ISOLATED   AREAS   IN    MISSISSIPPI   VALLEY.  735 
Harrison,56  in  1868,  describes  two  localities  in  Washington  County, 
where  between  the  horizontal  limestone  and  the  solid  porphyry  arc 
conglomerates  consisting  of  water-worn  pebbles  and  bowlders,  all  of 
porphyry,  cemented  together  by  a  calcareous  matrix.  Interstratified 
with  the  limestone  are  also  thin  layers  containing  water-worn  por- 
phyry pebbles.  It  is  therefore  concluded  that  the  porphyry  hills 
existed  as  such  before  the  Silurian  hills  were  deposited. 
Pumpelly,57  in  1873,  states  that  the  Archean  (Azoic)  rocks  of 
southeastern  Missouri  form  an  archipelago  of  islands  in  the  Lower 
Silurian  strata,  which  surround  them  as  a  whole  and  separate  them 
from  one  another.  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  extensive  surface  development  in  the  region  forming  the 
northern  part  of  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  Sweden.  The  rocks  overlying  them  be- 
long to  the  oldest  known  members  of  the  Silurian,  but  they  may 
be  the  deep-seated  equivalents  of  the  Potsdam  sandstone,  or  even 
older.  Before  the  deposition  of  the  Silurian  the  porphyries  and 
granite  had  undergone  an  enormous  amount  of  erosion,  an  amount 
at  least  several  times  as  great  as  they  have  suffered  since  that  remote 
time. 
The  surface  of  Iron  Mountain  has  disintegrated  and  decomposed 
in  mass,  the  entire  porphyry  hill  being  changed  to  a  cla}^  Disinte- 
gration has  often  taken  place  to  a  depth  of  certainly  more  than  50 
feet  in  the  granites  of  Madison  County.  The  iron  ore  of  Iron  Moun- 
tain is  a  residuary  deposit,  having  its  origin  in  the  gradual  removal 
of  the  existing  crystalline  rocks  and  leaving  behind  the  iron  ore. 
Pilot  Knob  is  composed  of  more  or  less  massively  bedded  porphy- 
ries, porphyry  conglomerates,  and  beds  of  hard  specular  ore.  These 
strata  strike  N.  50°  W.  and  dip  on  an  average  13°  southwest  by  south. 
The  top  of  the  knob  consists  of  stratified  porphyry  conglomerate  with 
a  thickness  of  140  feet.  This  rock  is  made  up  of  small  and  large, 
more  or  less  angular,  pebbles  of  porphyry  cemented  together  by  iron 
ore  and  containing  numerous  layers  and  bodies  of  ore.  At  the  base  of 
this  series  is  a  great  bed  of  ore  divided  into  two  parts  by  a  thin  slate 
seam.  Immediately  below  the  ore  is  porphyry,  which  continues  to  the 
base  of  the  hill. 
The  rocks  forming  the  southwestern  flank  of  Cedar  11  ill  are  Hie 
extension  of  the  conglomerates  and  ore  beds  of  Pilot  Knob.  Man- 
ganese ores  are  found  associated  with  the  porphyritic  rocks,  and  in 
sec.  16,  T.  33  N.,  R.  2  E.,  in  Reynolds  County,  the  manganese  ore  is 
one  of  the  members  of  a  series  of  bedded  porphyries.    At  thi>  locality 
