vanhise.]        ISOLATES    AREAS    OF    THE    MISSISSIPPI    VALLEY.  2C3 
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  clay.  Disintegration 
has  often  taken  place  to  a  depth  of  certainly  more  than  50  feet  in  the 
granites  of  Madison  county.  The  iron  ore  of  Iron  mountain  is  a  resid- 
uary 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  porphyries, 
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  frequent  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.  Im- 
mediately below  the  ore  is  porphyry,  which  continues  to  the  base  of 
the  hill. 
The  rocks  forming  the  southwestern  flank  of  Cedar  hill  are  the  exten- 
sion of  the  conglomerates  and  ore  beds  of  Pilot  knob.  Manganese  ores 
are  found  associated  with  the  i)orphyritic  rocks,  and  in  Sec.  16,  T.  33 
N.,  R.  2  E.,  in  Reynolds  county,  the  manganese  ore  is  one  of  the  mem- 
bers of  a  series  of  bedded  porphyries.  At  this  locality  metamorphic 
limestone  is  one  member  of  the  porphyry  series,  but  it  is  now  by  physical 
and  chemical  agents  greatly  changed  from  its  original  condition,  and  is 
very  manganiferous.  Another  member  of  this  same  succession  is  a  por- 
phyry-conglomerate or  breccia,  consisting  of  pebbles  of  red  and  com- 
pact porphyry  containing  grains  of  quartz  and  crystals  of  feldspar,  and 
cemented  by  porphyry  of  a  similar  character.  This  rock  resembles  the 
Caluniet-Hekla  conglomerate  of  lake  Superior. 
Schmidt,10  in  1873,  also  describes  the  iron-ore  deposits  of  Iron  moun- 
tain, Pilot  knob,  Shepherd  mountain,  Cedar  hill,  Buford  hill,  Big  Bogg 
mountain,  Lewis  mountain,  Cuthbertson  bank,  and  Hogan  mountain. 
The  succession  at  Cedar  hill  includes  slates  of  red-banded  porphyry, 
stratified  quartz-porphyry,  slates  of  red  porphyry,  green  porphyry, 
banded  jasper,  and  jasper  with  specular  ore. 
Shumard,17  in  1873,  states  that  granite  is  found  in  Laclede,  Craw- 
ford, and  Ste.  Genevieve  counties. 
Broadhead  and  Norwood,18  in  1874,  describe  granite  and  porphyry 
at  numerous  points  in  Madison  county..  On  the  west  side  of  St.  Fran- 
cois river  in  the  NW.  \  of  KE.  \  Sec.  33,  T.  34  K,  R.  5  E.,  there  is  an 
sxposure  of  sandstone  and  conglomerate  resting  directly  on  the  granite. 
Haworth,19  in  1888,  states  that  the  Archean  area  of  Missouri  covers 
n  irregularly  outlined  portion  of  no  less  than  ten  different  counties  and 
xtends  to  the  west  as  far  as  Texas  county,  to  the  north  and  northeast 
s  far  as  Washington,  St.  Francois  and  Ste.  Genevieve ;  to  the  east 
