264  CONTRIBUTIONS    TO   ECONOMIC   GEOLOGY,  1904.        [bull.  260. 
duction  of  spelter  and  in  part  have  gone  into  the  manufacture  of 
oxide.     In  amount  they  are  relatively  unimportant. 
The  Joplin  ores  occur  in  the  limestones  and  cherts  of  the  Boone 
formation,  which  is  of  Carboniferous  age.     They  are  closely  asso- 
ciated with  certain  fault  lines,  and  constitute  very  irregular  and 
much  modified  fissure  deposits.     They  consist  for  the  most  part  of 
limestone  and  chert  breccias,  partly  recemented  by  a  dark-colore 
secondary  chert  and  by  dolomite.     In  the  unaltered  and  secondaril 
enriched  ores  the  ore  minerals  are  blende  and  galena.     With  thes 
are  associated  minor  amounts  of  marcasite,  pyrite,  chalcopyrite,  and 
greenockite.     In  the  altered  ores  there  are  the  silicate  and  carbonate 
of  zinc,  the  carbonate  of  lead,  and  various  carbonates,  sulphates, 
oxides,  etc.,  of  the  associated  minerals.     The  bulk  of  the  ore  con- 
sists of  limestone,  chert,  dolomite,  blende,  and  galena. 
The  ore  bodies  are  characterized  by  extreme  irregularity.  While 
the  individual  ore  bodies  are  generally  arranged  along  a  more  or  less 
direct  line,  constituting  a  "  run,"  as  defined  by  W.  P.  Jenney,a  there 
are  many  minor  and  puzzling  irregularities.  Perhaps  the  best  illus- 
tration would  be  to  liken  the  single  ore  body  in  form  to  a  large, 
irregular,  and  very  knotty  potato.  The  ore  bodies  are  frequently 
80  to  100  feet  wide  and  40  to  00  feet  thick,  but  the  irregularities  are 
so  common  and  numerous  that  in  mining  they  can  be  followed  only 
with  great  difficulty. 
A  second  form  of  ore  body  which  is  becoming  increasingly  importan 
in  the  district  is  that  which  the  miner  knows  as  "  sheet  ground."     In 
this  there  has  been  a  somewhat  irregular  mineralization  of  an  individ- 
ual bed,  forming  a  blanket  vein.    Such  an  ore  body  is  frequently  found 
on  or  in  the  top  of  the  big  flint  bed  known  as  the  Grand  Falls  chert.  I 
In  single  mines  as  much  as  15  acres,  showing  a  face  of  ore  10  to  15  feet  I 
high,  have  been  mined.    These  ore  bodies  are  normally  of  lower  grade  1 
than  the  runs  and  are  more  thoroughly  cemented,  so  that  they  have 
been  less  mined.     They  constitute,  however,  a  very  important  reserve.  1 
The  ores  so  far  worked  are  all  found  near  the  surface,  no  mining  1 
having  as  yet  been  undertaken  at  depths  greater  than  250  feat,  and  J 
most  of  the  ore  having  been  found  within  150  feet  of  the  surface,  i 
Recent  development  work  has  emphasized  the  importance  of  carrying  3 
all  drill  holes  down  at  least  to  the  Grand  Falls  chert.     At  Aurora,  anl 
important  camp  east  of  the  main  Joplin  district,  new  runs  have  been 
found  below  the  ores  formerly  worked  and  trending  apparently  at  I 
right  angles  to  them.     Considerable  development  has  also  been  tak-  J 
ing  place  along  the  eastern  outcropping  edge  of  the  Cherokee,  the 
overlying  shale,  and  the  conditions  here  seem  to  be  not  unlike  those 
along  the  southern  border  of  the  Mesabi  iron  range,  where  the  ponded 
a  Jenney,  W.  P.,  Trans.  Am.  Inst.  Min.  Eny\,  vol.  22,  1894,  p.  189. 
