194  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,,  1904.        [bull.  26GJ 
"While  no  extended  development  has  been  done  in  the  northern  part 
of  the  area,  some  very  encouraging  results  have  been  obtained  in 
-hallow  openings,  and  that  part  of  the  belt  appears  to  be  the  most 
promising. 
It  is  possible  that  there  is  general  dissemination  of  the  cassiteril 
in  very  small  quantities  throughout  the  pegmatite,  but  such  does  not 
appear  to  be  the  case.     In  the  segregations,  or  ore  shoots,  the  amount 
of  cassiterite  varies  greatly,  from  a  small  fraction  of  1  per  cent  upi 
to   20   per  cent   or  more.     It   is  probable   that   a   number   of  shoots 
already  known  will  average  from  *j  per  cent  of  cassiterite  up,  and  will  1 
be  of  sufficient  size  and  persistency  to  prove  profitable  under  good  ' 
management.     There  seems  to  be  no  ground  for  expecting  any  marked 
change  in  the  value  of  the  deposits  considered  collectively,  as  depth 
i>  attained,  although  decided  fluctuations  will  probably  be  encounj    i 
tered  in  certain  individual  deposits. 
A  little  cassiterite  has  been  obtained  from  placers  near  the  Kossl 
and  near  the  Jones  mines,  but  systematic  search  for  stream  tin  has 
not  been  undertaken.     In  view  of  the  much  lower  cost  of  winning    j 
placer  tin  than  of  mining  it  from  lodes,  it  seems  that  this  part  of  thejj 
prospecting  ought  not  to  be  neglected. 
Of  the  second  shipment  from  the  Ross  mine,  comprising  nearly  18 
tons  of  ore.  analyses  by  Ledoux  &  Co.  show  that  about  seven-eighths    I 
averaged   70.44  per  cent  metallic  tin,  while  the  remaining  portion    j 
contained  36.50  per  cent,  the  total  shipment  thus  averaging  (>(;  per   \ 
cent  metal.     This  is  a  very  good  grade  for  lode  tin.     It  is  possible] 
that  ore-  from  other  parts  of  the  belt,  or  other  shipments  from  the 
same  mine,  may  not  equal  this  figure,  but  it  is  to  be  expected  that  j, 
careful  and  thorough  concentration  will  result  in  a  product  of  verw| 
good  grade. 
TESTS   FOR   CASSITERITE. 
Because  of  the  dark  color  and  metallic  luster  of  the  cassiterite  of  I 
this  region,  numerous  dark  heavy  minerals  have  been  mistaken  for  it.  ? 
Of  these,  the  iron  ores  ilmenite  and  magnetite  are  most  common.  It  fe 
may  therefore  be  of  use  to  note  here  one  or  two  simple  tests  for  cas-  J 
siterite.  Magnetite  can.  of  course,  be  detected  by  its  attraction  to  a  | 
common  magnet.  Ilmenite  is  also  feebly  magnetic  at  times.  The  I 
color  of  powdered  cassiterite  ranges  from  white  to  brown,  and  is  t 
never  black.  If  a  small  piece  of  the  mineral  in  question  is  ground  -f 
to  a  very  fine  powder  between  two  clean  pieces  of  steel  (the  face  of 
a  hammer  and  a  shovel  will  serve),  it  may  generally  be  safely  rejected  J 
as  not  containing  tin  if  the  powder  is  black.  If  brown  or  still  f 
lighter  in  color,  the  blowpipe  test  may  be  applied.  So  much  of  the 
finely  ground  material  as  can  be  piled  on  an  area  one-fourth  inch 
square  is  added  to  twice  its  bulk  of  powdered  charcoal  and  to  three 
