738  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
The  iron  ore  of  Iron  Mountain  and  most  of  the  other  localities  is  in 
veins  in  the  massive  rock,  probably  of  water-infiltrated  origin;  or  in 
a  residuary  mantle ;  or  as  concentrated  detritus  along  the  slopes  or 
ravines  of  the  porphyries.  In  the  two  latter  cases  the  ore  is  derived 
from  the  veins.  In  some  cases  this  concentration  occurred  before  or 
during  the  deposition  of  the  Cambrian  sandstones  and  limestones,  but 
in  other  cases  is  subsequent  to  the  deposition  of  these  rocks.  At  Pilot 
Knob  the  succession  from  the  base  upward  is :  Porphyry ;  conglomer- 
ate; a  slaty  ripple-marked  stratum  in  contact  with  the  ore  body;  main 
ore  body  19  to  29  feet  thick;  highly  ferruginous  slate,  1  to  3  feet 
thick;  heavy  beds  of  conglomerate  with  an  average  thickness  of  100 
feet.  The  pebbles  of  the  conglomerate  are  mainly  derived  from  the 
porphyries,  but  the  regularly  laminated  slate  and  ore  have  a  thin- 
bedded  structure,  which  is  such  as  to  lead  to  the  conclusion  that  they 
are  undoubtedly  of  sedimentary  origin. 
Winslow,65  in  1893,  places  in  the  Archean  the  granites,  porphyries, 
and  felsites  of  Missouri,  and  in  the  Algonkian  the  associated  con- 
glomerates, one  of  them  bearing  the  Pilot  Knob  iron  ore. 
Keyes,66  in  1895,  in  an  account  of  the  characteristics  of  the  Ozark 
Mountains,  briefly  describes  the  Archean  and  Algonkian  rocks  of  the 
region.  Archean  rocks  occur  at  the  east  and  west  ends  of  the  Ozark 
uplift.  The  best  known  of  the  areas  is  the  eastern  one,  the  Iron 
Mountain  district  of  southeastern  Missouri.  Here  the  largest  areas 
occur  in  the  vicinity  of  the  peak  from  which  the  district  takes  its 
name,  and  other  smaller  areas  are  scattered  over  a  considerable  range 
of  adjacent  territory.  The  Archean  rocks  in  the  Iron  Mountain  dis- 
trict are  granites  and  porphyries,  the  latter  predominating,  both  of 
which  are  broken  through  in  numerous  places  by  basic  intrusives. 
At  the  western  end  of  the  Ozark  uplift,  in  Indian  Territory,  are 
Archean  rocks,  principally  granites,  of  which  there  are  many  va- 
rieties, cut,  as  in  southeastern  Missouri,  by  dikes  of  basic  material.0 
Immediately  overlying  the  Archean  in  a  number  of  places  are  beds 
of  conglomerates  and  slates  provisionally  referred  to  the  Algonkian. 
These  appear  to  best  advantage  on  Pilot  Knob. 
Haavorth,67  in  1895,  maps  and  fully  describes  many  areas  of  pre- 
Cambrian  crystalline  rocks  of  Missouri.  These  occur  in  irregular 
areas  and  isolated  hills  extending  over  an  area  TO  miles  square  in  the 
southeastern  part  of  the  State.  The  rocks  consist  of  granites,  gran- 
ophyres,  and  porphyries,  which  are  occasionally  cut  by  diabase  dikes. 
Some  of  the  granophyres  are  located  between  the  granite  and  the 
porphyry  areas,  and  seem  to  be  a  connecting  link  between  them.  At 
other  times  they  are  in  contact  only  with  the  granite  or  with  the 
«  Pre-Cambrian  rocks  of  Indian  Territory  occur  only  in  tho  Arbuckle  uplift.  The  Ar 
buckle  uplift  in  southwestern  Indian  Territory  trends  west-northwest  and  east-southeast 
and  is  in  no  sense  to  be  connected  with  the  structure  of  the  Ozark  region. 
