268  CONTRIBUTIONS   TO   ECONOMIC    GEOLOGY,  1904.        I  bull.  260. 
The  origin  of  the  ores  has  never  been  satisfactorily  explained,  but 
studies  now  in  progress  are  expected  to  throw  new  light  upon  this 
interesting  problem  in  ore  genesis. 
The  mines  at  Franklin  furnace  are  worked  in  part  by  open  pits, 
but  the  underground  workings  are  also  very  extensive.  The  annual 
statements  to  the  State  geologist  of  New  Jersey,  authorized  by  the 
company,  indicate  that  since  1890  only  development  work  has  been  in 
progress  underground,  such  ore  as 'has  been  extracted  having  come 
from  drifts  and  raises  in  the  ore  body.  It  is  apparent,  therefore, 
that  the  present  reserves  must  be  very  large. 
The  suggestion  has  been  made  that  the  form  of  the  Franklin  ore 
body  and  its  blunt  underground  termination  indicate  that  its  eastern 
side  has  been  faulted  off,  but  this  theory  has  never  been  adequately 
tested  by  drilling. 
At  Sterling  Hill  the  average  grade  of  the  ore  is  said  to  be  lower 
than  at  Franklin  furnace,  but  from  surface  exposures  alone  it  is 
evident  that  large  amounts  of  ore  remain  to  be  mined,  though  the 
extent  of  the  deposit  in  depth  has  never  been  determined.  The  old 
mines  are  known,  however,  to  have  been  300  or  400  feet  deep. 
Exploration  with  diamond  drills  has  been  carried  on  by  various 
parties  in  the  neighborhood  of  Franklin  furnace,  but  thus  far  no 
indication  of  valuable  zinc  ores  has  been  reported. 
Virginia  and  Tennessee. — Zinc  is  found  in  two  districts  in  Vir- 
ginia, one  in  Albemarle  County  and  the  other  in  the  southwestern 
part  of  the  State,  adjacent  to  Tennessee.  In  the  latter  district  zinc 
and  lead  are  found  at  a  number  of  points  in  a  belt  extending  from 
Roanoke  nearly  to  Knoxville.  While  few  of  the  mines  are  steadily 
worked,  there  has  been  continuous  production  from  the  district  as  a 
whole  for  many  years.  Nearly  all  the  ore  is  locally  reduced.  The 
Bertha  works,  at  Pulaski,  is  the  only  spelter  plant  now  in  operation ; 
though  zinc  oxide  is  burned  and  lead  smelted  at  Austinville,  Va.,  by 
the  same  company.  Some  ore  has  been  shipped  to  the  Indiana  fur- 
naces for  reduction,  but  this  movement  is  not  important. 
Between  1891  and  1897,  the  only  years  for  which  figures  are  avail- 
able, the  spelter  production  of  this  district  averaged  about  3,500 
tons  per  year.  In  1893  the  district  as  a  whole  is  estimated  to  have 
produced  21,000  tons  of  ore.  In  1900  the  Tennessee  mines  alone  are 
estimated  to  have  produced  3,96S  tons.a  At  present  the  production 
is  very  small.  The  grade  of  concentrates  is  not  high  h — 10  to  50  perl 
cent  zinc — but  as  the  ore  is  of  unusual  purity  and  the  furnace  work 
very  good  there  is  produced  a  metal  which  commands  a  premium  in 
the  market. 
"  The  Mineral  Industry  for  10OO,  p.  665. 
*  Ingalls,  W.  R.,  Production  and  Properties  of  Zinc,  1902,  pp.  107-203. 
