﻿J. 
  P. 
  Kimball 
  — 
  Genesis 
  of 
  Iron-ores, 
  etc. 
  231 
  

  

  Art. 
  XXII. 
  — 
  Genesis 
  of 
  Iron-ores 
  by 
  Isomorphous 
  and 
  Pseu- 
  

   domorphous 
  Replacement 
  of 
  Limestone, 
  etc.; 
  by 
  James 
  P. 
  

   Kimball. 
  

  

  It 
  is 
  the 
  object 
  of 
  the 
  present 
  memoir 
  briefly 
  to 
  develop 
  the 
  

   following 
  proposition, 
  namely, 
  that 
  well 
  recognized 
  products 
  

   of 
  epigenesis, 
  like 
  siderite 
  and 
  ferro-calcite, 
  in 
  their 
  several 
  

   forms 
  and 
  wide 
  distribution 
  especially 
  on 
  a 
  petrographic 
  scale, 
  

   are 
  as 
  a 
  rule 
  also 
  products 
  of 
  direct 
  pseudomorphous 
  replace- 
  

   ment 
  of 
  isomorphous 
  calcic 
  carbonate, 
  like 
  limestone, 
  calcite, 
  

   calc-sinter, 
  calcareous 
  sediments, 
  calc-schutt, 
  etc. 
  This 
  prop- 
  

   osition 
  is 
  not 
  new. 
  But 
  some 
  of 
  the 
  conditions 
  remain 
  un- 
  

   settled. 
  So 
  also 
  some 
  of 
  the 
  deductions 
  which 
  have 
  been 
  

   thought, 
  or 
  may 
  seem, 
  to 
  follow. 
  These 
  it 
  is 
  my 
  purpose 
  

   briefly 
  to 
  discuss. 
  

  

  Contingent 
  to 
  this 
  proposition, 
  it 
  follows 
  that 
  secondary 
  or 
  

   indirect 
  replacement 
  of 
  calcic 
  carbonate 
  by 
  ferric 
  hydrate 
  is 
  

   wrought 
  through 
  alteration 
  of 
  pseudomorphous 
  siderite 
  or 
  ferro- 
  

   calcite, 
  and 
  also, 
  through 
  progressive 
  alteration, 
  by 
  ferric 
  oxide 
  

   and 
  even 
  magnetic 
  oxide. 
  Hence 
  proximate 
  derivation 
  from 
  

   siderite 
  of 
  many 
  occurrences 
  of 
  iron-ores 
  which 
  nevertheless 
  

   are 
  ultimate 
  products 
  of 
  indirect 
  or 
  progressive 
  pseudomor- 
  

   phism 
  of 
  calcic 
  carbonate 
  — 
  itself 
  often 
  a 
  product 
  of 
  epigenesis 
  

   from 
  basic 
  silicates. 
  Such 
  occurrences 
  may 
  therefore 
  be 
  re- 
  

   garded 
  as 
  instances 
  of 
  double 
  pseudomorphism, 
  sometimes 
  on 
  

   a 
  petrographic 
  scale 
  ; 
  that 
  is, 
  pseudomorphism 
  in 
  the 
  first 
  in- 
  

   stance 
  by 
  substitution 
  or 
  replacement 
  ; 
  in 
  the 
  second 
  instance 
  

   by 
  alteration. 
  

  

  Again, 
  ferric 
  hydrate 
  apparently 
  directly 
  pseudomorphous 
  

   after 
  limestone 
  is 
  produced 
  by 
  immediate, 
  perhaps 
  spontaneous, 
  

   oxidation 
  of 
  ferrous 
  carbonate, 
  resulting 
  from 
  interchange 
  or 
  

   double 
  decomposition 
  between 
  solutions 
  of 
  this 
  salt 
  or 
  ferrous 
  

   sulphate 
  and 
  calcic 
  carbonate 
  in 
  place. 
  All 
  these 
  permutations 
  

   proceed 
  from 
  the 
  same 
  reactions, 
  but 
  differ 
  in 
  results 
  according 
  

   to 
  atmospheric 
  environment 
  — 
  whether 
  oxidizing 
  or 
  not. 
  The 
  

   instable 
  salt 
  as 
  first 
  separated, 
  it 
  is 
  scarcely 
  necessary 
  to 
  add, 
  

   is 
  thrown 
  down 
  from 
  solutions 
  either 
  of 
  ferrous 
  carbonate 
  or 
  

   ferrous 
  sulphate 
  indifferently, 
  in 
  reaction 
  with 
  dissolved 
  calcic 
  

   carbonate 
  or 
  other 
  alkaline 
  mono-carbonates. 
  This 
  salt 
  how- 
  

   ever, 
  it 
  is 
  important 
  early 
  to 
  remark, 
  is 
  the 
  hydrous 
  salt, 
  from 
  

   which 
  geologists, 
  it 
  seems, 
  are 
  not 
  accustomed 
  to 
  distinguish 
  

   the 
  anhydrous 
  carbonate 
  which 
  is 
  almost, 
  if 
  not 
  altogether, 
  ex- 
  

   clusively 
  its 
  natural 
  form. 
  

  

  Other 
  adventitious 
  occurrences 
  of 
  brown 
  and 
  red 
  ferric 
  

   oxide 
  well 
  recognized 
  as 
  exotic, 
  that 
  is, 
  neither 
  in 
  original 
  

  

  Am. 
  Jour. 
  Sci. 
  — 
  Third 
  Series, 
  Vol. 
  XLII, 
  No. 
  249. 
  — 
  September, 
  1891. 
  

   16 
  

  

  