172  CONTRIBUTIONS    TO    ECONOMIC   OEOLOCY,  1904.         [bull.  260. 
exhausted,  and  none  of  the  veins  have  been  found  rich  enough  to 
work. 
In  the  Greenbushes  field  the  rocks  are  largely  crystalline,  granitic, 
and  gneissic,  cut  by  dikes  of  diorite,  tourmaline  granite  and  pegma- 
tite. Clay  slate,  dipping  at  a  high  angle,  underlies  much  of  the 
gravel. 
Some  of  the  pegmatite  dikes  contain  a  little  less  than  2  per  cent 
tin.  Veins  up  to  2^  feet  wide,  carrying  tin  and  much  tourmaline, 
occur  in  the  granite.  Zircon,  garnet,  monazite,  and  considerable 
amounts  of  niobates  and  tantalates  occur  with  the  tin. 
Placers  are  worked  in  both  residual  and  alluvial  gravels.  The  out- 
put of  western  Australia  for  1902  was  G94  tons  of  ore,  the  metallic 
content  of  which  is  not  given. 
TASMANIA. 
The  mountains  of  eastern  Australia  continue  into  Tasmania,  where 
they  are  also  tin  bearing. 
The  principal  producing  districts  are  Mount  Bischoff,  in  the  north- 
west part  of  the  island,  about  45  miles  from  the  coast ;  the  Stanley 
River  field,  on  the  river  of  that  name,  flowing  from  the  north  into  the 
Pieman  River,  south  of  Mount  Bischoff;  the  Mount  Heemskirk  de- 
posits, still  farther  south,  near  the  west  coast,  and  the  Blue  Tier,  a 
range  of  granite  mountains  south  of  the  Ringarooma  River,  in  the 
northeastern  part  of  the  island.  Tin  ore  also  occurs  in  other  por- 
tions of  the  island  in  smaller  quantities. 
In  general,  the  tin  ore  occurs  with  granite  or  granite-porphyry 
that  is  intruded  into  clay  slates,  sandstones,  limestones,  and  con- 
glomerates, which  are  frequently  metamorphosed  near  the  contact. 
Sheets  of  basalt  and  other  eifusive  rocks  sometimes  cover  both  granite 
and  sedimentaries,  but  have  no  genetic  connection  with  the  tin  de- 
posits. The  ore  occurs  in  veins  cutting  both  granite  and  sedimentary 
rocks,  in  acid  dikes,  and  as  impregnations  and  stockworks  in  the 
granite. 
At  Mount  Lyons  tin  ore  is  found  in  Silurian  rocks  18  miles  from 
the  nearest  known  outcrops  of  granite,  but  the  granite  is  probably 
not  far  below  the  surface. 
The  most  important  deposits  are  those  at  Mount  Bischoff,  in  the 
northwestern  part  of  the  island.  Here  quartz-bearing  porphyries,  in- 
truded through  Silurian  strata,  contain  impregnations  and  veins 
of  tin  ore,  accompanied  by  much  topaz.  The  dikes  are  often  so 
much  altered  that  the  groundmass  is  composed  of  topaz  and 
quartz.  Tin  ore  also  occurs  along  the  contact  of  the  slates  and  sand- 
stones with  the  igneous  rocks,  in  company  with  tourmaline,  fluorite, 
wolframite,  much  arsenic  and  iron  pyrites,  and  siderite.     Granite 
