ISOLATED    AREAS    IN    MISSISSIPPI   VALLEY.  739 
porphyry,  in  which  case  the  connections  are  traceable  in  one  direction 
only.  It  is  concluded  that  all  arc  different  facies  of  a  magma  belong- 
ing to  a  single  period  of  igneous  activity.  Associated  with  the  pre- 
Cambrian  arc  clastic  beds  occupying  small  areas,  as,  for  example  at 
the  summit  of  Pilot  Knob. 
Haworth,68  in  1896,  describes  and  maps  the  pre-Cambrian  geology 
of  the  area  of  the  Iron  Mountain  sheet  in  southeastern  Missouri, 
which  covers  portions  of  Iron.  St.  Francois,  and  Madison  comities. 
The  pre-Cambrian  rocks  are  crystalline  massive  Archean  rocks  and 
crystalline  stratified  Algonkian  rocks. 
The  Archean  rocks  in  general  form  the  uplands.  They  may  be 
divided  into  two  general  classes,  basic  eruptives  and  acidic  eruptives, 
including  granites  and  porphyries. 
The  basic  eruptives,  of  remarkably  uniform  character,  occur  prin- 
cipally in  the  southeastern  part  of  the  area,  usually  in  dikes  cutting 
through  the  granites  and  porphyries,  hut  in  a  few  cases  in  the  form 
of  bosses  almost  circular  in  outline.  The  general  trend  of  the  dikes 
is  northeast-southwest. 
The  granites  occur  mainly  in  two  large  areas,  though  they  arc 
found  occasionally  in  small  patches  within  the  porphyries.  The  two 
areas  are  the  GFraniteville  and  Stouts  Creek  or  St.  Francois  areas. 
The  porphyries  occur  in  numerous  large,  uniformly  distributed  areas, 
making  up  nearly  half  the  area  of  the  entire  sheet.  They  include 
what  have  been  called  by  other  writers  quartz  porphyry,  feldspar 
porphyry,  felsite,  felsophyre.  and  orthophyre. 
Numerous  observations  show  gradations  between  the  granites  and 
the  porphyries,  and  it  is  concluded  that  these  rocks  were  formed  from 
the  same  or  similar  magmas,  and  that  their  difference  in  texture  is 
due  to  crystallization  under  different  conditions. 
Algonkian  rocks  are  found  near  the  center  of  the  area,  capping  the 
Archean  rocks  of  Pilot  Knob.  They  comprise  conglomerates  and 
shilcs,  chiefly  the  former,  and  include  the  iron-ore  deposits  of  the 
locality.  The  pebbles  of  the  conglomerate  are  mostly  derived  from 
the  porphyry.  The  matrix  is  a  fine  felsitic  mass  mixed  intimately 
with  varying  amounts  of  hematite.  In  places  the  ore  forms  almost 
the  entire  body  of  the  rocks. 
Paleozoic  rock's  unconformably  overlie  tin1  crystallines,  and  dip 
away  from  the  Archean  hills. 
Keyes  and  Haworth,69  in  L896,  describe  mid  map  the  geology  of 
the  Mine  la  Motte  area,  which  includes  portion-  of  Ste.  Gene- 
vieve, Madison,  and  St.  Francois  counties,  Mo.  Archean  rock-,  de- 
scribed by  Haworth.  occupy  about  half  of  the  area  of  the  sheet,  form 
ing  the  nucleus  about  which  later  formations  are  exposed  in  conceu 
trie  belts.  They  are  granites  and  porphyries,  cut  by  dike-  of  diabase. 
The  acidic  rocks  greatly   predominate,   the  granite   making  up    fit  lb 
