734  PRE-CAMBRIAN    GEOLOGY    OF    NORTH   AMERICA. 
often  of  1,000  or  1,200  feet  above  the  level  of  the  Mississippi  River. 
The  sides  and  valleys  are  in  many  places  covered  with  sandstone  and 
limestone  in  such  quiet  relationship  as  to  show  that  their  deposition 
has  taken  place  since  the  Primitive  rocks  assumed  the  form  they 
now  have.  It  is  not  at  all  unusual  to  find  portions  or  fragments  of 
the  older  rocks  embedded  in  those  that  are  stratified.  This  occurs 
both  in  the  lowest  magnesian  limestone  and  in  the  overlying  sand- 
stone. The  Primitive  rock  is  broken  through  by  greenstone  dikes 
which  reach  the  surface  of  this  rock  yet  never  penetrate  the  overly- 
ing sandstone.  At  Iron  Mountain  is  a  layer  of  specular  oxide  of 
iron,  below  which  at  one  place  is  a  stratified  rock  which  may  have 
been  a  modified  granite.  The  iron-ore  deposit  is  often  in  the  form 
of  pebbles  in  various  sizes  up  to  a  foot  in  diameter.  In  the  inter- 
stices of  these  pebbles  is  a  reddish-brown  clay.  The  bed  of  iron 
in  the  thickest  point  opened  is  20  feet  thick.  At  the  summit  of  Pilot 
Knob  is  an  immense  mass  of  solid  ore  which  is  associated  with  por- 
phyry and  appears  to  pass  by  insensible  gradations  into  that  rock. 
Whitney,53  in  1854,  describes  Iron  Mountain  and  Pilot  Knob  as 
localities  in  which  are  found  eruptive  ores  of  Azoic  age.  Iron  Moun- 
tain is  a  flattened  dome-shaped  elevation,  composed  of  feldspathic 
porphyiy.  The  surface  of  the  mound  is  covered  with  loose  pieces 
of  ore,  which  is  in  some  places  in  a  layer  at  least  15  feet  thick.  Pilot 
Knob  is  mainly  composed  of  dark  siliceous  rock,  distinctly  bedded, 
dipping  to  the  south  at  an  angle  of  25°  or  30°.  For  about  two-thirds 
of  its  height  of  650  feet  quartz  rocks  predominate;  above  that  iron 
is  found  in  heavy  beds  alternating  with  siliceous  matter. 
Swallow,54  in  1855,  describes  the  granite,  greenstone,  and  por- 
phyry of  Missouri  as  igneous  rocks.  Red  feldspathic  granite,  spar- 
ingly micaceous,  occurs  in  sec.  15,  T.  34  N.,  R.  3  E.  Nearly  all  of 
the  hills  and  ridges  in  the  neighborhood  of  Iron  Mountain  and 
Pilot  Knob  are  wholly  or  in  part  formed  of  compact  reddish-purple 
feldspathic  porphyry.  The  porphyry  is  one  of  the  oldest  rocks  of 
the  State,  but  no  opportunity  occurred  for  determining  whether  it 
is  older  than  the  granite,  although  there  is  no  doubt  that  it  is  older 
than  the  greenstone,  as  the  latter  rock  is  said  to  occur  in  dikes  in  the 
porphyry.  The  porphyry  is  older  than  the  stratified  rocks  of  the 
region,  because  they  are  found  resting  upon  knobs  and  ridges  of 
porphyry  in  a  position  so  nearly  horizontal  as  to  preclude  the  idea 
that  they  were  deposited  before  the  upheaval  of  the  principal  masses 
which  form  the  hills.  Whether  the  slates  interstratified  with  the 
iron  near  the  top  of  Pilot  Knob  are  older  is  not  easily  determined. 
Swallow,55  in  1859,  states  that  in  one  locality  in  Laclede  County 
and  in  one  or  two  in  Crawford  County  are  granite  dikes  or  ridges 
which  rise  above  the  stratified  rocks. 
