﻿236 
  J. 
  P. 
  Kimball 
  — 
  Genesis 
  of 
  Iron-ores 
  by 
  Isomorphous 
  

  

  Upon 
  another 
  theory, 
  commonly 
  entertained 
  as 
  a 
  collateral 
  

   theory 
  by 
  the 
  same 
  geologists 
  who 
  employ 
  the 
  one 
  just 
  stated, 
  

   its 
  derivation 
  is 
  also 
  attributed 
  to 
  direct 
  deposition 
  through 
  

   volatilization 
  of 
  free 
  carbonic 
  acid 
  from 
  aqueous 
  carbonated 
  

   solution 
  — 
  likewise 
  in 
  atmospheres 
  of 
  hydro-carbon 
  gases 
  and 
  

   carbonic 
  anhydride. 
  

  

  (11.) 
  No 
  natural 
  occurrence 
  and 
  therefore 
  no 
  mineral 
  species 
  

   of 
  hydrous 
  ferrous 
  carbonate 
  seems 
  to 
  have 
  been 
  recognized 
  

   by 
  mineralogists. 
  A 
  moderately 
  instable 
  white, 
  earthy 
  amor- 
  

   phous 
  hydrate 
  said 
  by 
  Massieu 
  to 
  have 
  occurred 
  in 
  the 
  mineral 
  

   lode 
  of 
  Pontpeon, 
  France,* 
  seems 
  to 
  have 
  possessed 
  the 
  same 
  

   characteristics 
  as 
  an 
  occurrence 
  beneath 
  an 
  ochreous 
  deposit 
  of 
  

   a 
  carbonated 
  spring 
  near 
  Laacher-See 
  in 
  the 
  Eifel, 
  but 
  de- 
  

   scribed 
  by 
  Bischof 
  as 
  siderite 
  or 
  the 
  anhydrous 
  salt.f 
  The 
  

   same 
  locality 
  is 
  famous 
  for 
  exhalations 
  of 
  carbonic 
  acid. 
  

   Preservation 
  of 
  the 
  artificial 
  product 
  appears 
  to 
  be 
  impracti- 
  

   cable 
  except 
  in 
  an 
  atmosphere 
  displaced 
  by 
  carbonic 
  anhy- 
  

   dride, 
  or, 
  as 
  easily 
  supposable, 
  by 
  reducing 
  gases. 
  

  

  (12.) 
  Siderite 
  pseudomorphous 
  after 
  crystalline 
  anhydrous 
  

   calcic 
  carbonate 
  not 
  uncommonly 
  occurs 
  both 
  in 
  hexagonal 
  and 
  

   trimetric 
  forms, 
  though 
  isomorphous 
  only 
  in 
  the 
  former 
  case. 
  

   This 
  fact 
  goes 
  far 
  to 
  show 
  that 
  the 
  phenomena 
  of 
  replacement 
  

   of 
  calcic 
  carbonate 
  by 
  anhydrous 
  ferrous 
  carbonate 
  are 
  not 
  

   simply 
  those 
  of 
  isomorphism. 
  Yet 
  it 
  is 
  true 
  that 
  in 
  crystal- 
  

   line 
  as 
  well 
  as 
  in 
  amorphous 
  siderite 
  ferrous 
  carbonate 
  is 
  

   extremely 
  apt 
  to 
  be 
  partially 
  replaced 
  with 
  isomorphous 
  car- 
  

   bonates 
  of 
  lime, 
  magnesia, 
  manganese 
  and 
  zinc. 
  The 
  first 
  

   three, 
  and 
  sometimes 
  all 
  four, 
  of 
  these 
  carbonates 
  are 
  freely 
  

   developed 
  even 
  where 
  sparry 
  siderite 
  distinctly 
  occurs 
  as 
  a 
  

   product 
  of 
  epigenesis, 
  particularly 
  in 
  drusy 
  cavities 
  and 
  fissures 
  

   in 
  basic 
  rocks 
  inaccessible 
  to 
  atmospheric 
  air. 
  

  

  (13.) 
  The 
  much 
  greater 
  tendency 
  to 
  precipitation 
  of 
  ferric 
  

   hydrate 
  from 
  aqueous 
  solutions 
  of 
  ferrous 
  carbonate 
  than 
  of 
  

   the 
  salt 
  itself 
  by 
  dissipation, 
  as 
  assumed, 
  of 
  carbonic 
  acid, 
  is 
  

   well 
  exhibited 
  by 
  Roth 
  in 
  the 
  case 
  of 
  numerous 
  mineral 
  waters 
  

   and 
  deposits 
  of 
  mineral 
  springs, 
  as 
  well 
  as 
  the 
  relative 
  and 
  pro- 
  

   portional 
  precipitation 
  of 
  alkaline 
  and 
  manganous 
  carbonates.^ 
  

  

  The 
  existence 
  of 
  stable 
  siderite 
  in 
  calcareous 
  sinter 
  points 
  to 
  

   replacement 
  of 
  calcic 
  carbonate 
  previously 
  deposited. 
  Away 
  

   from 
  oxidizing 
  atmospheres, 
  anhydrous 
  ferrous 
  carbonate, 
  if 
  

   ever 
  directly 
  deposited, 
  which 
  there 
  seems 
  much 
  reason 
  to 
  

   doubt, 
  is 
  probably 
  by 
  reaction 
  of 
  solutions 
  of 
  ferrous 
  salts 
  

   with 
  these 
  anhydrous 
  carbonates, 
  and 
  at 
  ordinary 
  temperatures 
  

   in 
  no 
  other 
  way. 
  But 
  as 
  all 
  known 
  reactions 
  of 
  this 
  kind 
  

   result 
  in 
  hydrous 
  ferrous 
  carbonate 
  from 
  which 
  passage 
  into 
  

   the 
  anhydrous 
  carbonate 
  at 
  ordinary 
  temperatures 
  is 
  difficult 
  to 
  

  

  * 
  Compt. 
  Rend., 
  lix, 
  238. 
  f 
  Chem. 
  Geol., 
  Bd. 
  I, 
  1863, 
  550. 
  

  

  \ 
  Chem. 
  Geol., 
  Bd. 
  I, 
  565, 
  577. 
  

  

  