p.ain.]  LEAD   AND   ZINC    RESOURCES    OF    UNITED    STATES.  267 
into  zinc  oxide  by  the  direct  process  developed  by  S.  P.  Wetherill. 
The  ores  continued  to  be  used  mainly  for  oxide  up  to  the  develop- 
ment of  the  process  of  magnetic  concentration.  As  early  as  1891  or 
1892  experiments  with  the  then  existing  forms  of  magnetic  concen- 
trators showed  the  possibilities  of  this  form  of  ore  dressing,  produc- 
ing ore  of  sufficiently  high  grade  for  spelter  making.  By  1896  J.  P. 
Wetherill  had  developed  a  satisfactory  form  of  concentrator,  which 
was  patented  and  installed  in  that  year  at  the  Franklin  mines.  Since 
then  the  apparatus  has  been  improved,  and  an  entirely  new  concen- 
trating plant,  completed  in  1901,  has  a  capacity  of  1,000  tons  of 
ore  in  twenty-four  hours.  In  1901  the  total  cost  of  treatment  was 
estimated  at  40  cents  per  ton. 
The  output  for  190-1,  as  stated  by  James  B.  Tonking,  superin- 
tendent, was  250,000  tons.  Practically  everything  mined  is  used  in 
one  way  or  another.     The  products  of  the  mill  are  as  follows: 
(a)  Willemite  (30  to  34  per  cent  of  total),  containing  also  calcite, 
some  zincite,  and  small  amounts  of  other  impurities.  This  product 
assays  approximately  48  per  cent  zinc  and  is  suitable  for  high-grade 
spelter  free  from  lead  and  cadmium. 
(h)  Franklinite  product  (with  c  and  <1  forms  about  55  per  cent  of 
total),  consisting  of  magnetite  and  franklinite,  Avith  other  zinc  min- 
erals occurring  as  attached  particles.  This  product  is  used  for  the 
manufacture  of  zinc  oxide  and  the  cinder  is  smelted  for  spicgeleisen. 
(c)  Half-and-half,  containing  franklinite,  rhodonite,  garnet,  and 
other  silicates,  with  attached  particles  of  the  richer  zinc  minerals. 
This  product  contains  somewhat  more  zinc  than  6,  but  is  too  high  in 
silica  for  the  spiegel  furnaces.  It  is  used  entirely  for  the  manufacture 
:>f  zinc  oxide. 
(d)  The  dust  from  the  crushing  and  concentrating  plant  (about  4 
per  cent  of  the  total)  is  collected  and  shipped  with  the  other  material 
Cor  the  manufacture  of  oxide. 
(e)  The  tailings  from  the  nonmagnetic  portion  of  the  ore  (about  10 
per  cent  of  the  total)  still  contain  some  zinc,  but  not  enough  to  pay 
for  treatment.  This  material  is  shipped  for  use  in  concrete  construc- 
tion. 
The  ore  bodies  occur  in  pre-Cambrian,  coarsely  crystalline,  white 
imestone.  They  have  no  sharply  defined  walls  as  a  rule,  though  the 
passage  from  the  ore  into  clean  limestone  never  occupies  more  than  a 
few7  feet.  The  ore  material  has  a  structure  resembling  that  of  gneiss 
because  of  the  banded  arrangement  of  the  different  minerals,  and  this 
structure  is  cut  across  by  dikes  of  granite,  which  are  therefore  evi- 
dently later  than  the  formation  of  the  ore.  It  is  along  these  granite 
llikes  that  the  majority  of  rare  and  interesting  minerals  from  this 
Locality  occur. 
