b.un.]  LEAD    AND    ZINC    RESOURCES    OF    UNITED    STATES.  259 
entire  district  is  iioav  being  reinvestigated  by  Dr.  E.  R.  Buckley, 
State  geologist. 
In  St.  Francois  and  Madison  counties  there  are  two  great  dolomite 
formations,  to  which  Mr.  Winslow  gave  the  names  St.  Joseph  and 
Potosi,  in  St.  Francois  County,  and  which  Mr.  Keys"  called  the 
Fredericktown  dolomite  and  Lesueur  limestone,  in  Madison  County. 
The  lower,  now  called  the  Bonne  Terre,6  lies  on  the  La  Motte  sand- 
stone, which  is  of  irregular  thickness  and  constitutes  the  basal  member 
of  the  sedimentary  sequence.  Along  the  western  border  of  the  pro- 
ductive territory  the  granite  and  porphyry  hills  of  the  pre-Cambrian 
rise  through  and  above  the  sedimentary  beds.  To  the  east  of  the 
mining  district  these  older  rocks  rise  again,  so  that  the  sediments 
seem  to  have  been  laid  down  in  a  great  pre-Cambrian  erosion  trough. 
The  Bonne  Terre  limestone,  which  is  the  lead-bearing  member  of 
the  sequence,  is  a  noncherty  dolomite,  approximately  400  feet  thick 
where  uneroded.  At  the  base  the  dolomite  is  frequently  interbedded 
with  the  sandstone  of  the  La  Motte  formation,  and  for  as  much 
as  a  hundred  feet  above  the  base  dark,  shaly  material  is  common  in 
the  formation.  Chlorite,  from  the  decomposition  of  the  pre-Cam- 
brian crystallines,  and  probably  also  glauconite,  are  abundant  in 
this  portion  of  the  formation.  The  upper  member  of  the  formation 
is  also  shaly,  and  with  the  shale  are  thin  beds  of  conglomerate 
which  Nason c  has  called  the  "  Edgewise  beds,"  which  should  be 
taken  as  the  base  of  a  formation  intervening  between  the  Bonne  Terre 
and  the  true  Potosi.  In  this  formation  are  some  70  feet  of  non- 
cherty dolomite,  above  which  in  turn  are  the  characteristic  cherty 
dolomites  of  the  Potosi. 
The  ore  bodies  are  found  for  the  greater  part  in  the  lower  shaly 
portion  of  the  Bonne  Terre,  but  also  to  a  less  extent  in  the  upper 
shaly  beds ;  in  the  Bonne  Terre  mines  the  ore  continues  from  the  one 
member  to  the  other.  The  ore  bodies  are  famous  for  their  great 
size — measured  in  hundreds  of  feet  in  horizontal  dimensions  and  up 
to  a  hundred  in  vertical — and  for  the  almost  entire  freedom  of  the 
galena  from  mixture  with  other  metallic  minerals.  In  particular 
there  is  an  entire  absence  of  zinc,  though  elsewhere  in  the  Missis- 
sippi Valley  the  association  of  galena  and  blende  is  always  intimate. 
Small  quantities  of  copper,  nickel,  and  cobalt  occur,  and  on  the  Mine 
La  Motte  estate  these  are  sufficient  in  amount  to  warrant  separate 
treatment. 
It  is  possible  to  suggest  reasons  for  the  stratigraphic  localization  of 
these  great  ore  bodies,  but  so  far  no  entirely  satisfactory  hypothesis 
ins  been  advanced  to  explain  their  irregular  distribution  in  a  hori- 
°  Keyes,  C.  R.,  Geology  of  the  Mine  La  Motte  sheet,  1895,  pp.  48-53. 
6  For  full  discussion  of  the  stratigraphy,  see  Bull.  U.  S.  Geol.  Survey  No.  2G7,  in  press. 
c  Am,  Jour.  Sci.,  4th  ser.,  vol.  12,  1901,  p.  359. 
