ISOLATED   AREAS    IN    MISSISSIPPI   VALLEY.  737 
mantle  of  detrital  material,  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  resistant  and  heavier  masses  of  iron  ore 
have  been  concentrated  in  the  upper  slopes  of  the  ravine,  forming  a 
deposit  analogous  to  a  placer.  The  vast  amount  of  this  iron  ore  in 
these  ravines,  as  well  as  that  which  occurs  as  a  residuary  deposit  upon 
the  mound,  indicates  that  in  pre-Silurian  time  there  was  here  an  enor- 
mous erosion,  otherwise  this  quantity  could  not  have  accumulated 
from  the  relatively  sparse  and  small  veins  of  iron  ore  in  the  moun- 
tain. The  Pilot  Knob  iron-ore  bed  was  found  to  grade  upward  into 
a  conglomerate,  the  matrix  of  which  is  largely  composed  of  ore  and 
most  of  the  pebbles  of  which  are  porphyry.  The  whole  appearance  of 
the  deposit  is  that  of  a  detrital  one,  and  the  question  arises  whether 
this  bed  has  been  produced  from  the  erosion  of  earlier  vein  deposits 
in  the  porphyries,  such  as  are  found  in  Iron  Mountain.  Pilot  Knob 
itself  bears  the  same  relations  to  the  Silurian  as  does  Iron  Mountain, 
and  if  this  suggestion  as  to  the  origin  of  the  Pilot  Knob  ore  is  correct, 
it  implies  that  the  pre-Silurian  history  has  been  not' only  very  long 
but  complex. 
Ha  worth,03  in  1891,  describes  the  crystalline  rocks  in  the  vicinity 
of  Pilot  Knob,  Mo.  These  are  chiefly  porphyries,  felsites,  and  brec- 
cias. 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  litho- 
physse,  by  breccia,  by  scoria,  by  amygdaloids,  by  tuffs,  and  by  absence 
of  gradations  into  noncrystalline  rocks,  and  as  shown  by  the  micro- 
scope by  the  texture  of  the  groundmass  in  the  porphyries  and  brec- 
cias, 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  por- 
phyry or  felsite,  and  that  the  groundmasses  of  the  breccias  or  con- 
glomerates are  always  felsitic  or  porphyritic,  the  apparent  detrital 
fragments  being  merely  set  in  a  lava  of  a  similar  character. 
Nason,64  in  1892,  fully  describes  the  iron  ores  of  the  porphyry 
region  of  Missouri,  and  incidentally  treats  of  the  associated  rocks. 
The  porphyries  usually  show  evidence  of  bedding,  but  this  may  be 
that  of  igneous  flows.  The  Cambrian  lime-tones  and  sandstones 
flank   and   rest   unconformably    upon    the  granite-   and    porphyries. 
55721— Bull.  3G0— 09 47 
