308  CONTRIBUTIONS   TO    ECONOMIC    GEOLOGY,  1904.        [bull.  26$ 
In  the  crevice  deposits  the  ores  have  a  general  order  of  sequence  of 
deposition  as  follows,  beginning  next  to  the  wall  rock:  (1)  marea- 
site;  (2)  sphalerite  with  at  times  some  galena;  (3)  sphalerite,  frej 
quently  the  main  mass  o{  the  ore:  (  i)  cubes  of  galena;  (5)  calcitei 
(6)  barite;  (7)  open  cavity.  This  order,  while  not  universal,  i| 
exceedingly  common,  although  all  members  of  the  sequence  arj 
usually  not  present  in  any  one  crevice.  Not  uncommonly  the  deposit! 
have  been  fissured  and  the  cracks  filled  in  with  later  marcasite,  am 
at  times  there  i>  a  mixing,  apparently,  of  all  the  sulphides. 
The  best  kn«>\\  n  disseminated  deposits  occur  along  tributaries  of  tin 
Little  Platte  River,  3  or  1  miles  west  ami  southwest  oi  Platteville 
Here  this  type  oi  deposit,  lying  in  and  just  above'the  main  oil-rod 
layer,  is  less  rich  in  marcasite  than  are  the  flats  ami  pitches.  Thesj 
deposits  are  well  shown  in  the  Graham,  the  Capitola,  the  St.  Kosc 
and  the  Klondike  mines. 
The  ore-  thus  far  described  lie  below  the  level  oi  ground  water.  L 
the  crevices  above  this  level  are  layer-  of  smithsonite,  clearly  derive 
from  the  alteration  o(  sphalerite,  ami  galena,  the  latter  frequently  i 
large  masses.  Such  ore-  were  early  mined  in  this  region  ami  the 
-till  continue  to  he  mined,  although  now  of  much  less  relative  impoi 
tance  than  formerly. 
The  origin  of  the  ore  deposits  will  he  discussed  in  a  comprehensij 
report  on  the  lead  ami  zinc  deposits  oi  the  whole  Upper  Mississipj 
Valley  lead  and  zinc  district  It  is  sufficient  to  state  here  that  the  oi 
deposits  a  ithout  question  owe  their  origin  to  circulating  waters  whic 
have  leached  the  metallic  substances  from  the  surrounding  limeston 
mainly  the  Galena  limestone,  ami  deposited  them  in  their  preset 
positions.  At  least  one  important  agent  in  the  precipitation  of  the 
metallic  sulphides  was  the  carbonaceous  matter  in  the  rocks,  esp 
cially  in  the  oil-rock  horizon. 
MINING  AND   CONCENTRATING. 
In  former  years,  and  to-day  in  some  of  the  smaller  diggings,  mini! 
was  carried  on  in  a  very  primitive  manner  for  the  most  part.    The  oi 
were  mined  and  brought  to  the  surface  usually  by  hand  ami  we 
cleaned  by  cobbing  or  by  hand  jigging,     The  present  development 
the  region  is  due  in  considerable  pan  to  the  introduction  of  im 
modern    methods   of    mining   ami    oi   concentrating    the    ores.     T 
plant-  are,  from  the  nature  of  the  deposits,  comparatively  small,  V 
well  adapted  to  the  needs  of  the  region.     During  the  last  three  yej 
about  thirty  concent  ratine;-  mill-  have  been  erected.     These  are  simif 
to  those  used  in  Joplin  region,  and  make  a  successful  separation 
the  galena    from  the  sphalerite  and  the  marcasite,  hut   these  t\ 
because  oi  somewhat  similar  specific  gravity,  can  not  be  sueeossfu 
