264  PRE-CAMBRIAN   ROCKS    OP    NORTH    AMERICA.  [bull.  86. 
it  passes  through  Madison  county,  and  to  the  south  nearly  through 
Wayne  county,  but  only  a  small  portion  of  this  territory  is  covered  by 
Archean  rocks.  The  rocks  are  the  different  kinds  of  porphyry,  which 
predominate,  granite,  and  dikes  of  diabase  and  diabase-porphyry. 
Numerous  instances  were  observed  where  the  stratified  rocks  overlie 
the  massive  ones  and  are  nonconformable  with  them.  Nowhere  at  the 
contact  zone  was  metamorphosed  limestone  or  sandstone  observed. 
The  granites,  so  far  as  observed,  occur  on  low  ground,  while  the  hills 
are  almost  invariably  composed  of  porphyry.  At  numerous  places 
dikes  of  various  sizes  occur,  sometimes  in  the  granite  and  sometimes 
in  the  porphyry,  and,  as  stated  by  Broadhead,  sometimes  in  the  sand- 
stone. Detailed  descriptions  are  given  of  the  granites,  porphyries,  and 
dike  rocks. 
Haworth,20  in  1891,  describes  the  crystalline  rocks  in  the  vicinity 
of  Pilot  knob,  Missouri.  These  are  chiefly  porphyries,  felsites,  and 
breccias.  These  rocks  are  regarded  as  Archean  in  age,  because  there 
is  no  contact  metamorphism  between  them  and  the  surrounding  Pale- 
ozoic rocks,  and  because  in  the  Paleozoic  sandstones  and  limestones  are 
numerous  fragments  of  the  crystalline  rocks.  The  crystalline  rocks  are 
regarded  as  of  eruptive  origin,  as  shown  in  the  field  by  the  absence  of 
bedding,  by  flow  structure,  by  banded  structure,  by  lithophysse,  by 
breccia,  by  scoria,  by  amygdaloids,  by  tuffs,  and  by  absence  of  grada- 
tions into  noncry stall ine  rocks,  and  as  shown  by  the  microscope,  by 
the  texture  of  the  groundmass  in  the  porphyries  and  breccias,  by  flow 
structure  in  them,  by  magmatic  corrosion  of  porphyritic  crystals  and 
fragments  of  the  breccias,  and  by  other  phenomena.  The  laminated 
ferruginous  rocks  of  Pilot  knob  and  of  other  localities  are  regarded  as 
volcanic  breccias.  As  evidence  of  this  it  is  said  that  this  material 
passes  into  the  porphyry,  that  the  fragments  are  all  of  porphyry  or 
felsite,  and  that  the  groundmasses  of  the  breccias  or  conglomerates  are 
always  felsitic  or  porphyritic,  the  apparent  detrital  fragments  being 
merely  set  in  a  lava  of  a  similar  character. 
Pumpelly  and  Yan  Hise,21  in  1890,  find  that  at  Iron  mountain 
the  ore  (specular  hematite),  in  its  original  position,  occurs  in  the  form 
of  veins  in  the  porphyry.  These  veins  are  sometimes  of  very  consid- 
erable thickness,  running  as  high  as  30  feet.  They  vary  from  this  size 
to  those  much  smaller,  ramifying  through  and  cutting  the  porphyry  inj 
various  directions.  In  some  places  on  the  mound  between  the  stratified 
sandstone  and  the  porphyry  is  a  pre- Silurian  mantle  of  detrital  mate- 
rial, which  is  largely  composed  of  fragments  of  the  vein  ore.  The  chief; 
mining  at  the  present  time  is  from  a  mass  of  bowlders  of  the  iron  ore 
in  a  pre- Silurian  ravine.  In  the  process  of  disintegration  the  more  resist- 
ant and  heavier  masses  of  iron  ore  have  been  concentrated  in  the  uppe 
slopes  of  the  ravine,  forming  a  deposit  analogous  to  a  placer.  The  vas 
amount  of  this  iron  ore  in  these  ravines,  as  well  as  that  which  occurs  as 
residuary  deposit  upon  the  mound,  indicates  that  in  pre- Silurian  tim 
