. 
Tin  ore  has  been  found  in  auriferous  conglomerates  at  the  base  of 
the  coal  measures  at  Lankeys  Creek,  Reefton,  Milford,  and  Dusky 
Sounds,  on  the  west  coast  of  Otago. 
It  has  also  been  found  in  gravels  and  in  gneissic  granitoid  rocks  of 
the  Remarkable  Mountains,  Stewart  Island.  On  South  Island 
stream  tin  has  been  found  in  the  auriferous  gravels  of  Humphreys 
Gully,  near  Hokitika, 
