510  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1903.'  [bull.  225. 
the  rule  rather  than  the  exception  in  the  ground  so  far  mined.  Since 
only  the  clearest  fluorspar  can  be  mined,  the  total  size  of  the  zone  of 
mineralization  is  much  larger.  In  one  case  a  thickness  of  14  feet  of 
calcite  mingled  with  some  fluorspar  is  known  to  be  present  in  the  foot 
wall.  None  of  the  other  mines  in  Illinois  has  yet  been  developed 
enough  to  warrant  a  statement  as  to  the  size  of  the  ore  body,  but  sev- 
eral show  at  the  present  stage  well-defined  veins  of  mineral  of  a  thick- 
ness of  4  to  6  feet. 
Genesis  of  the  ores. — Studies  of  the  genesis  of  the  ores  are  now  being 
made,  but  are  not  yet  complete.  There  are  two  broadly  contrasted 
hypotheses:  (1)  Since  the  lead  and  zinc  deposits  of  the  Mississippi  Val- 
ley have  been  shown  by  repeated  investigations  to  represent  segrega- 
tions from  the  surrounding  rock  produced  by  the  action  of  ordinary 
underground  waters,  these  deposits,  which  carry  some  lead  and  some 
zinc,  have  been  held  to  belong  to  the  same  category;  (2)  the  presence 
in  quantity  of  minerals  either  entirely  absent  or  only  found  under 
exceptional  conditions  in  the  Mississippi  Valley,  the  structural  features 
of  the  ore  bodies,  not  found  elsewhere  in  the  ores  of  the  valley,  and 
the  presence  of  igneous  rocks  in  the  vicinity,  have  suggested  a  genetic 
relation  between  the  ores  and  the  igneous  rocks. 
In  this  connection  it  is  to  be  noted  that  the  eruptives  occurring  here 
are  of  the  same  type  as  those  found  in  connection  with  the  great 
fluorspar  deposits  of  the  north  of  England,  and  that  the  two  districts 
include  the  largest  known  bodies  of  fluorspar.  In  the  second  place, 
microscopic  examination  shows  the  presence  in  the  igneous  rocks  of 
unusual  quantities  of  apatite,  an  undoubtedly  primary  constituent, 
which  might  have  afforded  the  fluorine  necessary  to  the  formation 
of  these  ores.  No  attempt  will  be  made  at  this  place  to  determine 
finally  between  the  two  hypotheses  advanced,  but  it  ma}7  be  stated  that 
in  the  author's  opinion  the  latter  is  probably  correct  in  so  far  as  a 
general  genetic  relationship  between  the  ores  and  the  igneous  rocks  is 
postulated.  It  is  not  intended  to  convey  the  idea  that  each  ore  body 
is  related  to  a  particular  dike.  There  is  strong  field  evidence  against 
this,  but  the  widespread  presence  of  the  dikes,  together  with  the 
structural  features  mentioned,  indicate  in  his  judgment  the  derivation 
of  the  material  from  some  mass  of  igneous  rock  beneath  the  surface 
yet  within  reach  of  underground  waters  and  contributing  to  them. 
Future  production. — The  opinion  just  given  as  to  the  genesis  of  the 
ores  indicates  the  belief  of  the  author  that  they  will  prove  permanent 
in  depth  to  horizons  below  which  they  can  not  be  worked  because  of 
increasing  cost.  The  low  value  of  the  ore  places  a  somewhat  severe 
limitation  upon  the  future  depth  of  mining,  and  it  is  possible  that  this 
feature  alone  will  in  most  situations  preclude  work  to  depth  greater 
than  1,000  feet.  It  is  also  possible  that  changes  in  the  character  of 
the  country  rock  may  influence  disastrously  the  size  and  character  of 
