van  hiss.]       ISOLATED    AREAS    OF    THE    MISSISSIPPI    VALLEY.  2G5 
there  was  here  an  enormous  erosion  in  order  that  this  quantity  should 
accumulate  from  the  relatively  sparse  and  small  veins  of  iron  ore  in  the 
mountain.  The  Pilot  knob  iron-ore  bed  was  found  to  grade  upward  into 
a  conglomerate,  the  matrix  of  which  is  largely  composed  of  ore  and 
the  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  im- 
plies that  the  pre-Silurian  history  has  not  only  been  very  long,  but  com- 
plex. 
SUMMARY   OF   RESULTS. 
As  the  crystalline  rocks  of  central  Missouri  are  islands  surrounded 
by  Cambrian  sediments,  we  have  no  definite  means  of  determining  their 
age  except  that  they  wrere  in  their  present  condition  and  deeply  eroded 
before  the  overlying  rocks  were  deposited.  With  a  considerable  degree 
of  probability  they  may  therefore  be  referred  to  the  pre-Cambrian.  The 
series  is  mainly  an  eruptive,  porphyritic  one,  but  the  lavas  are  often- 
times bedded.  The  elastics  are  sometimes  porphyry-conglomerates, 
the  materials  of  which  have  evidently  been  derived  from  the  underly- 
ing porphyry  flows.  At  Pilot  knob  the  iron  ores  are  associated  with 
the  conglomerates. 
There  is,  then,  in  central  Missouri  a  pre-Cambrian  clastic  series,  and 
therefore  a  member  of  the  Algonkian  system.  Whether  any  of  the 
crystalline  rocks  are  older  than  the  Algonkian  there  is  no  certain  means 
of  judging.  There  are  also  no  certain  data  upon  which  to  parallelize 
this  Algonkian  series  with  the  Algonkian  series  of  the  nearest  pre- 
Cambrian  region,  that  of  lake  Superior.  Upon  the  whole,  the  litho- 
logical  character  of  the  series  more  nearly  resembles  that  of  the  Ke- 
weenawan than  any  other,  although  it  has  a  very  considerable  likeness 
to  the  Upper  Huronian.  This  is  indicated  by  the  porphyries  and  por- 
phyry-conglomerates, while  the  analogy  with  the  Upper  Huronian  is 
indicated  by  the  beds  of  iron  ore.  This  comparison  is  rather  strength- 
ened by  the  fact  that  the  Upper  Huronian  quartzite  outcrops  of  south- 
ern Wisconsin  are  associated  with  and  cut  by  porphyries.  But  a  refer- 
ence of  the  Missouri  rocks  either  to  the  Keweenawan  or  Upper  Huro- 
nian has  a  very  uncertain  value,  and  it  is  not  impossible  that  it  rather 
represents  the  period  of  erosion  which  separates  the  Keweenawan  and 
Upper  Huronian,  since  in  lithological  characters  it  combines  to  a  con- 
siderable extent  those  of  these  two  series. 
