vanhtsk]       ISOLATED    AREAS   OF    THE    MISSISSIPPI   VALLEY.  261 
descriptions  in  the  original  are  condensed,  and  to  include  nil  which 
bears  upon  structural  relations  would  greatly  extend  the  digesl  given. 
in  the  light  of  the  present  facts  it  would  seem  that  we  have  in  the 
Black  hills  a  set  of  slates  and  schists  as  yet  of  unknown  thickness  and 
undivided  into  series,  although  perhaps  capable  of  subdivision  by  closer 
work.  These  slates  and  schists  have  a  prominent  cleavage  which  is 
independent  of  the  structure.  The  different  parts  of  the  area  vary  in 
degree  of  crystallization,  and  this  crystalline  character  is  due  to  the 
intrusion  of  igneous  rocks;  but,  unlike  ordinary  contact  metamorphism, 
has  extended  from  the  granitic  areas  for  a  distance  of  several  miles. 
For  the  present  we  are  obliged  to  consider  the  whole  as  a  single  series. 
In  the  alternating  slaty  and  quartzose  characters  of  the  rocks,  in  the 
presence  of  siliceous  hematite  and  a  much  folded  condition,  the  present 
structures  being  in  a  vertical  attitude,  they  more  nearly  resemble  the 
Lower  Huronian  of  the  lake  Superior  region  than  any  other  rock  series, 
but  this  is  not  sufficient  warrant  for  the  positive  conclusion  that  the 
two  are  time  equivalents.  The  correlation  of  the  Black  hills  rocks  with 
the  Taconic  on  slight  lithological  evidence  as  is  done  by  Crosby,  or  a 
part  of  them  with  the  Coos  group  on  account  of  the  presence  of  stau- 
rolite,  can  be  considered  as  no  more  than  guesses. 
SECTION  II.  MISSOURI. 
LITERATURE. 
King,  (H.),10  in  1851,  states  that  the  Primitive  formation  is  met  with 
near  a  point  about  70  miles  south  of  St.  Louis  and  -'30  miles  west  of  the 
Misissippi.  It  consists  chiefly  of  granite,  syenite  and  porphyry,  and  rises 
in  cone-like  elevations  or  detached  ridges  to  the  height  often  of  1,000  or 
1,1*00  feet  above  the  level  of  the  Mississippi  river.  Its  sides  and  valleys 
are  frequently  covered  with  sandstone  and  limestone  in  such  quiet  re- 
lationship as  to  show  that  their  deposition  has  taken  place  since  the 
Primitive  rocks  assumed  the  form  they  now  have.  It  is  not  at  all  un- 
usual to  find  portions  or  fragments  of  tfce  older  rocks  imbedded  in  those 
that  are  stratified.  This  occurs  both  in  the  lowest  magnesian  limestone 
and  in  the  overlying  sandstone.  '  The  Primitive  rock  is  broken  through 
by  greenstone  dikes  which  reach  the  surface  of  this  rock,  yet  never 
penetrate  the  overlying  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  of  various  sizes  up  to  afoot  in  diameter.  In  the 
interstices  of  these  pebbles  is  a  reddish  brown  clay.  The  bed  of  iron 
in  the  thickest  point  opened  is  20  feet.  At  the  summit  of  Pilot  knob 
is  an  immense  mass  of  solid  ore  which  is  associated  with  porphyry  and 
appears  to  pass  by  insensible  gradations  into  that  rock. 
Whitney,11  in  1854,  describes  Iron  mountain  and  Pilot  knob  as 
localities  in  which  are  found  eruptive  ores  of  Azoic  a<re.     Iron  uioiiii- 
