﻿234 
  J. 
  P. 
  Kimball 
  — 
  Genesis 
  of 
  Iron-ores 
  hy 
  Isomorphous 
  

  

  of 
  carbonated 
  waters 
  ; 
  next 
  in 
  frequency, 
  from 
  solutions 
  of 
  

   ferrous 
  sulphate 
  in 
  reaction 
  with 
  calcic 
  carbonate 
  ; 
  and, 
  lastly, 
  

   from 
  like 
  reactions 
  with 
  ferrous 
  salts 
  from 
  reduction 
  of 
  ferric 
  

   silicates. 
  

  

  (6.) 
  The 
  stability 
  of 
  this 
  more 
  or 
  less 
  alterable 
  secondary 
  

   product 
  in 
  fissures 
  and 
  deep-seated 
  strata 
  in 
  an 
  atmosphere 
  of 
  

   carbonic 
  anhydride 
  or 
  reducing 
  gases, 
  was 
  also 
  long 
  since 
  

   pointed 
  out 
  by 
  Bischof 
  and 
  W. 
  B. 
  Rogers, 
  as 
  well 
  as 
  its 
  trans- 
  

   formation 
  into 
  ferric 
  hydrate 
  through 
  displacement 
  of 
  such 
  

   gases 
  by 
  atmospheric 
  air. 
  

  

  (T.) 
  The 
  frequent 
  occurrence 
  of 
  limonite 
  and 
  hematite 
  in 
  

   limestone 
  and 
  their 
  graduation 
  into 
  beds 
  of 
  this 
  sedimentary 
  

   material, 
  as 
  well 
  as 
  the 
  presence 
  of 
  similar 
  fossils 
  in 
  both, 
  are 
  

   facts 
  adduced 
  by 
  Bischof 
  to 
  justify 
  the 
  conclusion 
  that 
  iron-ore 
  

   deposits 
  of 
  this 
  description 
  have 
  had 
  their 
  origin 
  in 
  replace- 
  

   ment 
  of 
  limestone 
  beds.* 
  Yet, 
  as 
  by 
  him 
  remarked, 
  replace- 
  

   ment 
  of 
  amorphous 
  limestone 
  by 
  ferric 
  oxide 
  obviously 
  cannot 
  

   be 
  proved 
  mineralogically 
  as 
  in 
  the 
  case 
  of 
  rare 
  occurrences 
  of 
  

   incomplete 
  pseudomorphs 
  after 
  calc-spar, 
  like 
  the 
  specimen 
  

   originally 
  described 
  by 
  Blum. 
  But 
  every 
  geologist 
  has 
  never- 
  

   theless 
  observed 
  ultimate 
  replacement 
  of 
  limestone 
  by 
  brown 
  

   and 
  red 
  ferric 
  oxides, 
  whether 
  direct 
  or 
  indirect, 
  among 
  the 
  

   more 
  common 
  phenomena 
  of 
  weathering. 
  When 
  as 
  sometimes 
  

   happens 
  this 
  is 
  all 
  but 
  complete, 
  and 
  the 
  original 
  form 
  of 
  the 
  

   limestone 
  mass 
  is 
  preserved 
  in 
  situ, 
  the 
  replacement 
  is 
  likewise 
  

   seen 
  to 
  be 
  pseudomorphic 
  — 
  at 
  least 
  in 
  a 
  petrographic 
  sense. 
  

   Dana 
  has 
  given 
  a 
  good 
  pictorial 
  illustration 
  of 
  this 
  kind 
  in 
  

   describing 
  an 
  occurrence 
  in 
  the 
  Cone 
  ore-pit 
  at 
  West 
  Stock- 
  

   bridge.f 
  The 
  cut 
  is 
  here 
  reproduced. 
  Replacements 
  of 
  shells 
  

   and 
  parts 
  of 
  crinoids, 
  still 
  more 
  common, 
  are 
  likewise 
  pseudo- 
  

   morphic 
  in 
  the 
  same 
  limited 
  sense. 
  

  

  The 
  above 
  proposition 
  affords 
  grounds 
  

   for 
  a 
  ready 
  and 
  complete 
  explanation 
  of 
  

   the 
  common 
  association 
  of 
  iron 
  ores 
  with 
  

   limestone 
  as 
  far 
  from 
  accidental. 
  This 
  

   association 
  would 
  obviously 
  be 
  still 
  more 
  

   common 
  had 
  all 
  replacements 
  of 
  thin 
  

   limestone 
  beds 
  been 
  only 
  partially 
  ef- 
  

   fected, 
  as 
  in 
  replacements 
  of 
  thick 
  limestone, 
  which 
  are 
  

   necessarily 
  incomplete 
  or 
  relatively 
  superficial. 
  Occurrences 
  

   of 
  the 
  latter 
  kind 
  justify 
  the 
  conclusion 
  that 
  thin 
  beds 
  of 
  

   limestone 
  have 
  in 
  fact 
  in 
  numerous 
  instances 
  been 
  wholly 
  

   or 
  pseudomorphously 
  replaced. 
  Hence 
  frequent 
  occurrences 
  

   of 
  lenticular 
  beds 
  of 
  siderite 
  and 
  of 
  its 
  derivatives 
  in 
  place 
  

   of 
  thin 
  limestones, 
  of 
  which 
  no 
  trace 
  may 
  remain 
  except 
  

  

  *Bd. 
  Ill, 
  1866, 
  873. 
  

  

  f 
  T>is 
  Journal, 
  xiv, 
  1877, 
  p. 
  136. 
  See, 
  also, 
  Rep. 
  Tenth 
  Census, 
  xv, 
  292, 
  296, 
  

   297, 
  299, 
  396. 
  

  

  