2G2  PRE-CAMBRIAN   ROCKS   OF    NORTH   AMERICA.  [hull. SO. 
tain  is  a  flattened  dome-shaped  elevation,  composed  of  feldspathi< 
porphyry.  The  surface  of  the  mound  is  covered  with  loose  pieces  ol 
ore,  which  is  in  some  places  in  a  layer  at  least  15  feet  thick.  Pilot  knot 
is  mainly  composed  of  dark  siliceous  rock,  distinctly  bedded,  dipping  tc 
the  south  at  an  angle  of  25°  or  30°.  For  about  two-thirds  of  its  heigld 
of  G50  feet  quartz  rocks  predominate.  Above  that,  iron  is  found  in 
heavy  beds  alternating  with  siliceous  matter. 
Sw allow,12  in  1855,  describes  the  granite,  greenstone,  and  porphyry 
of  Missouri  as  igneous  rocks.  Bed  feldspathic  granite,  sparingly 
micaceous,  occurs  in  Sec.  15,  T.  34  ST.,  R>.  3  E.  Nearly  all  of  the  hills 
and  ridges  in  the  neighborhood  of  Iron  mountain  and  Pilot  knob  arc 
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  lattei 
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  inter- 
stratified  with  the  iron  near  the  top  of  Pilot  knob  are  older  is  not 
easily  determined. 
Swallow,  13  in  1859,  states  that  in  one  locality  in  Laclede  and  in  one 
or  two  in  Crawford  county  are  granite  dikes  or  ridges  which  rise 
above  the  stratified  rocks. 
Harrison,  u  in  1868,  describes  two  localities  in  Washington  county, 
where  between  the  horizontal  limestone  and  the  solid  porphyry  are 
conglomerates  consisting  of  water- worn  pebbles  and  bowlders  all  oi 
porpjryry,  cemented  together  by  a  calcareous  matrix.  In  terstra  tilled 
with  the  limestone  are  also  thin  layers  containing  water- worn  porphyry 
pebbles.  It  is  therefore  concluded  that  the  porphyry  hills  existed  as 
such  before  the  Silurian  hills  were  deposited. 
Pumpelly,  15  in  1873,  states  fchat  the  Archean  (Azoic)  rocks  of  south- 
eastern Missouri  form  an  archipelago  of  islands  in  the  Lower  Silurian 
strata  which  surrounds  them  as  a  whole  and  separates  them  from  each 
other.  They  appear  as  knobs  1,400  to  1,800  feet  above  the  sea,  and 
rising  300  to  700  feet  or  more  above  the  valleys.  The  rocks  consist 
chiefly  of  granites  and  felsitic  porphyries.  They  reach  their  most  ex] 
tensive  surface  development  in  the  region  forming  the  northern  part  oj 
Madison,  Iron,  and  Reynolds,  and  the  southern  part  of  St.  Francis  and 
Washington  counties.  This  series  is  the  near  equivalent  in  point  of 
age  to  the  iron-bearing  rocks  of  lake  Superior,  New  Jersey,  and  Swedenj 
The  rocks  overlying  them  belong  to  the  oldest  known  members  of  thd 
Silurian,  but  they  may  be  the  deep-seated  equivalents  of  the  Potsuamj 
sandstone  or  even  older.  Before  the  deposition  of  the  Silurian  the  porl 
phyries  and  granite  had  undergone  an  enormous  amount  of  erosion,  ail 
